Abstract

The authors investigated the association between folic acid supplementation and gestational hypertension. The study population included women with nonmalformed infants in the United States and Canada who were participating in the Slone Epidemiology Center Birth Defects Study between 1993 and 2000. Women were interviewed within 6 months after delivery about sociodemographic and medical factors, the occurrence of hypertension with or without preeclampsia, and multivitamin use in pregnancy. Relative risks, adjusted for weight, parity, twin pregnancy, diabetes, smoking, education, and family income, were estimated using Cox regression models. Of 2,100 women, 204 (9.7%) reported gestational hypertension (onset after the 20th week of gestation). The multivariate-adjusted relative risk of developing gestational hypertension during the month after folic acid supplementation, compared with not using folic acid during that same month, was 0.55 (95% confidence interval: 0.39, 0.79). This finding suggests that folic acid-containing multivitamins may reduce the risk of gestational hypertension.

Received for publication March 25, 2002; accepted for publication May 29, 2002.

Preeclampsia, clinically recognized by hypertension and proteinuria, is a leading cause of morbidity and mortality in pregnancy (1, 2). It is widely accepted that the disorder starts with poor placental perfusion associated with defective trophoblast invasion. Although the ultimate causes are still unclear (3), recent studies have shown an association between hyperhomocysteinemia and both gestational hypertension and preeclampsia (411). It is hypothesized that hyperhomocysteinemia might damage the vascular endothelium of the developing placenta by promoting oxidative stress, thereby increasing contractile response and the production of procoagulants and vasoconstrictors (3).

Although it has been shown that folic acid supplementation decreases plasma homocysteine concentrations (4, 12), it remains unknown whether folic acid supplements can prevent or ameliorate gestational hypertension and pre-eclampsia. We therefore investigated the association between folic acid supplementation and gestational hypertension and preeclampsia.

MATERIALS AND METHODS

Study population

Since 1976, the Slone Epidemiology Center has conducted a case-control surveillance program in which mothers of malformed infants ascertained in birth or tertiary care hospitals in the greater metropolitan areas of Boston, Massachusetts, Philadelphia, Pennsylvania, and Toronto, Canada, have been interviewed. Mothers of nonmalformed infants were included as potential controls since 1993; in addition, since 1998, a population-based sample of mothers of nonmalformed infants has been recruited from Massachusetts. A total of 2,151 mothers of nonmalformed infants ascertained between 1993 and 2000 served as the study population for the current analysis. Institutional review board approval was obtained from all participating institutions, and mothers provided informed consent before participation.

Assessment of exposure

Within 6 months of delivery of the infant, trained study nurses who were unaware of the hypothesis interviewed the mothers. Interviews were conducted primarily in the women’s homes before 1998 and by telephone afterward. The interview included questions on demographic characteristics, the mother’s medical and obstetric history, parents’ habits and occupations, and a detailed history of the use of medications (prescription and over-the-counter, including specific multivitamin brands from which we derived the folic acid constituents), from 2 months before conception through the entire pregnancy. We considered mothers to be exposed if they reported using either a folic acid supplement alone or a folic acid-containing multivitamin.

Outcome

We specifically asked women if a health care provider had diagnosed “high blood pressure” or “toxemia/preeclampsia” during their pregnancy, the dates when the condition started and ended, and whether they had used medications for those conditions. We defined gestational hypertension as self-reported hypertension starting after the 20th week of pregnancy; because hypertension diagnosed before that date may be unrelated to pregnancy, we excluded 51 such women from the analyses. Because of the different heritability, clinical manifestations, and prognosis of early and late-onset gestational hypertension, we classified gestational hypertension according to the timing of onset (13). Hypertension was considered early onset when it began between 20 and 32 weeks after the last menstrual period and late onset afterward. Preeclampsia was defined as self-reported “toxemia/preeclampsia” after the 20th week of pregnancy. Given the potential misclassification of hypertension with and without preeclampsia, we present the two diagnoses combined as gestational hypertension for the main analyses.

Gestational age was determined via questions aimed at estimating the date of the last menstrual period. Specifically, we asked the mother her due date and whether the due date was confirmed by an ultrasound examination. We also asked the date of her last menstrual period and whether her recall of this date was exact or an estimate. Most often we were able to obtain a due date based on ultrasound examination, from which we calculated the last menstrual period.

Data analysis

We considered our population as a retrospective cohort of women with identified completed pregnancies; for the analysis, all subjects contributed person-time from the last menstrual period until they developed gestational hypertension or preeclampsia, or delivered, whichever came first. The time scale was pregnancy days. Survival functions and cumulative incidence over time (1 – survival) were calculated using the Kaplan-Meier method.

Relative risks and 95 percent confidence intervals were estimated for gestational hypertension in relation to the use of folic acid supplementation using Cox regression models. Specific exposure characteristics associated with folic acid use during pregnancy included the fact that supplementation 1) may start at different gestational ages, 2) may be discontinued before delivery, and 3) may be affected by the diagnosis of gestational hypertension/preeclampsia (only folic acid use prior to the diagnosis is of interest). As a first step, we defined “early users” as women who were taking folic acid supplements by the end of the first trimester (12 weeks after the last menstrual period) and continued during the second trimester; “late users” as women who started taking folic acid during the second trimester (after 12 weeks and before 24 weeks after the last menstrual period); “early stoppers” as women who started taking folic acid at any time but discontinued before the end of the second trimester; and “nonusers” as women who did not take folic acid during the first two trimesters of pregnancy. We also considered folic acid supplementation to be a time-dependent covariate and estimated the relative risk of developing gestational hypertension in a given month for women who used folic acid during the prior month, as compared with women who did not.

We considered the following as potential confounders or effect modifiers: parity (primiparous, multiparous), prepregnancy weight (continuous), prepregnancy body mass index (weight (kg)/height (m)2) (continuous), smoking (never, during pregnancy, before pregnancy), age (<25, 25–30, 31–34, >35 years), education (<13, 13–15, >15 years), race (White, Black, other), diabetes (yes, no), and carrying a twin pregnancy (yes, no) (1, 10, 13, 14). We selected as confounders factors potentially related to the outcome but not affected by either the exposure or the outcome, and we retained in the models those actually associated with the outcome in our population.

RESULTS

Of the 2,100 women with nonmalformed infants who did not have chronic hypertension, 204 (9.7 percent) reported gestational hypertension; 157 (7.5 percent) did not report preeclampsia and 47 (2.2 percent) did (figure 1). The incidence increased with gestational time. The women’s baseline characteristics are shown in table 1.

Folic acid supplementation

Eighty-one percent of the women were taking folic acid supplements by the end of the first trimester and continued during the second trimester, 8 percent started later, 5 percent discontinued, and 7 percent were not taking folic acid supplements by the end of the second trimester. Maternal characteristics associated with early supplementation were higher education, higher income, and White race. The incidence of gestational hypertension was lower (9.2 percent) among early users than among nonusers (12.9 percent). Figure 2 presents the cumulative incidence for these two groups; the corresponding curves for late starters and early stoppers were intermediate. The multivariate-adjusted relative risk of gestational hypertension among early users of folic acid, compared with nonusers, was 0.61 (95 percent confidence interval (CI): 0.37, 1.00). To more fully account for changes in folic acid use during gestation, we considered exposure as time dependent (i.e., we took into account changes in supplement use throughout pregnancy); in the month following use, folic acid supplementation reduced the risk of developing gestational hypertension (relative risk (RR) = 0.55, 95 percent CI: 0.39, 0.79) (table 2). Folic acid was considered as a time-dependent covariate in subsequent analyses.

Supplementation reduced the risk of developing gestational hypertension without preeclampsia (RR = 0.53, 95 percent CI: 0.36, 0.80), particularly when the hypertension had an early onset (RR = 0.27, 95 percent CI: 0.10, 0.77). The relative risk of developing preeclampsia after folic acid supplementation was 0.63 (95 percent CI: 0.28, 1.45) (table 2).

As a proxy for disease severity, we divided gestational hypertension cases into those with and those without preterm delivery (before 37 weeks). The relative risk was 0.42 (95 percent CI: 0.15, 1.19) for cases with preterm delivery; for cases with term delivery it was 0.56 (95 percent CI: 0.38, 0.82). We also classified the outcome according to the use of medications for treatment of gestational hypertension. The relative risk was 0.33 (95 percent CI: 0.10, 1.07) for cases who required medication and 0.57 (95 percent CI: 0.39, 0.84) for nonmedicated hypertension.

Among women who took folic acid supplements, more than 90 percent took them daily, more than 90 percent used multivitamins (and not folic acid as a single component), and over 95 percent of the multivitamins contained at least 0.4 mg of folic acid. For supplementation with less than 1 mg, more than 1 mg, and undetermined amount of folic acid, the relative risk of gestational hypertension was 0.67 (95 percent CI: 0.40, 1.12), 0.55 (95 percent CI: 0.38, 0.81), and 0.55 (95 percent CI: 0.36, 0.85), respectively.

Other risk factors

Incidence estimates for gestational hypertension were higher among primiparous (14.0 percent) than among multiparous (7.9 percent) women. In the multivariate model the relative risk for primiparity was 2.04 (95 percent CI: 1.52, 2.73). However, the incidence of gestational hypertension among multigravidas whose prior pregnancies resulted in miscarriages or abortions approached that of primigravidas (table 1). In addition, among multigravidas, women who had delivered within 2 years of the current pregnancy had a lower incidence (7.2 percent) than did women with a larger interpregnancy interval (9.0 percent) (data not shown). To consider the potential effect modification of parity on the effect of folic acid, we analyzed the effect of folic acid on pregnancy hypertension within strata of parity. Among primigravidas, the relative risk associated with folic acid use for gestational hypertension was 0.42 (95 percent CI: 0.23, 0.75); among multigravidas, the corresponding relative risk was 0.63 (95 percent CI: 0.40, 0.99).

Compared with singleton pregnancies, carrying a multiple pregnancy was associated with a higher risk of gestational hypertension (RR = 5.00, 95 percent CI: 2.82, 8.88). Further, for each additional 5 kg in prepregnancy weight, the risk of gestational hypertension increased 15 percent (95 percent CI: 10, 20 percent). The relative risk associated with smoking during pregnancy was 0.86 (95 percent CI: 0.59, 1.25).

In 1998, we changed data collection from home to telephone interviews and, in the same year, folate fortification of cereal grains began in the United States. Therefore, we separately analyzed the effect of folic acid supplementation on the risk of gestational hypertension in data collected between 1993 and 1997 and between 1998 and 2000. The relative risks were 0.44 (95 percent CI: 0.28, 0.70) and 0.78 (95 percent CI: 0.44, 1.39), respectively. Results for other risk factors were almost identical for both calendar periods.

In an effort to disentangle the effect of folic acid from those of other components in multivitamins, we considered the associations between vitamins other than folic acid and the outcome of interest. Vitamins E and C were not associated with gestational hypertension (RRs = 1.10 and 0.91, respectively).

DISCUSSION

We found that folic acid supplementation during pregnancy was associated with a lower risk of gestational hypertension. Few studies have evaluated the relation between hypertensive disorders of pregnancy and multivitamin use. A 1938–1939 United Kingdom clinical trial (15) and a 1995–1996 Zimbabwe case-control study (11) found 30–50 percent reductions in preeclampsia associated with multivitamin supplementation, but neither distinguished between multivitamins and folic acid. A recent publication reported no association between hypertensive disorders of pregnancy and various nutrients in the diet, including folate, vitamin C, and vitamin E (16), but all women had received multivitamin supplements during their pregnancies, which might have precluded the detection of effects from dietary sources.

Of note, none of these studies presented results for early or late-onset hypertension, yet early and late-onset events, like hypertension with and without preeclampsia, have been shown to differ in their risk factors and prognoses (13). We attempted to explore these distinctions but were limited by both the small numbers and the potential cross-classification of the outcomes. Although replication of our findings is warranted, the effect of supplementation might be difficult to identify once populations have been exposed to foods fortified with folate, as suggested by our observations after 1998, when fortification started in the United States.

Other than chance, our findings may have at least two possible interpretations: folic acid supplementation may decrease the risk of gestational hypertension or, alternatively, methodological limitations may have created a spurious protective association. Apropos of methodological limitations, a potential weakness of the present study lies in the self-report of outcome information, which may include under- and overreporting of events, misclassification of the exact date of onset, and cross-classification of preeclampsia and gestational hypertension. We did not have access to obstetric notes in mothers’ medical records. However, information errors are likely to have been minimized by use of a carefully designed questionnaire that included specific questions on the onset of gestational hypertension and pre-eclampsia as diagnosed by a health care provider; in addition, interviews were conducted within 6 months of delivery by trained nurses who were unaware of the hypothesis under study. Further, the data offer evidence of the general validity of the outcome definitions. There are a few arguments supporting perhaps imperfect but still useful outcome data. First, the reported incidence of gestational hypertension with or without preeclampsia (14 percent) and the incidence of preeclampsia (4 percent) among nulliparous women in our population are similar to those described elsewhere (4). Second, the frequency and effect of other risk factors are strikingly similar to those consistently reported in the literature (13, 14).

Were misclassification of gestational hypertension to occur similarly among users and nonusers of multivitamins, such bias would favor the finding of no effect of folic acid supplementation; that is, it would result in an underestimate of a protective effect.

In the improbable situation of differential misclassification of the outcome among supplement users and nonusers, spurious associations could vary in direction. For example, if gestational hypertension were diagnosed more readily among women with better prenatal care, who are more likely to use supplements, such bias would result in a positive association between gestational hypertension and supplementation (i.e., an underestimate of the protective effect). It is difficult to identify a situation where differential misclassification could create a spurious protective association, but confounding could: If women not taking supplements during pregnancy were at an increased risk for disease because of factors other than folic acid (e.g., due to lack of prenatal care, socioeconomic factors, poor health care awareness, etc.), such confounding could result in the observed protective effect of multivitamins. However, our findings did not change with adjustment for socioeconomic factors, such as maternal education or family income. (Of note, over 99 percent of the women had had prenatal care during their pregnancies.)

Because women with fetal loss and women with malformed infants were excluded from analyses, we cannot generalize the effects of supplementation to the risk of gestational hypertension to those pregnancies.

In contrast to previous studies (11, 15), we considered only multivitamins that included folic acid. Nonetheless, we cannot fully distinguish the effect of folic acid itself from that of other components in multivitamins. If, as has been suggested (3), oxidative stress is involved in the pathophysiology of preeclampsia, antioxidants such as vitamins C and E could be responsible for the risk reduction (17). However, we found no meaningful association between these vitamins and gestational hypertension. Moreover, there are several lines of biologic evidence that tend to support the role of folic acid. First, hyperhomocysteinemia has been repeatedly associated with preeclampsia in pregnancy (4, 11), and folic acid supplementation reduces homocysteine levels (12, 18). Second, persons with genetic disturbances in the folate-mediated methylation pathway of homocysteine have increased plasma homocysteine levels (19) and might have an increased risk of preeclampsia (2022) (not all studies found the latter association) (2325). Third, folic acid supplementation improves endothelial function in adults with hyperhomocysteinemia (26).

These findings from pregnant women may have implications for cardiovascular diseases in nonpregnant adults. Women with preeclampsia in their first pregnancy have an increased risk of ischemic heart disease later in life (27), and hyperhomocysteinemia has been consistently associated with an increased risk of stroke, peripheral vascular disease, and myocardial infarction in nonpregnant adults (28). Although results from clinical trials are not yet available, observational data have suggested that folic acid might reduce the risk of various cardiovascular diseases (29). The present observations suggest that folic acid supplementation may offer benefit not only to pregnant women but to the larger population as well.

ACKNOWLEDGMENTS

The Slone Epidemiology Center Birth Defects Study is supported by the Charles E. Culpeper Biomedical Pilot Initiative grant, the National Heart, Lung, and Blood Institute grant HL50763, and the National Institute of Child Health and Human Development grant HD27697. Additional support for the Slone Epidemiology Center Birth Defects Study was provided by Rhone Poulenc Rorer (Collegeville, Pennsylvania), Hoechst Marion Roussel, Inc. (Kansas City, Missouri), Pfizer, Inc. (New York, New York), and the Glaxo-Wellcome Company (Research Triangle Park, North Carolina).

The authors thank Dawn Jacobs, Rachel Wilson, Fiona Rice, Rita Krolak, Sally Perkins, Mary Krieger, Kathleen Sheehan, Karen Bennett Mark, Deborah Kasindorf, Clare Coughlin, Joan Shander, Diane Gallagher, Valerie Hillis, Thomas Kelley, and Nastia Dynkin for their assistance.

Reprint requests to Sonia Hernández-Díaz, Slone Epidemiology Center, Boston University School of Public Health, 1010 Commonwealth Avenue, Boston, MA 02215 (e-mail: shernan@bu.edu).

FIGURE 1. Cumulative incidence (1 – survival) of gestational hypertension with and without preeclampsia by gestational days since the last menstrual period among mothers of nonmalformed infants, Slone Epidemiology Center Birth Defects Study, 1993–2000.

FIGURE 1. Cumulative incidence (1 – survival) of gestational hypertension with and without preeclampsia by gestational days since the last menstrual period among mothers of nonmalformed infants, Slone Epidemiology Center Birth Defects Study, 1993–2000.

FIGURE 2. Cumulative incidence (1 – survival) curve for gestational hypertension by gestational days since the last menstrual period among women who started folic acid supplementation during the first trimester of pregnancy and continued at least until the end of the second trimester versus those who did not use any folic acid supplementation during the first two trimesters, Slone Epidemiology Center Birth Defects Study, 1993–2000. Women who started after the first trimester and women who started but stopped during the first two trimesters are not included in this graph.

FIGURE 2. Cumulative incidence (1 – survival) curve for gestational hypertension by gestational days since the last menstrual period among women who started folic acid supplementation during the first trimester of pregnancy and continued at least until the end of the second trimester versus those who did not use any folic acid supplementation during the first two trimesters, Slone Epidemiology Center Birth Defects Study, 1993–2000. Women who started after the first trimester and women who started but stopped during the first two trimesters are not included in this graph.

TABLE 1.

Incidence of gestational hypertension according to women’s characteristics, Slone Epidemiology Center Birth Defects Study, 1993–2000

 Women’s characteristics Total no.* Gestational hypertension No. % Parity Primiparous 622 87 14.0 Multiparous 1,478 117 7.9 Full pregnancies 1,246 91 7.3 Miscarriages/abortions 232 26 11.2 No. of fetuses Single 2,057 191 9.3 Twins (two or more) 43 13 30.2 Diabetes No 2,021 195 9.7 Yes 79 9 11.4 Smokers Never 1,108 114 10.3 During pregnancy 443 43 9.7 Before pregnancy 548 47 8.6 Race White 1,775 175 9.9 Black 126 12 9.5 Other 199 17 8.5 Age (years) <25 411 41 10.0 25–30 622 72 11.6 31–35 742 65 8.8 >35 325 26 8.0 Education (years) <13 584 63 10.8 13–15 568 59 10.4 >15 949 82 8.6 Family income ($/year) <10,000 101 12 11.9 10,000–45,000 989 107 10.8 >45,000 857 75 8.8 Unknown 153 10 6.5 Folic acid supplements Nonusers 147 19 12.9 Early starters 1,691 156 9.2 Late starters 158 14 8.9 Early stoppers 104 15 14.4 Mean (SD†) Mean (SD) Prepregnancy weight (kg) 64.4 (13.2) 70.4 (15.2) Height (cm) 164.2 (6.8) 164.0 (6.7) Prepregnancy body mass index‡ 23.9 (4.7) 26.2 (5.5)  Women’s characteristics Total no.* Gestational hypertension No. % Parity Primiparous 622 87 14.0 Multiparous 1,478 117 7.9 Full pregnancies 1,246 91 7.3 Miscarriages/abortions 232 26 11.2 No. of fetuses Single 2,057 191 9.3 Twins (two or more) 43 13 30.2 Diabetes No 2,021 195 9.7 Yes 79 9 11.4 Smokers Never 1,108 114 10.3 During pregnancy 443 43 9.7 Before pregnancy 548 47 8.6 Race White 1,775 175 9.9 Black 126 12 9.5 Other 199 17 8.5 Age (years) <25 411 41 10.0 25–30 622 72 11.6 31–35 742 65 8.8 >35 325 26 8.0 Education (years) <13 584 63 10.8 13–15 568 59 10.4 >15 949 82 8.6 Family income ($/year) <10,000 101 12 11.9 10,000–45,000 989 107 10.8 >45,000 857 75 8.8 Unknown 153 10 6.5 Folic acid supplements Nonusers 147 19 12.9 Early starters 1,691 156 9.2 Late starters 158 14 8.9 Early stoppers 104 15 14.4 Mean (SD†) Mean (SD) Prepregnancy weight (kg) 64.4 (13.2) 70.4 (15.2) Height (cm) 164.2 (6.8) 164.0 (6.7) Prepregnancy body mass index‡ 23.9 (4.7) 26.2 (5.5)

* Of 2,151 women, we excluded 51 with a diagnosis of hypertension before the 20th week of pregnancy.

† SD, standard deviation.

‡ Weight (kg)/height (m)2.

TABLE 2.

Risk of developing gestational hypertension in the month after use of folic acid-containing supplements as compared with nonuse that month, Slone Epidemiology Center Birth Defects Study, 1993–2000*

 Folic acid supplementation Unadjusted RR† 95% CI† Adjusted RR‡ 95% CI Gestational hypertension Overall (n = 204) No use Reference Reference Use 0.59 0.41, 0.83 0.55 0.39, 0.79 Gestational hypertension without preeclampsia Overall (n = 157) No use Reference Reference Use 0.54 0.37, 0.80 0.53 0.36, 0.80 Early onset (n = 17) No use Reference Reference Use 0.25 0.09, 0.66 0.27 0.10, 0.77 Late onset (n = 140) No use Reference Reference Use 0.61 0.40, 0.94 0.61 0.39, 0.93 Preeclampsia Overall (n = 47) No use Reference Reference Use 0.79 0.35, 1.75 0.63 0.28, 1.45
 Folic acid supplementation Unadjusted RR† 95% CI† Adjusted RR‡ 95% CI Gestational hypertension Overall (n = 204) No use Reference Reference Use 0.59 0.41, 0.83 0.55 0.39, 0.79 Gestational hypertension without preeclampsia Overall (n = 157) No use Reference Reference Use 0.54 0.37, 0.80 0.53 0.36, 0.80 Early onset (n = 17) No use Reference Reference Use 0.25 0.09, 0.66 0.27 0.10, 0.77 Late onset (n = 140) No use Reference Reference Use 0.61 0.40, 0.94 0.61 0.39, 0.93 Preeclampsia Overall (n = 47) No use Reference Reference Use 0.79 0.35, 1.75 0.63 0.28, 1.45

* Cox model with time-dependent covariate.

† RR, relative risk; CI, confidence interval.

‡ Adjusted for geographic region, family income, and maternal age, prepregnancy weight, parity, twin pregnancy, diabetes, smoking, and education.

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