Abstract

Background: Although maternal folate insufficiency is a risk factor for fetal neural tube defects (NTDs), there is controversy about whether vitamin B12 (B12) insufficiency is also associated with an increased risk of NTDs.

Aim: To investigate whether low maternal B12 is associated with an increased risk of fetal NTDs.

Design: Systematic review.

Methods: A systematic search of Medline between 1980 and October 2002, with an examination of the citations of all retrieved studies. Studies were included that: (i) used a cohort or case‐control design; (ii) included case mothers with a prior or current NTD‐affected pregnancy; (iii) assessed a group of unaffected ‘controls’; and (iv) measured the vitamin B12 status of all participants.

Results: Overall, 17 case‐control studies were included, mean sample size 33 cases and 93 controls. In 5/6, mean amniotic fluid B12 concentration was significantly lower in case mothers than in controls. Of 11 that measured maternal serum or plasma B12, three observed a significantly lower mean concentration in case mothers vs. controls, while five others found a non‐significant lower trend in the case group. One study observed a significantly higher mean concentration of maternal serum methylmalonic acid among the maternal cases, while another found a non‐significant lower mean concentration of plasma holo‐transcobalamin. Five studies estimated the risk of NTDs in relation to low B12 or B12‐related metabolic markers: it was significantly increased in three studies, with a non‐significant trend in the fourth.

Discussion: There seems to be a moderate association between low maternal B12 status and the risk of fetal NTDs. However, several design limitations, and the inclusion of few study participants, may have under‐represented this. A large observational study, using reliable and valid indicators of B12 status in early pregnancy, could best assess the association between B12 insufficiency and the risk of fetal NTDs.

Introduction

Increased preconception intake of folic acid can significantly reduce the risk of fetal neural tube defects (NTDs).1 Although some countries have developed initiatives that promote maternal preconceptional use of folic acid tablets, and others have fortified their cereal grain foods with synthetic folates,2,3 fetal NTDs continue to affect approximately 6 in every 10 000 pregnancies.3 Maternal folate impairment is a demonstrated risk factor for fetal NTD, but a number of investigators have pointed to abnormalities of the homocysteine (tHcy) pathway, evidenced by elevations in plasma total tHcy, as the primary site of the metabolic defect.4 Vitamin B12 (B12), as methylcobalamin, is the key cofactor for methionine synthase, co‐contributing with folate to the capacity for remethylation of tHcy. What remains unknown is the degree to which low maternal B12 status increases the risk of NTDs, the subject of this systematic review.

Methods

We performed a systematic literature search of Medline between 1980 and October 2002. The following unlimited search expression was used: ‘(Neural tube defect OR spina bifida OR anencephaly) AND (vitamin B‐12 OR cobalamin OR methylmalonic acid or holotranscobalamin)’. We also examined the references of all studies for other potential citations, but did not search for unpublished studies. Reports that met all of the following criteria were accepted: (i) use of a cohort or case‐control design; (ii) inclusion of case mothers, defined as women with a prior or current NTD‐affected pregnancy; (iii) assessment of a group of unaffected ‘controls’; and (iv) assessment of B12 status among all participants, via determination of serum B12, methylmalonic acid (MMA) or holo‐transcobalamin (holoTC), the complex formed between B12 and its transport plasma‐binding protein, transcobalamin.

Data abstraction

The abstracted study data included the study period, the basic characteristics of the case mothers and controls, the method used to measure B12,5,6 whether the serum sample was obtained during pregnancy,7 and whether participants were taking a B12‐containing supplement at the time of specimen collection (Table 1). Using the data from each study, we presented the comparative mean vitamin concentrations among case mothers and controls (Table 2), as well as the odds ratio (OR) for a NTD in the presence of an abnormal level (Table 3). We evaluated whether folate levels or folic acid supplement use were factored into, or adjusted for, within this risk estimate, since folate insufficiency is a major risk factor for NTD.25 Statistical significance was set at p≤0.05. Because of dissimilarities in the design and participant characteristics of most studies, the data were not pooled.

Table 1

General features of 17 case‐control studies of maternal vitamin B12 insufficiency and the risk of NTD

Reference Country and time period Cases with an NTD‐affected pregnancy
 
Controls without an NTD
 
Was use of B12 supplements excluded prior to specimen collection?
 

 

 
General description
 
Were other concomitant anomalies excluded?
 
Period of specimen collection
 
General description
 
Period of specimen collection
 
Molloy 19858 Ireland 1981–1983 Current NTD birth No 88% were <20 weeks gestation Current non‐NTD birth <20 weeks gestation No 
Gardiki‐Kouidou 19889 UK 1982–1987 Current NTD pregnancy No 15–23 weeks, gestation Current non‐NTD pregnancy 15–23 weeks gestation No 
Weekes 199210 US before 1992 Current NTD pregnancy No 14–22 weeks gestation Current non‐NTD healthy infant 14–22 weeks gestation No (more than 37% took a supplement) 
Economides 199211 UK before 1991 Current terminated NTD pregnancy No 14–21 weeks gestation Current terminated non‐NTD pregnancy 14–21 weeks gestation No 
Mills 199212 Finland 1983–1989 Current NTD pregnancy No 96% were <17 weeks gestation Current non‐NTD pregnancy 94% were <17 weeks gestation Mostly (8% of cases and 15% of controls took a supplement) 
Wild 199313 UK before 1992 Prior NTD pregnancy No >12 months postartum Prior non‐NTD pregnancy >12 months postpartum No 
Kirke 199314 Ireland 1986–1990 Current NTD birth after 23 weeks gestation No 75% were <20 weeks gestation Current non‐NTD birth after 23 weeks gestation 75% were <20 weeks gestation No 
Steegers‐Theunissen 199415 Netherlands before 1993 Recent NTD pregnancy No >3 months postpartum Recent non‐NTD pregnancy >3 months postpartum Yes 
Steegers‐Theunissen 199616 Netherlands before 1994 Current NTD pregnancy No Second trimester Current non‐NTD pregnancy Second trimester Yes 
Adams 199517 US before 1994 Current NTD pregnancy No Second trimester Current non‐NTD pregnancy Second trimester No 
Wright 199518 Northern Ireland before 1995 Recent NTD pregnancy No 3–12 months postpartum Recent non‐NTD pregnancy 3–12 months postpartum No 
Wald 199619 International before 1993 Current NTD pregnancy No Before pregnancy Current non‐NTD pregnancy Before pregnancy Yes 
Van der Put 199720 Netherlands before 1997 Remote NTD pregnancy No Not pregnant 25% men and 75% women with no history of NTD Not pregnant No 
Steen 199821 US before 1997 Current NTD pregnancy No 14–18 weeks gestation Current non‐NTD pregnancy 14–18 weeks gestation No 
Dawson 199922 US before 1998 Current NTD pregnancy Yes 15–20 weeks gestation Current non‐NTD pregnancy 15–20 weeks gestation No (38% took a supplement) 
Wilson 199923 Canada before 1999 Prior NTD pregnancy Yes Not pregnant Healthy women, no history of NTD Not pregnant Yes 
Afman 200124 Netherlands before 2000 Prior NTD pregnancy Yes Not pregnant Healthy women, no history of NTD Not pregnant No 
Reference Country and time period Cases with an NTD‐affected pregnancy
 
Controls without an NTD
 
Was use of B12 supplements excluded prior to specimen collection?
 

 

 
General description
 
Were other concomitant anomalies excluded?
 
Period of specimen collection
 
General description
 
Period of specimen collection
 
Molloy 19858 Ireland 1981–1983 Current NTD birth No 88% were <20 weeks gestation Current non‐NTD birth <20 weeks gestation No 
Gardiki‐Kouidou 19889 UK 1982–1987 Current NTD pregnancy No 15–23 weeks, gestation Current non‐NTD pregnancy 15–23 weeks gestation No 
Weekes 199210 US before 1992 Current NTD pregnancy No 14–22 weeks gestation Current non‐NTD healthy infant 14–22 weeks gestation No (more than 37% took a supplement) 
Economides 199211 UK before 1991 Current terminated NTD pregnancy No 14–21 weeks gestation Current terminated non‐NTD pregnancy 14–21 weeks gestation No 
Mills 199212 Finland 1983–1989 Current NTD pregnancy No 96% were <17 weeks gestation Current non‐NTD pregnancy 94% were <17 weeks gestation Mostly (8% of cases and 15% of controls took a supplement) 
Wild 199313 UK before 1992 Prior NTD pregnancy No >12 months postartum Prior non‐NTD pregnancy >12 months postpartum No 
Kirke 199314 Ireland 1986–1990 Current NTD birth after 23 weeks gestation No 75% were <20 weeks gestation Current non‐NTD birth after 23 weeks gestation 75% were <20 weeks gestation No 
Steegers‐Theunissen 199415 Netherlands before 1993 Recent NTD pregnancy No >3 months postpartum Recent non‐NTD pregnancy >3 months postpartum Yes 
Steegers‐Theunissen 199616 Netherlands before 1994 Current NTD pregnancy No Second trimester Current non‐NTD pregnancy Second trimester Yes 
Adams 199517 US before 1994 Current NTD pregnancy No Second trimester Current non‐NTD pregnancy Second trimester No 
Wright 199518 Northern Ireland before 1995 Recent NTD pregnancy No 3–12 months postpartum Recent non‐NTD pregnancy 3–12 months postpartum No 
Wald 199619 International before 1993 Current NTD pregnancy No Before pregnancy Current non‐NTD pregnancy Before pregnancy Yes 
Van der Put 199720 Netherlands before 1997 Remote NTD pregnancy No Not pregnant 25% men and 75% women with no history of NTD Not pregnant No 
Steen 199821 US before 1997 Current NTD pregnancy No 14–18 weeks gestation Current non‐NTD pregnancy 14–18 weeks gestation No 
Dawson 199922 US before 1998 Current NTD pregnancy Yes 15–20 weeks gestation Current non‐NTD pregnancy 15–20 weeks gestation No (38% took a supplement) 
Wilson 199923 Canada before 1999 Prior NTD pregnancy Yes Not pregnant Healthy women, no history of NTD Not pregnant Yes 
Afman 200124 Netherlands before 2000 Prior NTD pregnancy Yes Not pregnant Healthy women, no history of NTD Not pregnant No 

NTD, neural tube defect.

Table 2

Results of case‐control studies of the mean concentration of vitamin B12 in maternal amniotic fluid or serum

Analyte and its source Reference Number of participants
 
B12 assay used Mean (SD) or median (range) concentration, pmol/l
 
p 

 

 
NTD cases
 
Non‐NTD controls
 

 
Cases
 
Controls
 

 
Amniotic fluid B12 Gardiki‐Kouidou 19889 26 65 RIA 110 (50–175) 162 (100–339) 0.04 
 Weekes 199210 47 RIA 140 (90) 600 (380) <0.001 
 Economides 199211 24 RIA 92 (74–177) 207 (81–760) <0.01 
 Steegers‐Theunissen 199516 27 31 RIA 379 (298) 481 (278) NS 
 Steen 199821 15 63 RIA 111 (109–325)* 398 (355–531)* 0.02 
 Dawson 199922 11 29 RIA 226 (190) 618 (285) <0.001 
Serum or plasma B12 Molloy 19858 32 384 Micro 219 (129–442) 204 (55–738) NS 
Economides 199211 24 RIA 151 (63–188) 170 (66–858) NS 
 Mills 199212 89 178 RIA 356 (119) 384 (142) NS 
 Wild 199313 29 29 RIA 331 (201–732) 361 (229–858) NS 
 Kirke 199314 81 247 Micro 179 (75–560) 218 (65–559) 0.001 
 Steegers‐Theunissen 199416 41 50 RIA 268 (100) 264 (115) NS 
 Adams 199517 33 132 Mass spec. 297 (NA) 319 (NA) NS 
 Wright 199518 15 15 RIA 148 (60) 218 (122) 0.004 
 Wald 199619 19 79 Micro 214 (NA) 236 (NA) 0.05 
 ven der Put 199720 60 94 RIA 245 (43–620) 255 (64–580) NS 
 Afman 200124 46 73 RIA 220 (70–1000) 220 (89–600) NS 
Other        
Serum MMA Adams 199517 33 132 Mass spec. 0.13 (NA) 0.10 (NA) 0.004 
Plasma holoTC** Afman 200124 46 73 RIA 41 (1–372) 50 (0.5–186) NS 
Analyte and its source Reference Number of participants
 
B12 assay used Mean (SD) or median (range) concentration, pmol/l
 
p 

 

 
NTD cases
 
Non‐NTD controls
 

 
Cases
 
Controls
 

 
Amniotic fluid B12 Gardiki‐Kouidou 19889 26 65 RIA 110 (50–175) 162 (100–339) 0.04 
 Weekes 199210 47 RIA 140 (90) 600 (380) <0.001 
 Economides 199211 24 RIA 92 (74–177) 207 (81–760) <0.01 
 Steegers‐Theunissen 199516 27 31 RIA 379 (298) 481 (278) NS 
 Steen 199821 15 63 RIA 111 (109–325)* 398 (355–531)* 0.02 
 Dawson 199922 11 29 RIA 226 (190) 618 (285) <0.001 
Serum or plasma B12 Molloy 19858 32 384 Micro 219 (129–442) 204 (55–738) NS 
Economides 199211 24 RIA 151 (63–188) 170 (66–858) NS 
 Mills 199212 89 178 RIA 356 (119) 384 (142) NS 
 Wild 199313 29 29 RIA 331 (201–732) 361 (229–858) NS 
 Kirke 199314 81 247 Micro 179 (75–560) 218 (65–559) 0.001 
 Steegers‐Theunissen 199416 41 50 RIA 268 (100) 264 (115) NS 
 Adams 199517 33 132 Mass spec. 297 (NA) 319 (NA) NS 
 Wright 199518 15 15 RIA 148 (60) 218 (122) 0.004 
 Wald 199619 19 79 Micro 214 (NA) 236 (NA) 0.05 
 ven der Put 199720 60 94 RIA 245 (43–620) 255 (64–580) NS 
 Afman 200124 46 73 RIA 220 (70–1000) 220 (89–600) NS 
Other        
Serum MMA Adams 199517 33 132 Mass spec. 0.13 (NA) 0.10 (NA) 0.004 
Plasma holoTC** Afman 200124 46 73 RIA 41 (1–372) 50 (0.5–186) NS 

NTD, neural tube defect; B12, vitamin B12; NS, not significant; RIA, radioimmunoassay; Micro, microbiological assay; Mass spec., mass spectrometry; NA, not available; MMA, methylmalonic acid; HoloTC, holotranscobalamin. *95%CI. **Derived as: HoloTC=(plasma vitamin B12)−(holo‐haptocorrin).

Table 3

Results of case‐control studies evaluating the risk of neural tube defects according to measures of maternal vitamin B12 status

Reference Number of participants
 
Analyte and its source Comparison Risk estimate
 

 
NTD cases
 
Non‐NTD controls
 

 

 
OR (95% CI) for NTD, cases vs. controls
 
Was the OR adjusted for maternal serum or plasma folate level?
 
Was the OR adjusted for maternal folic acid supplement use?
 
Molloy 19858 32 384 Serum B12 <185 pmol/l vs. ≥185 pmol/l 0.9 (0.4–1.9) No No 
Kirke 199314 81 247 Plasma B12 and folate ≤lower quartile vs. ≥upper quartile of both analytes 5.4 (1.2–25.2) No No 
Van der Put 199720 60 94 Serum B12 ≤5th centile vs. >95th centile 3.9 (1.3–11.9) No No 
Afman 200124 46 73 Plasma B12 ≤lower quartile vs. ≥upper quartile 1.8 (0.6–5.2) No No 
   Plasma holoTC ≤lower quartile us. ≥upper quartile of 2.9 (0.9–9.2) No No 
Adams 199517 33 132 Serum MMA ≥90th centile vs. <10th centile 13.3 (2.7–65.5) Yes No 
Reference Number of participants
 
Analyte and its source Comparison Risk estimate
 

 
NTD cases
 
Non‐NTD controls
 

 

 
OR (95% CI) for NTD, cases vs. controls
 
Was the OR adjusted for maternal serum or plasma folate level?
 
Was the OR adjusted for maternal folic acid supplement use?
 
Molloy 19858 32 384 Serum B12 <185 pmol/l vs. ≥185 pmol/l 0.9 (0.4–1.9) No No 
Kirke 199314 81 247 Plasma B12 and folate ≤lower quartile vs. ≥upper quartile of both analytes 5.4 (1.2–25.2) No No 
Van der Put 199720 60 94 Serum B12 ≤5th centile vs. >95th centile 3.9 (1.3–11.9) No No 
Afman 200124 46 73 Plasma B12 ≤lower quartile vs. ≥upper quartile 1.8 (0.6–5.2) No No 
   Plasma holoTC ≤lower quartile us. ≥upper quartile of 2.9 (0.9–9.2) No No 
Adams 199517 33 132 Serum MMA ≥90th centile vs. <10th centile 13.3 (2.7–65.5) Yes No 

NTD, neural tube defect; B12, vitamin B12; OR, odds ratio; HoloTC, holotranscobalamin; MMA, methylmalonic acid.

Results

The Medline search yielded 107 studies. Of these, 17 case‐control studies met the inclusion criteria, and contained original data (Table 1). The average study comprised 33 cases and 93 controls. Only five studies mentioned excluding most12 or all15,16,19,23 individuals who were taking a B12‐containing supplement at the time of biochemical testing (Table 1). In 5/6 studies, there was a significantly lower mean amniotic fluid B12 concentration in case mothers than in controls,9–11,21,22 with a trend in the same direction in the sixth study16 (Table 2). Among the controls in five of these studies, amniocentesis was performed for cytogenetic testing, either because of advanced maternal age9,10,16,21 or for reasons not given.22 In the sixth study, amniotic fluid samples were obtained at the time of pregnancy termination, for social reasons.11

Of 11 studies that measured maternal serum or plasma B12, eight observed either a significant14,18,19 or non‐significant trend11–13,17,20 lower mean concentration among the case mothers vs. controls, while three others did not8,15,24 (Table 2). There were no study features evident, such as the period of specimen collection (Table 1) or type of B12 assay used (Table 2), to explain these differences. For example, in five studies, participants were pregnant at the time of serum/plasma specimen collection,8,11,12,14,17 while in six studies they were not,13,15,18–20,24 but this did not seem to relate to whether there was an observed difference in mean serum B12 concentrations between case mothers and controls (Table 2).

As regards other measures, one study found a significantly higher mean concentration of serum MMA, a marker of B12 impairment, among maternal cases.17 In another case‐control study, a non‐significant lower mean concentration of plasma holoTC was observed24 (Table 2).

Five studies provided data on the estimate risk of NTD in relation to serum B12,8,14,20,24 holoTC24 or MMA17 (Table 3). An Irish group of investigators found no difference in the risk of NTDs between 32 case mothers and 384 controls, comparing serum B12 concentrations below and above 185 pmol/l (OR 0.9, 95% CI 0.4–1.9),8 while another group of researchers observed a significant associated risk between the lowest and highest quartile concentrations of combined plasma B12 and folate among 81 case mothers and 247 controls (OR 5.4, 95%CI 1.2–25.2.14 In a study from the Netherlands, with 60 case mothers and 94 controls, there was nearly a threefold increased risk for NTDs for a maternal serum B12 concentration below the 5th centile vs. that above the 95th centile (OR 3.9, 95% CI 1.3–11.9)20 (Table 3). A smaller Dutch study found a non‐significant increased risk for fetal NTD upon comparing the lower and upper quartile concentrations of plasma B12 (OR 1.8, 95%CI 0.6–5.2), and a borderline significant risk for holoTC (OR 2.9, 95%CI 0.9–9.2).24 In a US study with 33 case mothers and 132 controls, there was an increased risk for NTDs in the presence of a maternal serum MMA concentration above the 90th centile, compared to that below the 10th centile (adjusted OR 13.3, 95%CI 2.7–65.5).17 In only the latter study was maternal serum folate adjusted for in the risk estimate, but no study considered maternal folate supplement use (Table 3). Finally, in a sixth study, a genetic polymorphism of maternal methionine synthase reductase (MTRR 66A→G)—an enzyme that activates cobalamin‐dependent methionine synthase—in combination with low maternal serum B12, was associated with a four‐fold increased risk of NTDs (OR 4.8, 95% CI 1.5–15.8).23

Discussion

Some observational data suggest a moderately strong association between low maternal B12 status and the risk of fetal NTDs. The risk of NTDs was most pronounced when comparisons were made using more extreme cut‐points (e.g. 5th vs. 95th centiles) to define abnormal and normal levels.

Limitations to current knowledge

In addition to publication bias, these disparate findings can be explained by several factors. Since severe B12 insufficiency is rare among non‐vegan adults,26–28 only mild reductions in B12 concentrations would be expected in a study sample of young women. Hence, the detection of a small significant difference in the B12 status of case mothers and controls might be difficult, considering that the average study had only 33 cases and 93 controls, and that the measurement of B12 or its metabolites was done using assays with less than optimal accuracy.29–32 The collection of maternal specimens remote from the time of the index pregnancy, or many weeks after the period of embryonic risk for NTD formation, would also be expected to dilute any true relationship between B12 insufficiency and NTD risk. Similarly, measurement ‘contamination’ from recent B12 supplement use and the effect of specimen collection at various gestational periods of pregnancy33,34 might further underestimate the true effect of B12 insufficiency on the risk of NTD. The failure of most studies to adjust for maternal folate status may have further confounded matters, since B12 insufficiency may simply be a marker of concomitant folate impairment, while, on the other hand, high folate concentrations may mask the haematological signs of B12 insufficiency.

Nearly all studies reviewed herein found a lower concentration of amniotic fluid B12 among the maternal cases. But is the B12 status of the fetus accurately represented by that found in amniotic fluid, or is maternal serum B12 better? Among 76 women who underwent testing at approximately 17 weeks gestation, plasma B12 (320±130 pmol/l) was significantly lower than that measured in amniotic fluid (650±420 pmol/l), although both were highly correlated.35 Term newborns have B12 concentrations at least twice as high as their mothers, but the greatest concentration is found within the placental intervillous space.36 It seems that maternal B12 crosses the placenta into the fetal circulation in modest amounts,37 increasing with advanced gestational age,38 but perhaps, only once placental tissue stores have been saturated.39 Moreover, in pregnant rats, intrinsic factor‐cobalamin receptor (cubulin) activity declined 15‐fold in the visceral yolk sac membranes, but increased almost 20‐fold in the placental membranes, between the times of 14 and 19 days of gestation.40 Since concentrations of maternal serum or amniotic fluid B12 appear to be in a dynamic state of change throughout pregnancy, they may not accurately reflect that found within the early developing embryo, and thus, the respective NTD risk.

Ramifications for future research

This review does not resolve whether a deficiency of B12 or a related elevation in tHcy contributes to the formation of NTDs.4 To better understand whether maternal B12 insufficiency is a risk factor for NTD, a large observational study is needed. This study should use a reliable and valid indicator of B12 status, perhaps serum MMA,32 obtained in early pregnancy, and should adjust for maternal folate concentration. Such data may be of great clinical importance: Like folic acid, orally administered B12 appears to be a safe, simple, and inexpensive vitamin. If periconceptional maternal B12 deficiency too can be shown to increase the risk of NTDs in a consistent manner, with an attenuation of that risk with higher B12 intake, then there exists a rational basis for conducting a multicentre randomized controlled clinical trial comparing periconceptional B12 and folic acid supplements with folic acid alone. Until such data are made available, periconceptional folic acid remains the mainstay of NTD prevention, with consideration given to adding small doses of B12, such as that currently found in most multivitamin tablets.41

Address correspondence to Dr J.G. Ray. e‐mail: jray515445@aol.com

JGR is supported by a grant from the Physicians' Services Incorporated Foundation of Toronto, Ontario. HJB is an Established Investigator of the Netherlands Heart Foundation (D97.021).

References

1
Lumpy J, Watson L, Watson M, Bower C. Periconceptional supplementation with folate and/or multivitamins for preventing neural tube defects.
Cochrane Database Syst Rev
 
2001
; CD001056.
2
Honein MA, Paulozzi LJ, Mathews TJ, Erickson JD, Wong LY. Impact of folic acid fortification of the US food supply on the occurrence of neural tube defects.
JAMA
 
2001
;
285
:
2981
–6.
3
Ray JG, Meier C, Vermeulen MJ, Boss S, Wyatt PR, Cole DEC. Association of neural tube defects and folic acid food fortification.
Lancet
 
2002
;
360
:
2047
–8.
4
Mills JL, McPartlin JM, Kirke PN, Lee YJ, Conley MR, Weir DG, et al. Homocysteine metabolism in pregnancies complicated by neural‐tube defects.
Lancet
 
1995
;
345
:
149
–51.
5
Mollin DL, Anderson BB, Burman JF. The serum vitamin B12 level: its assay and significance.
Clin Haematol
 
1976
;
5
:
521
–46.
6
Lee DS, Griffiths BW. Human serum vitamin B12 assay methods: a review.
Clin Biochem
 
1985
;
18
:
261
–6.
7
Koebnick C, Heins UA, Dagnelie PC, Wickramasinghe SN, Ratnayaka ID, Hothorn T, Pfahlberg AB, Hoffmann I, Lindemans J, Leitzmann C. Longitudinal concentrations of vitamin B(12) and vitamin B(12)‐binding proteins during uncomplicated pregnancy.
Clin Chem
 
2002
;
48
:
928
–33.
8
Molloy AM, Kirke P, Hillary I, Weir DG, Scott JM. Maternal serum folate and vitamin B12 concentrations in pregnancies associated with neural tube defects.
Arch Dis Child
 
1985
;
60
:
660
–5.
9
Gardiki‐Kouidou P, Seller MJ. Amniotic fluid folate, vitamin B12 and transcobalamins in neural tube defects.
Clin Genet
 
1988
;
33
:
441
–8.
10
Weekes EW, Tamura T, Davis RO, Birch R, Vaughn WH, Franklin JC, Barganier C, Cosper P, Finley SC, Finley WH. Nutrient levels in amniotic fluid from women with normal and neural tube defect pregnancies.
Biol Neonate
 
1992
;
61
:
226
–31.
11
Economides DL, Ferguson J, Mackenzie IZ, Darley J, Ware II, Holmes‐Siedle M. Folate and vitamin B12 concentrations in maternal and fetal blood, and amniotic fluid in second trimester pregnancies complicated by neural tube defects.
Br J Obstet Gynaecol
 
1992
;
99
:
23
–5.
12
Mills JL, Tuomilehto J, Yu KF, Colman N, Blaner WS, Koskela P, Rundle WE, Forman M, Toivanen L, Rhoads GG. Maternal vitamin levels during pregnancies producing infants with neural tube defects.
J Pediatr
 
1992
;
120
:
863
–71.
13
Wild J, Schorah CJ, Sheldon TA, Smithells RW. Investigation of factors influencing folate status in women who have had a neural tube defect‐affected infant.
Br J Obstet Gynaecol
 
1993
;
100
:
546
–9.
14
Kirke PN, Molloy AM, Daly LE, Burke H, Weir DG, Scott JM. Maternal plasma folate and vitamin B12 are independent risk factors for neural tube defects.
Q J Med
 
1993
;
86
:
703
–8.
15
Steegers‐Theunissen RP, Boers GH, Trijbels FJ, Finkelstein JD, Blom HJ, Thomas CM, Borm GF, Wouters MG, Eskes TK. Maternal hyperhomocysteinemia: a risk factor for neural‐tube defects?
Metabolism
 
1994
;
43
:
1475
–80.
16
Steegers‐Theunissen RP, Boers GH, Blom HJ, Nijhuis JG, Thomas CM, Borm GF, Eskes TK. Neural tube defects and elevated homocysteine levels in amniotic fluid.
Am J Obstet Gynecol
 
1995
;
172
:
1436
–41.
17
Adams MJ Jr, Khoury MJ, Scanlon KS, Stevenson RE, Knight GJ, Haddow JE, Sylvester GC, Cheek JE, Henry JP, Stabler SP, et al. Elevated midtrimester serum methylmalonic acid levels as a risk factor for neural tube defects.
Teratology
 
1995
;
51
:
311
–17.
18
Wright ME. A case‐control study of maternal nutrition and neural tube defects in Northern Ireland.
Midwifery
 
1995
;
11
:
146
–52.
19
Wald NJ, Hackshaw AD, Stone R, Sourial NA. Blood folic acid and vitamin B12 in relation to neural tube defects.
Br J Obstet Gynaecol
 
1996
;
103
:
319
–24.
20
van der Put NM, Thomas CM, Eskes TK, Trijbels FJ, Steegers‐Theunissen RP, Mariman EC, De Graaf‐Hess A, Smeitink JA, Blom HJ. Altered folate and vitamin B12 metabolism in families with spina bifida offspring.
Q J Med
 
1997
;
90
:
505
–10.
21
Steen MT, Boddie AM, Fisher AJ, Macmahon W, Saxe D, Sullivan KM, Dembure PP, Elsas LJ. Neural‐tube defects are associated with low concentrations of cobalamin (vitamin B12) in amniotic fluid.
Prenat Diagn
 
1998
;
18
:
545
–55.
22
Dawson EB, Evans DR, Harris WA, Van Hook JW. Amniotic fluid B12, calcium, and lead levels associated with neural tube defects.
Am J Perinatol
 
1999
;
16
:
373
–8.
23
Wilson A, Platt R, Wu Q, Leclerc D, Christensen B, Yang H, Gravel RA, Rozen R. A common variant in methionine synthase reductase combined with low cobalamin (vitamin B12) increases risk for spina bifida.
Mol Genet Metab
 
1999
;
67
:
317
–23.
24
Afman LA, Van Der Put NM, Thomas CM, Trijbels JM, Blom HJ. Reduced vitamin B12 binding by transcobalamin II increases the risk of neural tube defects.
Q J Med
 
2001
;
94
:
159
–66.
25
Wald NJ, Law MR, Morris JK, Wald DS. Quantifying the effect of folic acid.
Lancet
 
2001
;
358
:
2069
–73.
26
Wright JD, Bialostosky K, Gunter EW, Carroll MD, Najjar MF, Bowman BA, Johnson CL. Blood folate and vitamin B12: United States, 1988–94.
Vital Health Stat
 
1998
;
11
:
1
–78.
27
Ray JG, Vermeulen MJ, Boss SC, Cole DE. Increased red cell folate concentrations in women of reproductive age after Canadian folic acid food fortification.
Epidemiology
 
2002
;
13
:
238
–40.
28
Krajcovicova‐Kudlackova M, Blazicek P, Kopcova J, Bederova A, Babinska K. Homocysteine levels in vegetarians versus omnivores.
Ann Nutr Metab
 
2000
;
44
:
135
–8.
29
Grinblat J, Marcus DL, Hernandez F, Freedman ML. Folate and vitamin B12 levels in an urban elderly population with chronic diseases. Assessment of two laboratory folate assays: microbiologic and radioassay.
J Am Geriatr Soc
 
1986
;
34
:
627
–32.
30
Ray JG, Cole DE, Boss SC. An Ontario‐wide study of vitamin B12, serum folate, and red cell folate levels in relation to plasma homocysteine: is a preventable public health issue on the rise?
Clin Biochem
 
2000
;
33
:
337
–43.
31
Holleland G, Schneede J, Ueland PM, Lund PK, Refsum H, Sandberg S. Cobalamin deficiency in general practice: assessment of the diagnostic utility and cost‐benefit analysis of methylmalonic acid determination in relation to current diagnostic strategies.
Clin Chem
 
1999
;
45
:
189
–98.
32
McMullin MF, Young PB, Bailie KE, Savage GA, Lappin TR, White R. Homocysteine and methylmalonic acid as indicators of folate and vitamin B12 deficiency in pregnancy.
Clin Lab Haematol
 
2001
;
23
:
161
–5.
33
Metz J, McGrath K, Bennett M, Hyland K, Bottiglieri T. Biochemical indices of vitamin B12 nutrition in pregnant patients with subnormal serum vitamin B12 levels.
Am J Hematol
 
1995
;
48
:
251
–5.
34
Walker MC, Smith GN, Perkins SL, Keely EJ, Garner PR. Changes in homocysteine levels during normal pregnancy.
Am J Obstet Gynecol
 
1999
;
180
:
660
–4.
35
Tamura T, Weekes EW, Birch R, Franklin JC, Cosper P, Davis RO, Finley SC, Finley WH. Relationship between amniotic fluid and maternal blood nutrient levels.
J Perinat Med
 
1994
;
22
:
227
–34.
36
Giugliani ER, Jorge SM, Goncalves AL. Serum vitamin B12 levels in parturients, in the intervillous space of the placenta and in full‐term newborns and their interrelationships with folate levels.
Am J Clin Nutr
 
1985
;
41
:
330
–5.
37
Perez‐D'Gregorio RE, Miller RK. Transport and endogenous release of vitamin B12 in the dually perfused human placenta.
J Pediatr
 
1998
;
132
:
S35
–42.
38
Graber SE, Scheffel U, Hodkinson B, McIntyre PA. Placental transport of vitamin B12 in the pregnant rat.
J Clin Invest
 
1971
;
50
:
1000
–4.
39
Baker H, Frank O, Deangelis B, Feingold S, Kaminetzky HA. Role of placenta in maternal‐fetal vitamin transfer in humans.
Am J Obstet Gynecol
 
1981
;
141
:
792
–6.
40
Ramanujam KS, Seetharam S, Seetharam B. Regulated expression of intrinsic factor‐cobalamin receptor by rat visceral yolk sac and placental membranes.
Biochim Biophys Acta
 
1993
;
1146
:
243
–6.
41
Czeizel AE. Periconceptional folic acid containing multivitamin supplementation.
Eur J Obstet Gynecol Reprod Biol
 
1998
;
78
:
151
–61.