Variability in predicted rates of Down syndrome associated with elevated maternal serum alpha-fetoprotein levels in older women.

Quantitative predictions of rates of Down syndrome offspring as a function of maternal serum alpha-fetoprotein (MSAFP) values and of maternal age were reviewed. Comparisons were made of 35-year-equivalent-risk values by maternal age, that is, MSAFP values (at various maternal ages) that predict the risk (of a Down syndrome offspring) equivalent to that of an average 35-year-old woman irrespective of knowledge of MSAFP. This boundary was chosen because MSAFP values that predict a greater risk than this point for younger women or a lower risk for older women are likely in many jurisdictions to alter a decision about amniocentesis that would be reached without knowledge of MSAFP. There were marked differences among available studies in these 35-year-equivalent-risk values for older women. For instance, for age 41, the values among studies vary from 1.5 MoM to 2.1 MoM and the predicted proportion of 41-year-old women likely to be affected clinically by MSAFP determinations by the above criterion is 4.4%-18%, depending on which rate schedule is used. At age 39, the variation is 1.1 to 1.9 MoM or 7.1% to 38%. Possible explanations for the variation include sampling fluctuation. Relatively few data on Down syndrome have been used to calculate the parameters of the gaussian distributions used by the studies reviewed. It is suggested that great caution be used before employing published rate schedules for genetic counseling of older women, at least until sufficient data are collected and the nature of the discrepancies among studies is clarified.     

Associations of Maternal Diabetes During Pregnancy with Overweight in Offspring: Results from the Prospective TEDDY Study

Objective: This study aimed to determine the relationship between different forms of, and potential pathways between, maternal diabetes and childhood obesity at different ages. Methods: Prospective cohort data from The Environmental Determinants of Diabetes in the Young (TEDDY) study, which was composed of 5,324 children examined from 0.25 to 6 years of age, were analyzed. Cross‐sectional and longitudinal analyses taking into account potential confounders and effect modifiers such as maternal prepregnancy BMI and birth weight z scores were performed. Results: Offspring of mothers with gestational diabetes mellitus (GDM) or type 1 diabetes mellitus (T1DM) showed a higher BMI standard deviation score and increased risk for overweight and obesity at 5.5 years of age than offspring of mothers without diabetes. While these associations could be substantially explained by maternal prepregnancy BMI in offspring of mothers with GDM, significant associations disappeared after adjustment for birth weight z scores in offspring of T1DM mothers. Furthermore, overweight risk became stronger with increasing age in offspring of mothers with diabetes compared with offspring of mothers without diabetes. Conclusions: Maternal diabetes is associated with increased risk of offspring overweight, and the association appears to get stronger as children grow older. Indeed, intrauterine exposure to maternal T1DM may predispose children to later obesity through increased birth weight, while maternal BMI is more important in children exposed to GDM.     

SELECTION ON MENOPAUSE IN TWO PREMODERN HUMAN POPULATIONS: NO EVIDENCE FOR THE MOTHER HYPOTHESIS

Evolutionary theory suggests that natural selection should synchronize senescence of reproductive and somatic systems. In some species, females show dramatic discordance in senescence rates in these systems, leading to a clear menopause coupled with prolonged postreproductive life span. The Mother Hypothesis proposes that menopause evolved to avoid higher reproductive‐mediated mortality risk in late‐life and ensure the survival of existing offspring. Despite substantial theoretical interest, the critical predictions of this hypothesis have never been fully tested in populations with natural fertility and mortality. Here, we provide an extensive test, investigating both short‐ and long‐term consequences of mother loss for offspring, using multigenerational demographic datasets of premodern Finns and Canadians. We found no support for the Mother Hypothesis. First, although the risk of maternal death from childbirth increased from middle age, the risk only reached 1–2% at age 50 and was predicted to range between 2% and 8% by 70. Second, offspring were adversely affected by maternal loss only in their first two years (i.e., preweaning), having reduced survival probability in early childhood as well as ultimate life span and fitness. Dependent offspring were not affected by maternal death following weaning, either in the short‐ or long‐term. We suggest that although mothers are required to ensure offspring survival preweaning in humans, maternal loss thereafter can be compensated by other family members. Our results indicate that maternal effects on dependent offspring are unlikely to explain the maintenance of menopause or prolonged postreproductive life span in women.     

The MTRR 66A>G polymorphism and maternal risk of birth of a child with Down syndrome in Caucasian women: a case–control study and a meta-analysis

We performed a large case–control study and a meta-analysis of the literature to address the role of the methionine synthase reductase (MTRR) c.66A>G polymorphism as a maternal risk factor for the birth of a child with Down Syndrome (DS) among Caucasian women. A total of 253 mothers of a DS child (MDS) and 298 control mothers of Italian origin were included in the case–control study. The meta-analysis of previous and present data involved a total of seven studies performed in Caucasian populations (971 MDS and 1,387 control mothers). Results from the meta-analysis indicated overall a positive significant association between MTRR c.66A>G genotype [OR 1.36 (95 % CI 1.10–1.68), dominant model] and allele frequencies [OR 1.26 (95 % CI 1.04–1.51), allele contrast model] and maternal risk of birth of a child with DS. A sensitivity analysis revealed some interesting differences between Europeans, Caucasians of European descent, and inhabitants of Mediterranean regions, suggesting the possibility of population-specific modifying factors. The case–control study revealed association of the polymorphism with increased folate levels, and a possible interaction with the methionine synthase (MTR) c.2756A>G one, that resulted in a borderline significant maternal risk of birth of a child with DS for the double heterozygous MTR 2756AG/MTRR 66AG genotype [OR 1.79 (95 % CI 1.00–3.18)]. Overall, present data suggest that the MTRR c.66A>G polymorphism represents a risk factor for the birth of a child with DS among white Caucasian women. However, the combined presence of other genetic factors and interactions with geographic and environmental ones, can modify the effect of the single polymorphism alone, leading to population specific effect sizes.     

The Risk of Schizophrenia and Child Psychiatric Disorders in Offspring of Mothers with Lung Cancer and Other Types of Cancer: A Danish Nationwide Register Study

Background

Maternal immune responses and brain-reactive antibodies have been proposed as possible causal mechanisms for schizophrenia and some child psychiatric disorders. According to this hypothesis maternal antibodies may cross the placenta and interact with the developing CNS of the fetus causing future neurodevelopmental disorders. Therefore, we investigated if children of mothers with cancer might be at higher risk of developing psychiatric disorders, with particular focus on small-cell lung cancer, which is known to induce production of antibodies binding to CNS elements.

Methods

Nationwide population-based registers were linked, including the Danish Psychiatric Central Register and The Danish Cancer Registry. Data were analyzed as a cohort study using survival analysis techniques. Incidence rate ratios (IRRs) and accompanying 95% confidence intervals (CIs) were used as measures of relative risk.

Results

In general, parental cancer was not associated with schizophrenia in the offspring (IRR, 0.98; 95% CI, 0.95-1.01). Furthermore, we found no temporal associations with maternal cancer in general; neither around the pregnancy period. However, maternal small-cell lung cancer increased the risk of early-onset schizophrenia and maternal small-cell lung cancer diagnosed within 20 years after childbirth increased the risk of schizophrenia. Parental cancer was not associated with child psychiatric disorders (IRR, 1.01; 95% CI, 0.98-1.05) except for the smoking related cancers. There was a significantly increased risk of child psychiatric disorders in offspring of both mothers (IRR, 1.35; 95% CI, 1.16-1.58) and fathers (IRR, 1.47; 95% CI, 1.30-1.66) with lung cancer of all types.

Conclusions

In general, parental cancer did not increase the risk of schizophrenia nor of child psychiatric disorders. However, maternal small-cell lung cancer increased the risk of schizophrenia in subgroups; and lung cancer in general increased the risk of child psychiatric disorders, which could be due to risk factors associated with parental smoking.     

Intergenerational transmission of insulin resistance and type 2 diabetes

Studies in women with type 1 or type 2 diabetes mellitus (DM) and their children suggest that the in utero ‘diabetic’ environment in which the fetus develops can increase the risk of diabetes in the child, in a non-genetic but heritable fashion. Studies in rodents provide strong evidence for maternal transmission of diabetes, but are based primarily on a model type 1 DM and there is no standard animal model of type 2 DM in pregnancy or of gestational diabetes mellitus (GDM), although those reported uniformly show glucose intolerance in the offspring. Rodent models of diet-induced obesity have relevance to current upward trends in maternal obesity and GDM, although maternal glucose homeostasis is not always assessed and elements of the diet may have an independent influence. The mechanisms by which maternal type 2DM evokes a higher risk of the disorder in the offspring are likely to result from epigenetic modification in early life of pathways of pancreatic β cells and of liver and muscle insulin signalling pathways. Also, epigenetic processes associated with hormonal imbalance may lead to irreversible ‘reordering’ of hypothalamic neural networks in fetal/neonatal life, permanently alter energy balance and lead to obesity with associated insulin resistance.     

Maternal Height and Preterm Birth: A Study on 192,432 Swedish Women

BackgroundThere is increasing evidence that lower maternal stature is associated with shorter gestational length in the offspring. We examined the association between maternal height and the likelihood of delivering preterm babies in a large and homogeneous cohort of Swedish women.MethodsThis study covers antenatal data from the Swedish Medical Birth Register on 192,432 women (aged 26.0 years on average) born at term, from singleton pregnancies, and of Nordic ethnicity. Continuous associations between women''s heights and the likelihood of preterm birth in the offspring were evaluated. Stratified analyses were also carried out, separating women into different height categories.ResultsEvery cm decrease in maternal stature was associated with 0.2 days shortening of gestational age in the offspring (p<0.0001) and increasing odds of having a child born preterm (OR 1.03), very preterm (OR 1.03), or extremely preterm (OR 1.04). Besides, odds of all categories of preterm birth were highest among the shortest women but lowest among the tallest mothers. Specifically, women of short stature (≤155 cm or ≤-2.0 SDS below the population mean) had greater odds of having preterm (OR 1.65) or very preterm (OR 1.47) infants than women of average stature (-0.5 to 0.5 SDS). When compared to women of tall stature (≥179 cm), mothers of short stature had even greater odds of giving birth to preterm (OR 2.07) or very preterm (OR 2.16) infants.ConclusionsAmong Swedish women, decreasing height was associated with a progressive increase in the odds of having an infant born preterm. Maternal short stature is a likely contributing factor to idiopathic preterm births worldwide, possibly due to maternal anatomical constraints.     

Maternal predator-exposure has lifelong consequences for offspring learning in threespined sticklebacks

Learning is an important form of phenotypic plasticity that allows organisms to adjust their behaviour to the environment. An individual''s learning performance can be affected by its mother''s environment. For example, mothers exposed to stressors, such as restraint and forced swimming, often produce offspring with impaired learning performance. However, it is unclear whether there are maternal effects on offspring learning when mothers are exposed to ecologically relevant stressors, such as predation risk. Here, we examined whether maternal predator-exposure affects adult offsprings’ learning of a discrimination task in threespined sticklebacks (Gasterosteus aculeatus). Mothers were either repeatedly chased by a model predator (predator-exposed) or not (unexposed) while producing eggs. Performance of adult offspring from predator-exposed and unexposed mothers was assessed in a discrimination task that paired a particular coloured chamber with a food reward. Following training, all offspring learned the colour-association, but offspring of predator-exposed mothers located the food reward more slowly than offspring of unexposed mothers. This pattern was not driven by initial differences in exploratory behaviour. These results demonstrate that an ecologically relevant stressor (predation risk) can induce maternal effects on offspring learning, and perhaps behavioural plasticity more generally, that last into adulthood.     

Maternal exposure to predation risk decreases offspring antipredator behaviour and survival in threespined stickleback

1. Adaptive maternal programming occurs when mothers alter their offspring's phenotype in response to environmental information such that it improves offspring fitness. When a mother's environment is predictive of the conditions her offspring are likely to encounter, such transgenerational plasticity enables offspring to be better-prepared for this particular environment. However, maternal effects can also have deleterious effects on fitness.2. Here, we test whether female threespined stickleback fish exposed to predation risk adaptively prepare their offspring to cope with predators. We either exposed gravid females to a model predator or not, and compared their offspring's antipredator behaviour and survival when alone with a live predator. Importantly, we measured offspring behaviour and survival in the face of the same type of predator that threatened their mothers (Northern pike).3. We did not find evidence for adaptive maternal programming; offspring of predator-exposed mothers were less likely to orient to the predator than offspring from unexposed mothers. In our predation assay, orienting to the predator was an effective antipredator behaviour and those that oriented, survived for longer.4. In addition, offspring from predator-exposed mothers were caught more quickly by the predator on average than offspring from unexposed mothers. The difference in antipredator behaviour between the maternal predator-exposure treatments offers a potential behavioural mechanism contributing to the difference in survival between maternal treatments.5. However, the strength and direction of the maternal effect on offspring survival depended on offspring size. Specifically, the larger the offspring from predator-exposed mothers, the more vulnerable they were to predation compared to offspring from unexposed mothers.6. Our results suggest that the predation risk perceived by mothers can have long-term behavioural and fitness consequences for offspring in response to the same predator. These stress-mediated maternal effects can have nonadaptive consequences for offspring when they find themselves alone with a predator. In addition, complex interactions between such maternal effects and offspring traits such as size can influence our conclusions about the adaptive nature of maternal effects.     

Predictors of trisomy 21 in the offspring of older and younger women

BACKGROUND: Advanced maternal age is the only well‐established risk factor for trisomy 21, yet the majority of affected individuals are born to younger women. To identify factors associated with the risk of trisomy 21 in the offspring of younger and older women, we analyzed data for cases with trisomy 21 from the Texas Birth Defects Registry for 1999 to 2007. METHODS: Data were analyzed separately for younger (i.e., <35 years of age at delivery; n = 2306) and older (i.e., ≥35 years of age at delivery; n = 1811) women using Poisson regression. RESULTS: After adjustment for maternal age and several other covariates, the prevalence of trisomy 21 in the offspring of women in both maternal age groups was higher in male than in female infants and in offspring of women who were Hispanic (compared with non‐Hispanic white women) or who had at least one previous liveborn child compared to those with none. In the offspring of older women only, the prevalence of trisomy 21 was also significantly higher when the father was 20to 24 years old (compared with 25 to 29 years old; adjusted prevalence ratio [aPR], 2.27; 95% confidence interval [CI], 1.47–3.49) and Hispanic (compared with non‐Hispanic white; aPR, 1.34; 95% CI, 1.13–1.58) and among women with less than a high school education (compared with greater than high school). CONCLUSIONS: This study identified several factors, in addition to maternal age, that were associated with trisomy 21 risk. In general, these factors were similar for both maternal age groups, although paternal characteristics were significantly associated with risk of trisomy 21 only in offspring of older women. Birth Defects Research (Part A), 2012. © 2011 Wiley Periodicals, Inc.     

CTG instability in myotonic dystrophy: molecular genetic analysis of families from south-eastern France with characteristics of intergenerational variation in CGT repeat numbers.

We report clinical, genetical and genealogical findings in 149 French families from the Rh?ne-Alpes area studied over a 5-year period. There was a significant excess of DM females compared to DM males with (CTG) repeat sizes between 1-2 kb. The mean maternal (CTG) repeat size was higher than paternal repeat size. Anticipation phenomenom was significantly higher after maternal than after paternal transmission. A significant correlation between parental (CTG) repeat size and intergenerational variation both in paternal and maternal transmissions was observed. The anticipation phenomenom was more important for sons than daughters particularly after maternal transmission. The mean (CTG) repeat size in mothers of CDM cases was about twice that of mothers of NCDM children. The risk of giving birth to a CDM child increased considerably when the number of maternal (CTG) repeats was over 300 (CTG). A significant excess of DM females was observed. They had on average 24% fewer children than male patients. Paternal transmission (63.6%) of DM occurred more frequently than maternal transmission (52.7%).     

Inherited ring chromosomes: an analysis of published cases

Summary A review of case reports on patients with ring chromosome revealed 30 individuals (plus two fetuses) who inherited the ring from a total of 23 carrier parents (21 mothers and 2 fathers). The proportion of cases with inherited rings, among all patients with a ring, was calculated to be 5.6% as an upper limit. However, because of a propable difference in survival and fertility between individuals with transmitted and de novo rings, and because of the preferential publication of cases involving inherited rings (and thus a publication bias), the proportion of inherited rings should in reality be no more than 1%. Out of 30 transmitted rings, there were 9 where parent and child were both mosaics, suggesting an inherited instability of the chromosome involved leading to de novo re-formation of the ring in the second generation. The relatively mild clinical manifestations of ring chromosomes, in general, was found to be even more striking in familial cases. In half of the offspring the phenotype was very similar to that of the parent. However, in about a third of cases the offspring were more severely (mentally) affected. This fact should be considered in genetic counseling of clinically normal women who carry a ring chromosome.     

Clients'' interpretation of risks provided in genetic counseling.

Clients in 544 genetic counseling sessions who were given numeric risks of having a child with a birth defect between 0% and 50% were asked to interpret these numeric risks on a five-point scale, ranging from very low to very high. Whereas clients' modal interpretation varied directly with numeric risks between 0% and 15%, the modal category of client risk interpretation remained "moderate" at risks between 15% and 50%. Uncertainty about normalcy of the next child increased as numeric risk increased, and few clients were willing to indicate that the child would probably or definitely be affected regardless of the numeric risk. Characteristics associated with clients' "pessimistic" interpretations of risk, identified by stepwise linear regression, included increased numeric risk, discussion in depth during the counseling session of whether they would have a child, have a living affected child, discussion of the effects of an affected child on relationships with client's other children, and seriousness of the disorder in question (causes intellectual impairment). Client interpretations are discussed in terms of recent developments in cognitive theory, including heuristics that influence judgments about risks, and implications for genetic counseling.     

Size of the unstable CTG repeat sequence in relation to phenotype and parental transmission in myotonic dystrophy.

A clinical and molecular analysis of 439 individuals affected with myotonic dystrophy, from 101 kindreds, has shown that the size of the unstable CTG repeat detected in nearly all cases of myotonic dystrophy is related both to age at onset of the disorder and to the severity of the phenotype. The largest repeat sizes (1.5-6.0 kb) are seen in patients with congenital myotonic dystrophy, while the minimally affected patients have repeat sizes of < 0.5 kb. Comparison of parent-child pairs has shown that most offspring have an earlier age at onset and a larger repeat size than their parents, with only 4 of 182 showing a definite decrease in repeat size, accompanied by a later age at onset or less severe phenotype. Increase in repeat size from parent to child is similar for both paternal and maternal transmissions when the increase is expressed as a proportion of the parental repeat size. Analysis of congenitally affected cases shows not only that they have, on average, the largest repeat sizes but also that their mothers have larger mean repeat sizes, supporting previous suggestions that a maternal effect is involved in the pathogenesis of this form of the disorder.     

Orofacial clefts, parental cigarette smoking, and transforming growth factor-alpha gene variants.

Results of studies to determine whether women who smoke during early pregnancy are at increased risk of delivering infants with orofacial clefts have been mixed, and recently a gene-environment interaction between maternal smoking, transforming growth factor-alpha (TGFa), and clefting has been reported. Using a large population-based case-control study, we investigated whether parental periconceptional cigarette smoking was associated with an increased risk for having offspring with orofacial clefts. We also investigated the influence of genetic variation of the TGFa locus on the relation between smoking and clefting. Parental smoking information was obtained from telephone interviews with mothers of 731 (84.7% of eligible) orofacial cleft case infants and with mothers of 734 (78.2%) nonmalformed control infants. DNA was obtained from newborn screening blood spots and genotyped for the allelic variants of TGFa. We found that risks associated with maternal smoking were most elevated for isolated cleft lip with or without cleft palate, (odds ratio 2.1 [95% confidence interval 1.3-3.6]) and for isolated cleft palate (odds ratio 2.2 [1.1-4.5]) when mothers smoked > or =20 cigarettes/d. Analyses controlling for the potential influence of other variables did not reveal substantially different results. Clefting risks were even greater for infants with the TGFa allele previously associated with clefting whose mothers smoked > or =20 cigarettes/d. These risks for white infants ranged from 3-fold to 11-fold across phenotypic groups. Paternal smoking was not associated with clefting among the offspring of nonsmoking mothers, and passive smoke exposures were associated with at most slightly increased risks. This study offers evidence that the risk for orofacial clefting in infants may be influenced by maternal smoke exposures alone as well as in combination (gene-environment interaction) with the presence of the uncommon TGFa allele.     

Patterns of maternal transmission in bipolar affective disorder.

The mode of inheritance of bipolar affective disorder (BPAD) appears complex, and non-Mendelian models of inheritance have been postulated. Two non-Mendelian phenomena, genomic imprinting and mitochondrial inheritance, may contribute to the complex inheritance pattern seen in BPAD. Both imprinting and mitochondrial inheritance share the feature of differential expression of the phenotype, depending on the parent of origin. In this study we tested the hypothesis of a parent-of-origin effect on the transmission of BPAD. We examined the frequency and risk of affective disorder among relatives in a sample of 31 families ascertained through treated probands with BPAD and selected for the presence of affected phenotypes in only one parental lineage. Three specific comparisons were performed: (1) the observed frequency of transmitting mothers versus transmitting fathers; (2) the observed frequency and lifetime risk of BPAD among the maternal versus the paternal relatives of probands; and (3) the observed frequency and lifetime risk of BPAD for the offspring of affected mothers compared with the offspring of affected fathers. We observed a higher than expected frequency of affected mothers (P < .04), a 2.3-2.8-fold increased risk of illness for maternal relatives (P < .006), and a 1.3- 2.5-fold increased risk of illness for the offspring of affected mothers (P < .017).In seven enlarged pedigrees, fathers repeatedly failed to transmit the affected phenotype to daughters or sons. Taken together, these findings indicate a maternal effect in the transmission of BPAD susceptibility and suggest that molecular studies of mtDNA and imprinted DNA are warranted in patients with BPAD.     

Socio-Cultural Disparities in GDM Burden Differ by Maternal Age at First Delivery

AimsSeveral socio-cultural and biomedical risk factors for gestational diabetes mellitus (GDM) are modifiable. However, few studies globally have examined socio-cultural associations. To eliminate confounding of increased risk of diabetes in subsequent pregnancies, elucidating socio-cultural associations requires examination only of first pregnancies.MethodsData for all women who delivered their first child in Victoria, Australia between 1999 and 2008 were extracted from the Victorian Perinatal Data Collection. Crude and adjusted GDM rates were calculated. Multivariate logistic regression was used to examine odds of GDM within and between socio-cultural groups.ResultsFrom 1999 to 2008, 269,682 women delivered their first child in Victoria. GDM complicated 11,763 (4.4%) pregnancies and burden increased with maternal age, from 2.1% among women aged below 25 years at delivery to 7.0% among those aged 35 years or more. Among younger women, GDM rates were relatively stable across socioeconomic levels. Amongst older women GDM rates were highest in those living in most deprived areas, with a strong social gradient. Asian-born mothers had highest GDM rates. All migrant groups except women born in North-West Europe had higher odds of GDM than Australian-born non-Indigenous women. In all ethnic groups, these differences were not pronounced among younger mothers, but became increasingly apparent amongst older women.ConclusionsSocio-cultural disparities in GDM burden differ by maternal age at first delivery. Socio-cultural gradients were not evident among younger women. Health and social programs should seek to reduce the risk amongst all older women to that of the least deprived older mothers.     

Maternal smoking during pregnancy predicts adult offspring cardiovascular risk factors - evidence from a community-based large birth cohort study

Background

Maternal smoking during pregnancy is associated with offspring obesity. However, little is known about whether maternal smoking in pregnancy predicts other offspring cardiovascular risk factors including waist circumference (WC), waist-hip-ratio (WHR), pulse rate (PR), systolic (SBP), and diastolic blood pressure (DBP).

Methods

We studied a sub-sample of 2038 (50% males) young adults who were born in Brisbane, Australia to investigate the prospective association of maternal smoking during pregnancy with young adult cardiovascular risk factors. We compared offspring mean BMI, WC, WHR, SBP, DBP and PR and the risk of being overweight and obese at 21 years by three mutually exclusive categories of maternal smoking status defined as never smoked, smoked before and/or after pregnancy but not in pregnancy or smoked during pregnancy and other times.

Results

Offspring of mothers who smoked during pregnancy had greater mean BMI, WC, WHR and PR and they were at greater risk of being obese at 21 years compared to offspring of those mothers who never smoked. The mean of these risk factors among those adult offspring whose mothers stopped smoking during pregnancy, but who then smoked at other times in the child''s life, were similar to those mothers who never smoked. These results were independent of a range of potential confounding factors.

Conclusion

The findings of this study suggest a prospective association of maternal smoking during pregnancy and offspring obesity as well as PR in adulthood, and reinforce the need to persuade pregnant women not to smoke.     

Folate status and neural tube defects

Periconceptional folic acid supplementation prevents approximately 70% of neural tube defects (NTDs). While most women carrying affected fetuses do not have deficient blood folate levels, the risk of having an NTD affected child is inversely correlated with pregnancy red cell folate levels. Current research is focused on the discovery of genetic abnormalities in folate related enzymes which might explain the role of folate in NTD prevention. The first candidate gene to emerge was the C677T variant of 5,10-methylenetetrahydrofolate reductase. Normal subjects who are homozygous for the mutation (TT) have red cell folate status some 20% lower than expected. It is now established that the prevalence of the TT genotype is significantly higher among spina bifida cases and their parents. Nevertheless, our studies show that the variant does not account for the reduced blood folate levels in many NTD affected mothers. We conclude that low maternal folate status may in itself be the most important risk factor for NTDs and that food fortification may be the only population strategy of benefit in the effort to eliminate NTDs.