Reproductive Factors, Subfertility, and Risk of Neural Tube Defects: A Case-Control Study Based on the Oxford Record Linkage Study Register
David Whiteman1,
Michael Murphy1,
Kate Hey1,
Maeve O'Donnell2 and
Michael Goldacre2
1 Imperial Cancer Research Fund General Practice Research Group, Institute of Health Sciences, University of Oxford, Oxford, United Kingdom.
2 Unit of Health Care Epidemiology, Institute of Health Sciences, University of Oxford, Oxford, United Kingdom.
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ABSTRACT
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Periconceptual exposure to subfertility treatments is increasingly common, raising concerns about the possibility of malformations in the offspring. The authors conducted a case-control study to determine whether subfertility or its treatment was associated with increased risk of neural tube defects (NTDs). Cases were 694 women diagnosed with an NTD-affected pregnancy in Oxfordshire or West Berkshire, England, between 1970 and 1987. Cases were individually matched on maternal year of birth and year of index pregnancy to controls randomly selected from a computerized database. Data on demographic, reproductive, and obstetric factors were abstracted from patient hospital records. Overall, the period prevalences of subfertility and of subfertility treatment were 7% and 3%, respectively. No evidence was found that the risk of NTD-affected pregnancies was increased by either subfertility (odds ratio (OR) = 1.2, 95% confidence interval (CI): 0.7, 2.1) or its treatment (OR = 0.9, 95% CI: 0.4, 2.0). After adjustment, NTD-affected pregnancies were associated with female offspring (OR = 2.3, 95% CI: 1.8, 3.1), multiple birth (OR = 4.8, 95% CI: 1.2, 18.8), and higher numbers of pregnancies (p for trend = 0.005). The findings from this large, population-based study were wholly consistent with those from smaller studies that found no increased risk of NTD associated with exposure to fertility treatments but reported associations with various pregnancy outcomes. Am J Epidemiol 2000;152:8238.
case-control studies; fertility agents; infertility; medical record linkage; neural tube defects; pregnancy
Abbreviations:
CI, confidence interval; NTD, neural tube defect; OR, odds ratio.
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INTRODUCTION
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Neural tube defects (NTDs) are congenital malformations resulting from complete or partial failure of the neural tube to close in the developing embryo. There is now strong epidemiologic evidence that a relative folate deficiency in the periconceptual period is causally related to many cases of NTD (1
). Maternal and fetal genotypes may also alter risk (2
), and there may be a teratogenic role for other agents during embryogenesis. Since the 1970s, there has been an escalation in use of hormonal therapies to assist conception in subfertile women, raising concerns regarding possible risks associated with these treatments (3
). Case reports of NTDs arising in pregnancies conceived following treatment with clomiphene citrate have lent anecdotal support to the possibility that these agents may cause iatrogenic malformations of the neural tube (4
, 5
).
We used register data compiled from population-based sources spanning three decades to undertake a case-control study in central England. We investigated the possible association between treatment for subfertility, various reproductive factors, and occurrence of NTD in offspring.
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MATERIALS AND METHODS
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Subjects
We ascertained cases from a comprehensive, local NTD register, the construction of which has been described previously (6
). Eligible cases were defined as women whose pregnancies were affected by one or more of the following: anencephaly, encephalocele, spina bifida aperta, or spina bifida occulta; these NTDs occurred alone or in combination with other defects in a liveborn or stillborn child, late miscarriage, or a terminated pregnancy and were diagnosed in Oxfordshire or West Berkshire, England, between 1970 and 1987 inclusive. NTD cases were identified from three principal sources: the Oxford Record Linkage Study (6
), the local
-fetoprotein screening program, and the abortions/congenital malformations data set held by the former Office of Population Censuses and Surveys (now the Office for National Statistics). Further cases were ascertained from pediatric surgery unit records, perinatal pathology reports, the regional genetics unit, and home birth and delivery suite registers. Capture-recapture estimates have suggested a high degree of completeness (>90 percent) of the register as a sampling frame for cases (7
). In all, 694 index pregnancies met the eligibility criteria.
For each woman with an NTD-affected pregnancy, we randomly selected a woman from the Oxford Record Linkage Study database, matched on exact maternal age and year of NTD event. Eligible were women whose pregnancies were listed in the "maternity" file as a delivery (liveborn or stillborn) or women listed in the "general hospital" file as having pregnancies with an abortive outcome. Women whose pregnancies had terminated were excluded from the control group. In every instance in which a case's pregnancy was terminated following diagnosis of NTD, the matched control's fetus was required to be at least of the same gestational age.
Exposure information
An experienced research nurse used a standardized form to abstract data from the clinical case notes of all subjects. Both general hospital and maternity records were reviewed, including notes from inpatient admissions, outpatient attendances at specialist clinics, and referral letters from general practitioners. "Blinding" of the research nurse was not attempted. Background information pertaining to occupation, marital status, and smoking history was collected, and social class was assigned according to Office of Population Censuses and Surveys occupational classifications (8
). For the index pregnancy and all previous pregnancies, information was recorded on the number of infants delivered and their gender, weight, gestation, coincident diagnoses, and outcome (liveborn, stillborn, spontaneous abortion/miscarriage, or termination). We adopted the same definition of subfertility as used by local fertility specialists during the period of the study, which determined eligibility for investigation and treatment: repeated failure to conceive over 12 months for women with normal menstrual cycles or 6 months for women with ovarian failure as defined by amenorrhea or oligomenorrhea lasting at least 6 months in the absence of pregnancy. A separate abstraction form was used for patients with a history of subfertility, on which timing, treatment, and etiology details were recorded. As a further check, we also matched participants against the nominal roll of a cohort of women who gave birth locally after experiencing an episode of subfertility (9
).
Statistical analysis
In matched case-control analyses, we calculated the odds ratio and 95 percent confidence interval of conceiving an NTD-affected pregnancy associated with various exposures. We separately calculated odds ratios for lesions of the rostral and caudal neural tube but excluded the very few pregnancies affected only with spina bifida occulta (n = 26, 3.7 percent). For factors relating to conception (such as treatment for subfertility), we distinguished between exposures during the index and previous pregnancies and adopted as a common reference group those who had never been exposed to the factor under consideration. Adjustment was undertaken by including terms for potentially confounding variables in a conditional logistic regression model (10
). To test for linear trend, ordinal categorical variables were included in the model as continuous data, and the score test was used as an approximation of the Mantel extension chi-square with 1 degree of freedom (11
). All analyses were conducted by using SAS software, version 6.12 (12
).
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RESULTS
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Description of cases
Characteristics of the women who had NTD-affected pregnancies are shown in table 1. The overall prevalence of rostral lesions of the neural tube was 49.2 percent; the remainder were caudal lesions of varying severity. Of the 694 NTD pregnancies, 145 (20.9 percent) occurred with at least one other recorded diagnosis, and 115 (16.6 percent) were associated with other congenital malformations (table 1). Nearly 40 percent of index pregnancies resulted in liveborn infants, while approximately 30 percent of pregnancies were stillborn and 30 percent were terminated. In this series, the outcome of an NTD-affected pregnancy was strongly determined by the year of conception; the proportion of births (live- and stillbirths) declined from 97 to 25 percent during the study period, with a corresponding rise in the proportion of terminated pregnancies. NTD offspring were similarly prevalent among women in social class III (odds ratio (OR) = 1.1, 95 percent confidence interval (CI): 0.8, 1.5) and social classes IV and V (OR = 1.0, 95 percent CI: 0.7, 1.4) compared with women in social classes I and II.
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TABLE 1. Characteristics of women whose pregnancies were affected by neural tube defects, Oxford, England, 19701987
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Fertility factors
Approximately 7 percent of the women in this sample had a documented history of subfertility, and this finding was the same for cases and controls both for the index pregnancy and all previous pregnancies (table 2). There was no difference regarding history of subfertility treatment among cases and controls, with approximately 3 percent of women having a record of receiving treatment. The most common subfertility treatments were clomiphene citrate (2.9 percent) and human chorionic gonadotropin (0.9 percent). Other treatments, including cyclofenil, human menopausal gonadotropin, bromocriptine, and tamoxifen, were recorded for fewer than 0.5 percent of the subjects. No evidence was found that women with NTD-affected pregnancies had different exposures to any specific subfertility treatments than women with non-NTD pregnancies. The risk estimates were not meaningfully altered after adjustment for potentially confounding factors such as poor reproductive history (not shown).
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TABLE 2. Distribution of subfertility factors and associated odds ratios* of pregnancy affected by neural tube defects for women in Oxford, England, 19701987
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Reproductive factors
Female offspring were more than twice as likely as males to harbor an NTD (table 3). Multiple birth in the index pregnancy was also strongly associated with risk of NTD, although the number of such pregnancies was small. Modest associations were observed between the outcome of the preceding pregnancy and risk of NTD in the index pregnancy. Compared with those who delivered a livebirth in the preceding pregnancy, those who had a stillbirth delivery or a miscarriage had a 1.2-fold and 1.5-fold increased risk, respectively, of an NTD-affected pregnancy. Age at first birth was associated with risk of NTD, although the relation was not linear. Compared with women who first gave birth before age 20 years, women in their late twenties and early thirties had a decreased risk, whereas women aged more than 35 years had a substantially increased risk of having an NTD-affected pregnancy.
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TABLE 3. Distribution of reproductive factors and associated odds ratios* of pregnancy affected by neural tube defects for women in Oxford, England, 19701987
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We observed a trend of increasing risk of NTD with increasing number of pregnancies (p for trend = 0.005); for women with three or more previous pregnancies, the risk of NTD was significantly elevated 1.7 times over that for women with no previous pregnancies (table 3). A history of ever having a miscarriage was associated with an increased risk of NTD (OR = 1.5, 95 percent CI: 1.1, 2.1), and higher point estimates were observed for more frequent occurrences. These associations persisted when analyses were restricted to cases whose offspring had no other congenital malformations (n = 579) (not shown).
We conducted multivariable conditional logistic regression analyses to adjust for possible confounding, since the reproductive factors under investigation were often associated with each other as well as with NTD. Gender of the offspring, multiple birth in the index pregnancy, number of prior pregnancies, and age at first pregnancy all retained their association with NTD after we included all terms in the regression model (table 4).
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TABLE 4. Adjusted odds ratios* of pregnancy affected by neural tube defects associated with reproductive factors for women in Oxford, England, 19701987
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DISCUSSION
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This study found that while some reproductive factors were associated with the risk of NTD, neither the existence of nor the treatment for subfertility appeared to be a risk factor for NTD. We undertook analyses in which affected offspring were categorized as spinal and anencephalic types and as those with "pure" NTD and other associated malformations, but there was no evidence that women whose pregnancies were affected by these outcomes had greater exposure to any subfertility treatments, and clomiphene citrate in particular, than matched controls did.
A limitation of this study was the small number of women who had subfertility exposures; fewer than 7 percent of the participants had a documented history of subfertility, and fewer than 3 percent had been treated for subfertility. Nevertheless, this study had 80 percent power to detect an odds ratio of 2 or more at an
level of 5 percent, and the prevalence of subfertility was similar to that for other populations (13
). Moreover, our finding of no association between subfertility treatments and risk of NTD generally agrees with published data from other North American and European studies (13
15
).
In this United Kingdom population, NTD-affected pregnancies were most strongly associated with female offspring and with multiple birth in the index pregnancy. Our estimate of a more than twofold increased risk of NTDs among female offspring accords with other studies from the British Isles (16
), although it may be somewhat inflated owing to the high sex ratio of controls. Several explanations have been proposed for the preponderance of affected female infants, ranging from anomalous X-chromosome inactivation (17
) to higher abortion frequencies for affected male fetuses with mutations of the 5,10-methylene tetrahydrofolate reductase gene (18
). While we have no genotypic data to confirm or refute these hypotheses, our observed gender ratio is at least consistent with such speculations. We also found that women with NTD-affected pregnancies were significantly more likely than controls to have suffered a miscarriage in the preceding pregnancy. Similar findings have been reported by other (19
21
) but not all (13
) investigators. Importantly, this effect was observed among those offspring who had no other congenital malformations, suggesting that it is not simply due to an association with hereditary NTD syndromes. We found no association with social class, which may reflect the sharp declines in NTD rates in the United Kingdom during the study period, although misclassification cannot be excluded in our data.
The greatest threat to the validity of this study stems from possible sources of bias in abstracting data from patient records. For example, the extent of missing information for some potentially confounding exposures (such as maternal ethnicity and smoking history (22
)) rendered them unsuitable for analysis. However, ascertainment of key reproductive exposures was almost complete, and the broadly similar rates of missing data for cases and controls across a range of demographic variables provide some evidence that information quality was unbiased with respect to outcome. It is possible that abstraction of exposure information from the case notes was systematically different for cases and controls, since the abstractor was not blind to case-control status, although we consider this possibility unlikely. Previous studies have also been criticized for having inadequate control groups, but selection bias should not have affected this study because subject ascertainment was population based.
In conclusion, we found no evidence that a history of subfertility, nor its successful treatment, contributed to an increased risk of NTD. However, we did find that women who had poor prior reproductive outcomes, female offspring, or multiple births in the index pregnancy were at higher risk of such anomalies. Whether these effects are independent of maternal or fetal folate metabolism is not clear (2
), but they might usefully be considered further.
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ACKNOWLEDGMENTS
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This study was supported by grants from the Association for Spina Bifida and Hydrocephalus (ASBAH) and the Oxford University Medical School Medical Research Fund. Dr. Whiteman was supported by a Nuffield Medical Research Fellowship from the University of Oxford.
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NOTES
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Correspondence to Dr. Michael Murphy, ICRF General Practice Research Group, Institute of Health Sciences, University of Oxford, Old Road, Headington, Oxford OX3 7LF, United Kingdom (e-mail: michael.murphy{at}dphpc.ox.ac.uk).
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Received for publication August 26, 1999.
Accepted for publication January 14, 2000.