Abstract

The authors examined the relation between fish consumption during pregnancy and fetal growth among 44,824 women from the Danish National Birth Cohort (1996–2002). They evaluated the associations between consumption of total fish, fatty fish, and lean fish in midpregnancy and birth weight, birth length, and head circumference among singleton full-term infants. Fish consumption was ascertained by food frequency questionnaire. The birth of infants classified below the 10th percentile for gestational age and gender was significantly increased among women who consumed more than 60 g of fish per day, as compared with women who consumed 5 g or less per day. Adjusted odds ratios were 1.24 (95% confidence interval (CI): 1.03, 1.49) for birth weight and 1.21 (95% CI: 1.01, 1.43) for head circumference. The adjusted odds ratio was borderline significant for birth length (odds ratio = 1.20, 95% CI: 1.00, 1.45). These increases in risk were followed by small decreases in average values for these growth measures. Furthermore, the inverse association for total fish consumption could be explained by consumption of fatty fish, while no association was found for lean fish. These results indicate that consumption of fatty fish, a known route of exposure to persistent organic pollutants, could be associated with reduced fetal growth.

Fish contains many nutritionally important components, including marine n-3 polyunsaturated fatty acids, protein, vitamin D, and minerals such as selenium and iodine. Previous observational studies (1–4) have shown a positive association between fish consumption and birth weight and prolonged gestation. While the observed increase in birth weight was related to both increased length of gestation and increased fetal growth, much attention has been given to the role of marine n-3 fatty acids, particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which have shown positive associations with prolonged gestation in several randomized controlled trials (5–8). Other trials, however, have found no association (9–11). In a large randomized controlled trial focusing on fish oil supplements and length of gestation conducted within the Danish National Birth Cohort (12), no association was found, and it appears that the importance of these fatty acids in length of gestation is far from clear. In a recent observational study from the United States (13), fish consumption was even found to be inversely associated with fetal growth, while no association was observed with length of gestation. Another recent observational study from Iceland (14) found a positive association between fish consumption and fetal growth but an inverse association with high intake of fish liver oil.

Fish consumption is also a well-known route of exposure to pollutants such as methyl mercury (15) and persistent organic pollutants; exposure to the latter comes mainly from consumption of fatty fish (16, 17). However, only a few studies have shown a direct association between these contaminants and reduced fetal growth (18). An ecologic study from Sweden (17) found that women from the eastern coast, who eat fish from the contaminated Baltic Sea, had a higher risk of giving birth to low birth weight (<2,500 g) infants than did women on the western coast, where the locally caught fish are less contaminated (19). A study from the Faroe Islands (15) also showed an inverse association between serum levels of EPA and fetal growth, where exposure to methyl mercury and polychlorinated biphenyls was high through consumption of whale meat and blubber.

Our objectives in the present study were to examine the association between fish consumption and fetal growth among singleton full-term infants and to determine the importance of type of fish in this association by distinguishing between fatty fish and lean fish. We used birth weight, length, and head circumference adjusted for gestational age as measures of fetal growth.

MATERIALS AND METHODS

Population and study design

This study was based on data from the Danish National Birth Cohort, whose structure has been described elsewhere (20). In brief, 101,046 pregnant women from throughout Denmark were recruited between 1996 and 2002. All pregnant women living in Denmark who were fluent in Danish were eligible for recruitment. Recruitment took place around weeks 6–10 of gestation during the first antenatal visit to the general practitioner.

It was estimated that during the study period, approximately 30 percent of all deliveries in Denmark were covered by the cohort (20). Nohr et al. (21) concluded that this participation rate should not result in biased estimates of association between lifestyle factors and health outcomes, such as smoking and risk of small-for-gestational-age birth.

Data collection in this study consisted of a recruitment form handed out at the first antenatal visit, a self-administered food frequency questionnaire (22) sent by mail around week 25 of gestation, and two computer-assisted telephone interviews of 10–15 minutes' duration each, administered around weeks 12 and 30 of gestation. Information on birth outcomes was obtained through linkage to the Medical Birth Registry, which includes all deliveries occurring in the country.

Dietary assessment

Information on fish consumption was collected through the food frequency questionnaire (22). The questionnaire was a modified form of the questionnaire used by the Danish Cancer Registry (23) and has been validated for use in pregnancy (24). The questionnaire solicited information on frequency and type of fish consumed, either as a meal or with bread.

Definition of the fish variable

When evaluating the association between fish consumption and fetal growth, we used total fish intake quantified in grams per day. Consumption of fish as a meal and consumption of fish with bread were combined using assumptions on standard portion sizes (25). Average fish consumption in the cohort was 27 g/day (standard deviation, 23), and we divided consumption into five categories: 0–5, >5–20, >20–40, >40–60, and >60 g/day. By comparison, an average consumption of 47 g/day has been reported among Icelandic pregnant women (14), and among US pregnant women an average of 6.4 servings per month has been reported (26), which corresponds to approximately 32 g/day (assuming that one serving equals 150 g).

When examining type of fish, we used frequency of meals and divided fish consumption into fatty fish and lean fish. Salmon, herring, mackerel, trout, and Greenland halibut (Reinhardtius hippoglossoides) were classified as fatty fish, while cod, pollack, plaice, flounder, garfish, and similar species were classified as lean fish. According to this definition, the fat contribution ranged from 0.6 g (cod) to 2.7 g (garfish) per 100 g of fresh weight for lean fish and from 6.7 g (trout) to 24.4 g (autumn herring) per 100 g of fresh weight for fatty fish, based on the Danish food composition tables (27). For both types of fish, consumption was divided into four categories: 0, 1, 2–3, and ≥4 meals per month.

The outcome variables

Birth weight, birth length, and head circumference were measured by the midwife who attended the birth. The date of birth was extracted from the Danish Civil Registration System. Gestational age was assessed from the last menstrual period, on the basis of information recorded on the recruitment form (gestation week 6–10) and during the first telephone interview (gestation week 12). If this estimate was uncertain because of irregular or abnormally long (>32 days) or short (<24 days) menstrual cycles, gestational age was calculated from the expected date of delivery, which is most often based on ultrasound scanning, provided by the woman during the second telephone interview (gestation week 30). If this information was missing or led to unrealistic gestational age estimates (>308 days), we used length of gestation assessed at delivery by the midwife and reported to the Medical Birth Registry. For our gestational age estimates, 47 percent, 52 percent, and 1 percent were based on information on the last menstrual period, information obtained from the second telephone interview, and the Medical Birth Registry, respectively.

To identify children born small for gestational age, we used the growth reference curves provided by the British Child Growth Foundation (28, 29). The growth reference curves were used to calculate z scores, standardized for gestational age and gender, for birth weight, birth length, and head circumference. For a given growth measure, z scores were calculated for all singleton births in the cohort. On the basis of that distribution, we defined infants whose z scores were below the 10th percentile as small for gestational age.

Mother-child pairs available for analysis

A total of 101,042 women were registered in the Danish National Birth Cohort during early pregnancy. Of those women, 92,892 and 87,902 participated in interviews conducted around gestational weeks 12 and 30, respectively, and provided information on maternal characteristics. The number of women filling out the food frequency questionnaire was 70,183. Combining information from these three sources reduced the number of available women to 66,120. Restriction to singleton term births resulted in a study sample of 57,946 women, and of those, covariate information was available for 50,618. Given the hypothesis that use of fish oil supplements might increase birth weight and the analytical difficulty of distinguishing between the separate effects of food- and supplement-derived nutrients, we also excluded women taking fish oil supplements; this reduced the data set to 47,635 women. Due primarily to missing data on birth outcomes and to observations falling out of the realistic ranges for birth weight (1,000–6,000 g), birth length (35–65 cm), and head circumference (25–45 cm) for singleton term infants, the final data set contained 44,824 women.

Statistical methods

We used linear regression to analyze continuous birth outcomes and logistic regression for dichotomous birth outcomes. For pairwise comparison of group means, we employed Student's t test, and for pairwise comparison of dichotomous outcomes, we used chi-squared tests. We identified and included as covariates a set of nine nondietary factors that are well-recognized determinants of fetal growth: gestational age (in days), infant gender (binary variable), maternal parity (binary variable: nulliparous vs. parous), maternal age (<20, 20–40, or >40 years), maternal height (<160, 160–169, 170–179, or >179 cm), maternal prepregnancy body mass index (weight (kg)/height (m)2; <18.5, 18.5–24.9, 25–29.9, 30–34.9, 35–40, or >40), maternal smoking (never smoking, occasional smoking, daily smoking of <15 cigarettes, or daily smoking of ≥15 cigarettes), paternal height (<170, 170–179, 180–189, or >189 cm), and familial socioeconomic status (six occupational categories). Familial socioeconomic status was based on the occupation of both parents if they were living together; otherwise, it was based on the mother's occupation only. Because of the correlation between intake of food and intake of energy, we additionally included mother's total energy intake (in quintiles) as a covariate, since it is generally important to distinguish between the separate effects of food and energy intake (30). For statistical analyses, we used SAS software, version 9.1 (SAS Institute Inc., Cary, North Carolina).

RESULTS

For the 44,824 women available for analyses, fish consumption contributed on average 1.7 percent of their total energy intake, 5.7 percent of their total protein intake, and 2.7 percent of their total fat intake. Since most of the women reported occasional consumption of fish with bread, only 2.6 percent had a fish intake of zero. However, 8 percent of the women had total consumption of 5 g/day or less, which corresponds to less than one meal per month.

The characteristics of birth outcomes and covariates with respect to total fish consumption are shown in table 1. Across categories of increased fish consumption, there was a considerable decrease in the percentage of smokers, and women of high socioeconomic status tended to eat more fish than women of lower status. Women consuming fish regularly also tended to be slightly older, having a lower prepregnancy body mass index and a higher energy intake and greater parity. Except for maternal age and prepregnancy body mass index, the changes across categories for these characteristics were found to be positively associated with fetal growth.

TABLE 1.

Parental and infant characteristics according to maternal fish consumption during pregnancy (n = 44,824), Danish National Birth Cohort, 1996–2002

 Fish consumption (g/day) p value 
 ≤5 (8.0%) >5–20 (35.9%) >20–40 (36.4%) >40–60 (14.0%) >60 (5.6%) 
Continuous variables (mean and standard deviation
Gestational age (days)* 280.2 (8.1) 280.2 (8.0) 280.3 (7.9) 280.1 (7.9) 280.1 (7.9) 0.43† 
Birth weight (g) 3,597 (498) 3,610 (492) 3,627 (483) 3,625 (500) 3,612 (493) <0.001† 
Birth length (cm) 52.3 (2.3) 52.4 (2.2) 52.5 (2.2) 52.4 (2.2) 52.4 (2.2) 0.02† 
Head circumference (cm) 35.3 (1.6) 35.3 (1.6) 35.4 (1.6) 35.3 (1.6) 35.3 (1.6) 0.96† 
Mother's age (years) 27.9 (4.2) 28.7 (4.1) 29.5 (4.1) 29.9 (4.2) 29.6 (4.5) <0.001† 
Mother's height (cm) 168.2 (6.2) 168.7 (6.1) 169.0 (6.0) 169.1 (6.1) 169.0 (6.1) <0.001† 
Prepregnancy body mass index‡ 24.2 (4.7) 23.8 (4.3) 23.3 (3.9) 23.1 (4.0) 23.1 (4.0) <0.001† 
Mother's total energy intake (MJ/day) 9.6 (2.8) 9.9 (2.6) 10.6 (2.6) 11.5 (2.7) 12.5 (3.3) <0.001† 
Father's height (cm) 181.5 (7.1) 181.8 (7.0) 182.2 (6.9) 182.2 (6.9) 182.0 (6.9) <0.001† 
Discrete characteristics (%
Male infant gender (%) 50.2 51.0 51.7 50.1 50.6 0.18§ 
Parous mother (%) 48.9 50.7 54.8 56.2 57.6 <0.001§ 
Maternal smoking (%) 30.6 24.4 22.1 21.1 25.3 <0.001§ 
Familial socioeconomic status (row %)¶      <0.001§ 
    High 5.2 31.5 40.3 17.0 6.1  
    Intermediate 6.7 35.5 38.3 14.4 5.2  
    Skilled worker(s) 10.4 39.7 33.0 11.9 5.1  
    Unskilled worker(s) 11.5 38.5 31.4 12.4 6.3  
    Student(s) 7.2 34.1 37.5 14.1 7.0  
    Not working 11.4 34.1 34.1 11.8 8.7  
 Fish consumption (g/day) p value 
 ≤5 (8.0%) >5–20 (35.9%) >20–40 (36.4%) >40–60 (14.0%) >60 (5.6%) 
Continuous variables (mean and standard deviation
Gestational age (days)* 280.2 (8.1) 280.2 (8.0) 280.3 (7.9) 280.1 (7.9) 280.1 (7.9) 0.43† 
Birth weight (g) 3,597 (498) 3,610 (492) 3,627 (483) 3,625 (500) 3,612 (493) <0.001† 
Birth length (cm) 52.3 (2.3) 52.4 (2.2) 52.5 (2.2) 52.4 (2.2) 52.4 (2.2) 0.02† 
Head circumference (cm) 35.3 (1.6) 35.3 (1.6) 35.4 (1.6) 35.3 (1.6) 35.3 (1.6) 0.96† 
Mother's age (years) 27.9 (4.2) 28.7 (4.1) 29.5 (4.1) 29.9 (4.2) 29.6 (4.5) <0.001† 
Mother's height (cm) 168.2 (6.2) 168.7 (6.1) 169.0 (6.0) 169.1 (6.1) 169.0 (6.1) <0.001† 
Prepregnancy body mass index‡ 24.2 (4.7) 23.8 (4.3) 23.3 (3.9) 23.1 (4.0) 23.1 (4.0) <0.001† 
Mother's total energy intake (MJ/day) 9.6 (2.8) 9.9 (2.6) 10.6 (2.6) 11.5 (2.7) 12.5 (3.3) <0.001† 
Father's height (cm) 181.5 (7.1) 181.8 (7.0) 182.2 (6.9) 182.2 (6.9) 182.0 (6.9) <0.001† 
Discrete characteristics (%
Male infant gender (%) 50.2 51.0 51.7 50.1 50.6 0.18§ 
Parous mother (%) 48.9 50.7 54.8 56.2 57.6 <0.001§ 
Maternal smoking (%) 30.6 24.4 22.1 21.1 25.3 <0.001§ 
Familial socioeconomic status (row %)¶      <0.001§ 
    High 5.2 31.5 40.3 17.0 6.1  
    Intermediate 6.7 35.5 38.3 14.4 5.2  
    Skilled worker(s) 10.4 39.7 33.0 11.9 5.1  
    Unskilled worker(s) 11.5 38.5 31.4 12.4 6.3  
    Student(s) 7.2 34.1 37.5 14.1 7.0  
    Not working 11.4 34.1 34.1 11.8 8.7  
*

Sample was restricted to term births.

Two-sided p value for association as determined by Spearman's correlation coefficient.

Weight (kg)/height (m)2.

§

Two-sided p value from chi-squared test for measure of association.

Percentages for each level of familial socioeconomic status. Familial socioeconomic status was based on the occupation of both parents if they were living together; otherwise, it was based on the mother's occupation only. Of the 44,824 observations, 24.2% of families were classified as high status, 31.1% as intermediate status, 26.8% as skilled worker(s), 11.9% as unskilled worker(s), 3.8% as student(s), and 2.2% as not working.

For birth characteristics, slight increases in birth weight and length were observed with increased fish consumption, while no association was observed with head circumference or gestational age among these full-term births.

Total fish consumption

When examining the three measures of fetal growth as continuous outcomes (table 2, upper half), we obtained in the unadjusted analyses a positive association (p for trend < 0.05) with respect to birth weight but no association with respect to birth length or head circumference. For birth weight and length, the estimates were significant for moderate consumption, but significance was lost for the highest consumption group. After covariate adjustment, the association was reversed, and fish consumption showed a weak but inverse association with fetal growth for birth length and head circumference and a borderline significant association for birth weight. In the highest consumption group, the estimates were −25.2 g (95 percent confidence interval (CI): −47.4, −3.0) for decrease in birth weight, −0.08 cm (95 percent CI: −0.18, 0.02) for decrease in birth length, and −0.11 cm (95 percent CI: −0.18, −0.03) for decrease in head circumference.

TABLE 2.

Associations between total fish consumption and measures of fetal growth (n = 44,824) before and after covariate adjustment, Danish National Birth Cohort, 1996–2002

Total fish intake (g/day) % of subjects Birth weight Birth length Head circumference 
  Continuous growth measures 
  Increase in birth weight (g)
 
95% CI*
 
Increase in birth length (cm)
 
95% CI
 
Increase in head circumference (cm)
 
95% CI
 
Unadjusted        
    ≤5 8.0 Referent  Referent  Referent  
    >5–20 35.9 12.9 −4.8, 30.7 0.08 0.00, 0.17 0.02 −0.04, 0.07 
    >20–40 36.4 30.2 12.5, 47.9 0.13 0.05, 0.22 0.06 0.00, 0.12 
    >40–60 14.0 28.4 8.3, 48.6 0.10 0.00, 0.19 0.00 −0.06, 0.07 
    >60 5.6 14.7 −10.3, 39.7 0.07 −0.05, 0.19 −0.04 −0.12, 0.04 
        p for trend†   0.02  0.28  0.34 
Adjusted‡        
    ≤5 8.0 Referent  Referent  Referent  
    >5–20 35.9 −12.6 −28.1, 2.9 −0.03 −0.10, 0.04 −0.04 −0.09, 0.02 
    >20–40 36.4 −11.6 −27.3, 4.0 −0.05 −0.12, 0.03 −0.02 −0.07, 0.03 
    >40–60 14.0 −14.0 −31.9, 3.9 −0.08 −0.16, 0.01 −0.07 −0.13, −0.01 
    >60 5.6 −25.2 −47.4, −3.0 −0.08 −0.18, 0.02 −0.11 −0.18, −0.03 
        p for trend   0.09  0.04  0.005 
  Dichotomized growth measures
 
  SGA* for birth weight
 
SGA for birth length
 
SGA for head circumference
 
  OR*
 
95% CI
 
OR
 
95% CI
 
OR
 
95% CI
 
Unadjusted        
    ≤5 8.0    
    >5–20 35.9 0.94 0.82, 1.06 0.89 0.78, 1.01 0.97 0.86, 1.10 
    >20–40 36.4 0.82 0.72, 0.93 0.79 0.69, 0.90 0.89 0.79, 1.01 
    >40–60 14.0 0.86 0.74, 1.00 0.86 0.74, 0.99 0.90 0.78, 1.04 
    >60 5.6 0.99 0.83, 1.18 1.03 0.86, 1.23 1.08 0.92, 1.28 
        p for trend   0.26  0.96  0.85 
Adjusted§        
    ≤5 8.0 1.00  1.00  1.00  
    >5–20 35.9 1.08 0.95, 1.23 1.01 0.88, 1.15 1.04 0.92, 1.18 
    >20–40 36.4 1.02 0.90, 1.17 0.95 0.83, 1.09 0.99 0.87, 1.12 
    >40–60 14.0 1.10 0.95, 1.29 1.05 0.89, 1.23 1.02 0.88, 1.17 
    >60 5.6 1.24 1.03, 1.49 1.20 1.00, 1.45 1.21 1.01, 1.43 
        p for trend   0.08  0.07  0.20 
Total fish intake (g/day) % of subjects Birth weight Birth length Head circumference 
  Continuous growth measures 
  Increase in birth weight (g)
 
95% CI*
 
Increase in birth length (cm)
 
95% CI
 
Increase in head circumference (cm)
 
95% CI
 
Unadjusted        
    ≤5 8.0 Referent  Referent  Referent  
    >5–20 35.9 12.9 −4.8, 30.7 0.08 0.00, 0.17 0.02 −0.04, 0.07 
    >20–40 36.4 30.2 12.5, 47.9 0.13 0.05, 0.22 0.06 0.00, 0.12 
    >40–60 14.0 28.4 8.3, 48.6 0.10 0.00, 0.19 0.00 −0.06, 0.07 
    >60 5.6 14.7 −10.3, 39.7 0.07 −0.05, 0.19 −0.04 −0.12, 0.04 
        p for trend†   0.02  0.28  0.34 
Adjusted‡        
    ≤5 8.0 Referent  Referent  Referent  
    >5–20 35.9 −12.6 −28.1, 2.9 −0.03 −0.10, 0.04 −0.04 −0.09, 0.02 
    >20–40 36.4 −11.6 −27.3, 4.0 −0.05 −0.12, 0.03 −0.02 −0.07, 0.03 
    >40–60 14.0 −14.0 −31.9, 3.9 −0.08 −0.16, 0.01 −0.07 −0.13, −0.01 
    >60 5.6 −25.2 −47.4, −3.0 −0.08 −0.18, 0.02 −0.11 −0.18, −0.03 
        p for trend   0.09  0.04  0.005 
  Dichotomized growth measures
 
  SGA* for birth weight
 
SGA for birth length
 
SGA for head circumference
 
  OR*
 
95% CI
 
OR
 
95% CI
 
OR
 
95% CI
 
Unadjusted        
    ≤5 8.0    
    >5–20 35.9 0.94 0.82, 1.06 0.89 0.78, 1.01 0.97 0.86, 1.10 
    >20–40 36.4 0.82 0.72, 0.93 0.79 0.69, 0.90 0.89 0.79, 1.01 
    >40–60 14.0 0.86 0.74, 1.00 0.86 0.74, 0.99 0.90 0.78, 1.04 
    >60 5.6 0.99 0.83, 1.18 1.03 0.86, 1.23 1.08 0.92, 1.28 
        p for trend   0.26  0.96  0.85 
Adjusted§        
    ≤5 8.0 1.00  1.00  1.00  
    >5–20 35.9 1.08 0.95, 1.23 1.01 0.88, 1.15 1.04 0.92, 1.18 
    >20–40 36.4 1.02 0.90, 1.17 0.95 0.83, 1.09 0.99 0.87, 1.12 
    >40–60 14.0 1.10 0.95, 1.29 1.05 0.89, 1.23 1.02 0.88, 1.17 
    >60 5.6 1.24 1.03, 1.49 1.20 1.00, 1.45 1.21 1.01, 1.43 
        p for trend   0.08  0.07  0.20 
*

CI, confidence interval; SGA, small for gestational age; OR, odds ratio.

Two-sided p value.

Adjusted for gestational age, infant gender, parity, maternal age, maternal height, prepregnancy body mass index, energy intake, smoking, familial socioeconomic status, and paternal height.

§

Adjusted for the same covariates as in the upper half of the table, apart from gestational age and gender, which were adjusted for in the z scores.

Maternal smoking, height, parity, and energy intake were the covariates most responsible for reversing the association observed in the unadjusted analyses in table 2. As an example, the unadjusted estimate for birth weight (upper half of table) for the highest fish category was 14.7 g (95 percent CI: −10.3, 39.7). Adjusting only for smoking resulted in an estimate of −1.4 g (95 percent CI: −26.1, 23.2), and adding parity to the regression model produced an estimate of −17.8 g (95 percent CI: −42.1, −6.4). When maternal height and energy intake were also included, the estimate became −32.7 g (95 percent CI: −57.1, −8.4). Further adjustment for prepregnancy body mass index increased the estimate to −24.6 g (95 percent CI: −48.8, −0.4). Inclusion of the five remaining covariates then resulted in the fully adjusted estimate of −25.2 g (95 percent CI: −47.4, −3.0).

It has previously been reported (2) that smoking might be an effect modifier with respect to fish consumption and fetal growth. We investigated the possibility of effect modification with respect to smoking, parity, and prepregnancy body mass index. To simplify the analyses, we categorized the variables in the following way: smoking as yes versus no, parity as nulliparous versus parous, and prepregnancy body mass index as <18.5, 18.5–24.9, or ≥25. We used only the lowest and highest categories of fish consumption (≤5 g/day and >60 g/day). We tested for effect modification by including the variable for total fish, the covariate of interest, and a term for interaction between the two variables, both with and without adjustment for the nine remaining covariates. In all cases, the interaction term was nonsignificant (data not shown).

For the dichotomous growth measures (table 2, lower half), the odds ratio estimates for birth weight and length in the unadjusted analyses showed a significantly decreased risk of small-for-gestational-age birth for moderate fish consumption, but the estimates for the highest consumption group were not significant. For the adjusted analyses, the odds ratio estimates were centered around 1, except for the highest consumption group, where the odds ratio estimates showed a significantly increased risk of being small for gestational age for birth weight (odds ratio = 1.24, 95 percent CI: 1.03, 1.49) and head circumference (odds ratio = 1.21, 95 percent CI: 1.01, 1.43) and a borderline-significant risk for birth length (odds ratio = 1.20, 95 percent CI: 1.00, 1.45). In both unadjusted and adjusted analyses, tests for linear trend were not significant.

Fatty fish versus lean fish

The distributions of fatty and lean fish consumption are shown in table 3. The majority of the women (n = 24,205) did not consume any fatty fish, while consumption of lean fish was more general. High consumption (≥4 meals/month) of fatty fish and lean fish was observed for 2,977 and 10,720 women, respectively. Of those, 1,789 had high consumption of both types. However, the correlation between fatty fish and lean fish was only moderate (Spearman's r = 0.33, p < 0.001), which indicates that analyzing the two variables separately is justifiable.

TABLE 3.

Cross-classification of fish meals according to type of fish consumed (n = 44,824), Danish National Birth Cohort, 1996–2002

Lean* fish consumption (no. of meals per month) Fatty† fish consumption (no. of meals per month) 
2–3 ≥4 Row total 
No. %‡ No. %‡ No. %‡ No. %‡ No. 
7,809 17.4 1,234 2.8 820 1.8 182 0.4 10,045 22.4 
5,052 11.3 1,745 3.9 1,194 2.7 246 0.5 8,237 18.4 
2–3 7,698 17.2 4,094 9.1 3,270 7.3 760 1.7 15,822 35.3 
≥4 3,646 8.1 2,333 5.2 2,952 6.6 1,789 4.0 10,720 23.9 
Column total 24,205 54.0 9,406 21.0 8,236 18.4 2,977 6.6 44,824 100 
Lean* fish consumption (no. of meals per month) Fatty† fish consumption (no. of meals per month) 
2–3 ≥4 Row total 
No. %‡ No. %‡ No. %‡ No. %‡ No. 
7,809 17.4 1,234 2.8 820 1.8 182 0.4 10,045 22.4 
5,052 11.3 1,745 3.9 1,194 2.7 246 0.5 8,237 18.4 
2–3 7,698 17.2 4,094 9.1 3,270 7.3 760 1.7 15,822 35.3 
≥4 3,646 8.1 2,333 5.2 2,952 6.6 1,789 4.0 10,720 23.9 
Column total 24,205 54.0 9,406 21.0 8,236 18.4 2,977 6.6 44,824 100 
*

Cod, pollack, plaice, flounder, garfish, and similar species were classified as lean fish.

Salmon, herring, mackerel, trout, and Greenland halibut (Reinhardtius hippoglossoides) were classified as fatty fish.

Percentage of total study population.

For fatty fish, using continuous growth measures (table 4, upper half), we found a significant inverse association for birth weight and head circumference in the unadjusted analyses, while all three growth measures showed an inverse association in the adjusted analyses. The estimates for the highest consumption group in the adjusted analyses were significant for all three growth measures and were similar in magnitude to the estimates for total fish consumption. With respect to the dichotomous growth measures (table 4, lower half), in the adjusted analyses we found a significantly increased risk of small size for gestational age for birth weight and birth length among women who consumed fatty fish four times per month or more as compared with those who did not consume fatty fish, while the increase for head circumference was nonsignificant. Corresponding odds ratios were 1.18 (95 percent CI: 1.03, 1.35) for birth weight, 1.22 (95 percent CI: 1.05, 1.40) for birth length, and 1.10 (95 percent CI: 0.97, 1.25) for head circumference. A test for linear trend was significant for birth weight and length in the adjusted analyses.

TABLE 4.

Associations between consumption of fatty fish during pregnancy and measures of fetal growth (n = 44,824), before and after adjustment for covariates, Danish National Birth Cohort, 1996–2002

Fatty* fish intake (no. of meals/month) % of subjects Birth weight Birth length Head circumference 
  Continuous growth measures
 
  Increase in birth weight (g)
 
95% CI†
 
Increase in birth length (cm)
 
95% CI
 
Increase in head circumference (cm)
 
95% CI
 
Unadjusted        
    0 54.1 Referent  Referent  Referent  
    1 21.0 3.0 −8.7, 14.7 0.09 0.03, 0.14 0.01 −0.03, 0.04 
    2–3 18.4 −1.7 −13.9, 10.6 0.08 0.02, 0.14 −0.02 −0.06, 0.02 
    ≥4 6.6 −30.7 −49.3, −12.0 −0.07 −0.16, 0.02 −0.12 −0.18, −0.06 
        p for trend‡   0.005  0.94  <0.001 
Adjusted§        
    0 54.1 Referent  Referent  Referent  
    1 21.0 −4.8 −15.1, 5.5 0.03 −0.01, 0.08 −0.01 −0.04, 0.03 
    2–3 18.4 −10.1 −20.9, 0.8 0.00 −0.05, 0.05 −0.04 −0.07, 0.00 
    ≥4 6.6 −27.5 −43.8, −11.1 −0.10 −0.18, −0.03 −0.11 −0.16, −0.05 
        p for trend   <0.001  0.03  <0.001 
  Dichotomized growth measures
 
  SGA† for birth weight
 
SGA for birth length
 
SGA for head circumference
 
  OR†
 
95% CI
 
OR
 
95% CI
 
OR
 
95% CI
 
Unadjusted        
    0 54.1    
    1 21.0 0.90 0.82, 0.98 0.89 0.81, 0.98 0.97 0.89, 1.06 
    2–3 18.4 0.93 0.85, 1.02 0.99 0.90, 1.09 1.02 0.94, 1.12 
    ≥4 6.6 1.12 0.98, 1.28 1.13 0.98, 1.30 1.10 0.97, 1.26 
        p for trend   0.42  0.18  0.15 
Adjusted¶        
    0 54.1     
    1 21.0 0.97 0.89, 1.06 0.97 0.88, 1.07 1.00 0.91, 1.09 
    2–3 18.4 1.00 0.91, 1.10 1.09 0.99, 1.21 1.05 0.96, 1.15 
    ≥4 6.6 1.18 1.03, 1.35 1.22 1.05, 1.40 1.10 0.97, 1.25 
        p for trend   0.04  0.003  0.12 
Fatty* fish intake (no. of meals/month) % of subjects Birth weight Birth length Head circumference 
  Continuous growth measures
 
  Increase in birth weight (g)
 
95% CI†
 
Increase in birth length (cm)
 
95% CI
 
Increase in head circumference (cm)
 
95% CI
 
Unadjusted        
    0 54.1 Referent  Referent  Referent  
    1 21.0 3.0 −8.7, 14.7 0.09 0.03, 0.14 0.01 −0.03, 0.04 
    2–3 18.4 −1.7 −13.9, 10.6 0.08 0.02, 0.14 −0.02 −0.06, 0.02 
    ≥4 6.6 −30.7 −49.3, −12.0 −0.07 −0.16, 0.02 −0.12 −0.18, −0.06 
        p for trend‡   0.005  0.94  <0.001 
Adjusted§        
    0 54.1 Referent  Referent  Referent  
    1 21.0 −4.8 −15.1, 5.5 0.03 −0.01, 0.08 −0.01 −0.04, 0.03 
    2–3 18.4 −10.1 −20.9, 0.8 0.00 −0.05, 0.05 −0.04 −0.07, 0.00 
    ≥4 6.6 −27.5 −43.8, −11.1 −0.10 −0.18, −0.03 −0.11 −0.16, −0.05 
        p for trend   <0.001  0.03  <0.001 
  Dichotomized growth measures
 
  SGA† for birth weight
 
SGA for birth length
 
SGA for head circumference
 
  OR†
 
95% CI
 
OR
 
95% CI
 
OR
 
95% CI
 
Unadjusted        
    0 54.1    
    1 21.0 0.90 0.82, 0.98 0.89 0.81, 0.98 0.97 0.89, 1.06 
    2–3 18.4 0.93 0.85, 1.02 0.99 0.90, 1.09 1.02 0.94, 1.12 
    ≥4 6.6 1.12 0.98, 1.28 1.13 0.98, 1.30 1.10 0.97, 1.26 
        p for trend   0.42  0.18  0.15 
Adjusted¶        
    0 54.1     
    1 21.0 0.97 0.89, 1.06 0.97 0.88, 1.07 1.00 0.91, 1.09 
    2–3 18.4 1.00 0.91, 1.10 1.09 0.99, 1.21 1.05 0.96, 1.15 
    ≥4 6.6 1.18 1.03, 1.35 1.22 1.05, 1.40 1.10 0.97, 1.25 
        p for trend   0.04  0.003  0.12 
*

Salmon, herring, mackerel, trout, and Greenland halibut (Reinhardtius hippoglossoides) were classified as fatty fish.

CI, confidence interval; SGA, small for gestational age; OR, odds ratio.

Two-sided p value.

§

Adjusted for gestational age, infant gender, parity, maternal age, maternal height, prepregnancy body mass index, energy intake, smoking, familial socioeconomic status, and paternal height.

Adjusted for the same covariates as in the upper half of the table, apart from gestational age and gender, which were adjusted for in the z scores.

When we evaluated consumption of lean fish (table 5), the results were almost opposite those for fatty fish. In the unadjusted analyses, the estimates showed an increase in birth weight, birth length, and head circumference and a decreased risk of being small for gestational age for these growth measures. After adjustment for covariates, the estimates become nonsignificant and no association was observed for either continuous growth measures or dichotomized growth measures.

TABLE 5.

Associations between consumption of lean fish during pregnancy and measures of fetal growth (n = 44,824), before and after covariate adjustment for covariates, Danish National Birth Cohort, 1996–2002

Lean* fish intake (no. of meals/month) % of subjects Birth weight Birth length Head circumference 
  Continuous growth measures
 
  Increase in birth weight (g)
 
95% CI†
 
Increase in birth length (cm)
 
95% CI
 
Increase in head circumference (cm)
 
95% CI
 
Unadjusted        
    0 24.7 Referent  Referent  Referent  
    1 20.1 30.1 15.8, 44.4 0.12 0.05, 0.19 0.09 0.04, 0.13 
    2–3 35.9 42.7 30.4, 55.0 0.16 0.10, 0.21 0.06 0.02, 0.10 
    ≥4 19.3 44.7 31.3, 58.0 0.12 0.06, 0.19 0.07 0.03, 0.11 
        p for trend‡   <0.001  0.004  0.04 
Adjusted*        
    0 24.7 Referent  Referent  Referent  
    1 20.1 9.1 −3.4, 21.6 0.03 −0.03, 0.09 0.04 0.00, 0.09 
    2–3 35.9 9.4 −1.4, 20.2 0.01 −0.04, 0.06 0.00 −0.04, 0.04 
    ≥4 19.3 1.9 −10.0, 13.8 −0.05 −0.11, 0.01 −0.01 −0.05, 0.03 
        p for trend   0.82  0.06  0.16 
  Dichotomized growth measures
 
  SGA for birth weight
 
SGA for birth length
 
SGA for head circumference
 
  OR
 
95% CI
 
OR
 
95% CI
 
OR
 
95% CI
 
Unadjusted        
    0 24.7    
    1 20.1 0.88 0.79, 0.97 0.91 0.81, 1.01 0.84 0.76, 0.93 
    2–3 35.9 0.81 0.74, 0.88 0.87 0.79, 0.96 0.92 0.84, 1.00 
    ≥4 19.3 0.79 0.71, 0.87 0.93 0.84, 1.03 0.87 0.79, 0.96 
        p for trend   <0.001  0.33  0.06 
Adjusted§        
    0 24.7 1.00  1.00  1.00  
    1 20.1 0.96 0.86, 1.06 0.99 0.88, 1.11 0.89 0.79, 1.00 
    2–3 35.9 0.94 0.86, 1.03 1.01 0.91, 1.11 1.00 0.92, 1.09 
    ≥4 19.3 0.96 0.86, 1.06 1.10 0.99, 1.22 0.98 0.89, 1.08 
        p for trend   0.51  0.05  0.66 
Lean* fish intake (no. of meals/month) % of subjects Birth weight Birth length Head circumference 
  Continuous growth measures
 
  Increase in birth weight (g)
 
95% CI†
 
Increase in birth length (cm)
 
95% CI
 
Increase in head circumference (cm)
 
95% CI
 
Unadjusted        
    0 24.7 Referent  Referent  Referent  
    1 20.1 30.1 15.8, 44.4 0.12 0.05, 0.19 0.09 0.04, 0.13 
    2–3 35.9 42.7 30.4, 55.0 0.16 0.10, 0.21 0.06 0.02, 0.10 
    ≥4 19.3 44.7 31.3, 58.0 0.12 0.06, 0.19 0.07 0.03, 0.11 
        p for trend‡   <0.001  0.004  0.04 
Adjusted*        
    0 24.7 Referent  Referent  Referent  
    1 20.1 9.1 −3.4, 21.6 0.03 −0.03, 0.09 0.04 0.00, 0.09 
    2–3 35.9 9.4 −1.4, 20.2 0.01 −0.04, 0.06 0.00 −0.04, 0.04 
    ≥4 19.3 1.9 −10.0, 13.8 −0.05 −0.11, 0.01 −0.01 −0.05, 0.03 
        p for trend   0.82  0.06  0.16 
  Dichotomized growth measures
 
  SGA for birth weight
 
SGA for birth length
 
SGA for head circumference
 
  OR
 
95% CI
 
OR
 
95% CI
 
OR
 
95% CI
 
Unadjusted        
    0 24.7    
    1 20.1 0.88 0.79, 0.97 0.91 0.81, 1.01 0.84 0.76, 0.93 
    2–3 35.9 0.81 0.74, 0.88 0.87 0.79, 0.96 0.92 0.84, 1.00 
    ≥4 19.3 0.79 0.71, 0.87 0.93 0.84, 1.03 0.87 0.79, 0.96 
        p for trend   <0.001  0.33  0.06 
Adjusted§        
    0 24.7 1.00  1.00  1.00  
    1 20.1 0.96 0.86, 1.06 0.99 0.88, 1.11 0.89 0.79, 1.00 
    2–3 35.9 0.94 0.86, 1.03 1.01 0.91, 1.11 1.00 0.92, 1.09 
    ≥4 19.3 0.96 0.86, 1.06 1.10 0.99, 1.22 0.98 0.89, 1.08 
        p for trend   0.51  0.05  0.66 
*

Cod, pollack, plaice, flounder, garfish, and similar species were classified as lean fish.

CI, confidence interval; SGA, small for gestational age; OR, odds ratio.

Two-sided p value.

§

Adjusted for gestational age, infant gender, parity, maternal age, maternal height, prepregnancy body mass index, energy intake, smoking, familial socioeconomic status, and paternal height.

Adjusted for the same covariates as in the upper half of the table, apart from gestational age and gender, which were adjusted for in the z scores.

To check the stability of our estimates for fatty and lean fish and to account for their collinearity, we included those variables simultaneously in the regression model. This mutual adjustment had only a minor impact on estimates, and the same conclusions were reached for all three growth measures (data not shown).

DISCUSSION

In a large cohort of pregnant women, high intake of fatty fish was found to be inversely related to birth weight, birth length, and head circumference. The average change in these measures of fetal growth was small, but it was associated with an increased risk of children being born small for gestational age with respect to birth weight, birth length, and head circumference. No association was observed for lean fish, and the inverse association observed for total fish consumption can be explained by intake of fatty fish.

It is difficult to compare these results directly with those of other studies, since information on type of fish consumed is often sparse and the exposure variable is assessed in different ways. At least two other studies focusing on intake of marine n-3 fatty acids, which can be regarded as a marker for fish consumption (31), have found inverse associations with fetal growth. Grandjean et al. (15) found an inverse association between serum levels of EPA and birth weight in a Faroese fishing community, where fish consumption was high. However, in that study, a marine diet consisted not only of fish but also of whale meat and blubber—substances that are potentially high in persistent organic pollutants and mercury, which may influence fetal growth (32–34). A study by Oken et al. (13) focused on quantified EPA and DHA estimated by food frequency questionnaire. They found the sum of EPA and DHA to be associated with reduced fetal growth. In attempting to compare our results with those of that study, we could not in our data distinguish between the association of EPA plus DHA with fetal growth, on the one hand, and the association of fatty fish with fetal growth on the other, because of high collinearity.

Thorsdottir et al. (14) found a positive association of fish consumption with birth length and head circumference among Icelandic women. In Iceland, as in the Faroe Islands, fish consumption is mostly based on locally caught fish, which is predominantly lean fish. With respect to lean fish, consumption in the Icelandic study was considerably higher than in our cohort. Thorsdottir et al. (14) also found a decreased birth length and head circumference among women consuming a high amount of cod liver oil (>8.7 g/day), a product which is supposed to have been purified with respect to persistent organic pollutants. However, despite purification, fish oil may contain substantial amounts of these pollutants (35, 36). We excluded women taking fish oil supplements (5 percent), but for total fish consumption, the women in the highest category had an average intake of 7.6 g of fish fat per day. We therefore have evidence that high amounts of marine fats, consumed through either intake of fish oil (14), consumption of marine mammals (15), or intake of fatty fish (as in this study), might have an inverse association with fetal growth. However, it is not clear whether this inverse association is due to industrial contaminants, fatty acid composition, or some other constituent in the fish fat.

One explanation for the inverse association observed for fatty fish might be contamination by persistent organic pollutants. A role for methyl mercury is less likely, since methyl mercury is also present in lean fish (36), because of protein binding. With the exception of a few studies (32, 33), most studies focusing on persistent organic pollutants such as polychlorinated biphenyls have failed to find an association with fetal growth (18). Because of the complexity and cost of measuring these substances, these studies usually contain few subjects and have low statistical power. In addition, the possibility cannot be excluded that certain subgroups might be more sensitive to these substances than others, which small studies would fail to detect (37). Although many of the studies in the literature focus on polychlorinated biphenyls, other compounds such as hexachlorobenzene have been shown to be associated with reduced fetal growth (38) and are potentially present in fish. However, the absence of information regarding levels of persistent organic pollutants in our study makes the above arguments speculative.

The inverse association observed for fatty fish might also be related to the marine n-3 fatty acids. In postnatal feeding trials with preterm infants, Carlson et al. (39) observed reduced growth among infants who were given formula fortified with marine n-3 fatty acids as compared with controls. It was hypothesized that the growth-retarding effect was mediated through EPA by displacement of arachidonic acid, but this hypothesis has not been substantiated (40).

The strengths of this study include the facts that we had prospective data on a large number of pregnant women, that cohort members were recruited throughout Denmark, and that we gathered detailed information on both fish consumption and maternal characteristics. However, the results of this study are subject to certain limitations. As with all observational studies, we cannot rule out the possibility that the observed association resulted from the influence of unadjusted or unmeasured confounders. Another limitation is that fish consumption in our cohort was strongly associated with lifestyle (smoking), socioeconomic status, and maternal factors such as parity and prepregnancy body mass index, which are strong predictors of fetal growth. We did check the stability of our estimates after covariate adjustment by using restricted cubic spline regression for continuous variables as an alternative (41), but changes in estimates were minor. The consistency between the unadjusted and adjusted results for fatty fish also makes it less likely that our results are due to residual confounding.

In this large cohort of pregnant Danish women, women who consumed fatty fish four times per month or more had a higher risk of giving birth to children who were small for gestational age with respect to birth weight, birth length, and head circumference, while consumption of lean fish had no association with these growth measures. We conclude that for pregnant women, the type of fish consumed is important, and moderate consumption of fatty fish should be encouraged. Further studies distinguishing between fatty fish and lean fish in relation to fetal growth are warranted.

Abbreviations

    Abbreviations
  • CI

    confidence interval

  • DHA

    docosahexaenoic acid

  • EPA

    eicosapentaenoic acid

Financial support for this study was obtained from the Nordic Academy for Advanced Study, the Nordic Working Group on Fishery Research, and the Early Nutrition Programming Project (EARNEST) (European Union Sixth Framework Programme, project FOOD-CT-2005-007036). The March of Dimes Birth Defects Foundation supported the collection of the dietary data. Financial support for the Danish National Birth Cohort was obtained from the Danish National Research Foundation, the Pharmacy Foundation, the Egmont Foundation, the Augustinus Foundation, the Health Foundation, the European Union (grant QLK1-2000-00083), the Danish Medical Research Foundation, and the Heart Foundation.

The managerial team of the Danish National Birth Cohort consists of Drs. Jørn Olsen (Chair), Mads Melbye, Anne Marie Nybo Andersen, Sjurdur F. Olsen, Thorkild I. A. Sørensen, and Peter Aaby.

Conflict of interest: none declared.

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