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Prepregnancy Obesity and Fetal Death: A Study Within the Danish National Birth Cohort

Nohr, Ellen Aagaard MHS; Bech, Bodil Hammer MD; Davies, Michael John PhD; Frydenberg, Morten PhD; Henriksen, Tine Brink PhD; Olsen, Jorn PhD

doi: 10.1097/01.AOG.0000172422.81496.57
Original Research

Objective: To examine the association between high prepregnancy body mass index and fetal death, allowing for the effects of gestational age, weight gain, and maternal diseases in pregnancy.

Methods: Prepregnancy body mass index (BMI) and fetal death were examined in the Danish National Birth Cohort among 54,505 pregnant women who participated in a comprehensive interview during the second trimester. Pregnancy outcomes were obtained from registers and medical records. Cox regression analyses with delayed entry and time-dependent covariates were used to estimate the risk of fetal death.

Results: Compared with normal-weight women (18.5 ≤ BMI < 25), the risks of fetal death among obese women (BMI ≥ 30), expressed as adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) were as follows: before week 14: 0.8 (0.5–1.4), weeks 14–19: 1.6 (1.0–2.5), weeks 20–27: 1.9 (1.1–3.3), weeks 28–36: 2.1 (1.0–4.4), weeks 37–39: 3.5 (1.9–6.4), and weeks 40+: 4.6 (1.6–13.4). Overweight women (25 ≤ BMI < 30) also experienced a higher risk after 28 weeks, and especially after 40 weeks of gestation (HR 2.9, 95% CI 1.1–7.7). Analysis of stillbirth (fetal death at 28+ completed weeks of gestation) indicated that the effects were not due to obesity-related diseases in pregnancy, nor was weight gain associated with stillbirth. The increased risk of stillbirth among overweight and obese women was partly attributable to inadequate placental function (crude odds ratios 2.1, 95% CI 1.0–4.4, and 5.2, 95% CI 2.5–10.9, respectively).

Conclusion: Prepregnancy obesity was associated with an increasing excess risk of fetal death with advancing gestation, and placental dysfunction may be a possible contributing factor.

Level of Evidence: II-2

Prepregnancy obesity is associated with an increasing excess risk of fetal death with advancing gestation.

From the Danish Epidemiology Science Centre, Institute of Public Health, University of Aarhus, Aarhus, Denmark; Department of Obstetrics and Gynaecology, University of Adelaide, Queen Elizabeth Hospital, Woodville, Australia; Department of Biostatistics, Institute of Public Health, and Perinatal Epidemiology Research Unit, University of Aarhus, Aarhus, Denmark; and Department of Epidemiology, School of Public Health, University of California, Los Angeles, California.

Ellen Aagaard Nohr is supported by a grant from the Health Insurance Foundation (No. 2002B020). The Danish National Research Foundation has established the Danish Epidemiology Science Centre, which initiated and created the Danish National Birth Cohort. The cohort is furthermore a result of a major grant from this Foundation. Additional support for the Danish National Birth Cohort is obtained from the Pharmacy Foundation, the Egmont Foundation, the March of Dimes Birth Defects Foundation, and the Augustinus Foundation.

Corresponding author: Ellen Aagaard Nohr, The Danish Epidemiology Science Centre, Institute of Public Health, Vennelyst Boulevard 6, Building 260, University of Aarhus, DK-8000 Aarhus, Denmark; e-mail: ean@soci.au.dk.

The obesity epidemic affects childbearing women in many countries. In the United States, 54% of women aged 20–39 are either overweight or obese,1 and Europe seems to follow a similar pattern, albeit with some delay.2

Prepregnancy obesity, defined as a body mass index (BMI) of 30 or above, has been associated with several severe pregnancy and delivery complications, including stillbirth.3–6 Stillbirth remains a serious reproductive failure, with a frequency of 2–5 per 1,000 births, and constitutes more than half of all perinatal deaths.7,8

Biological pathways for the excess risk are conjectural, and it is not possible to disentangle the intrinsic effect of body fat from the effect of obesity as a metabolic disorder. Diseases associated with obesity in pregnancy that may mediate an association with stillbirth are gestational diabetes, hypertension, and preeclampsia. However, the metabolic and vascular abnormalities of these conditions can be found in obese pregnant women without a clinical disease.9,10

Both obesity and stillbirth have been associated with prolonged pregnancy,11–13 but little is known about the risk of stillbirth in obese women with 40 completed weeks of gestation recorded. The association between obesity and spontaneous abortions has shown inconsistent results,14,15 and little is known about the temporal pattern of the association between BMI and fetal death.

We therefore conducted a large prospective cohort study to examine the association between high prepregnancy body mass index and fetal death while considering the effects of gestational age, weight gain, and maternal diseases in pregnancy.

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MATERIALS AND METHODS

Data were obtained from the Danish National Birth Cohort, which is a nationwide study of pregnant women and their offspring.16 The cohort included women who intended to carry the pregnancy to term and spoke Danish well enough to participate in 4 telephone interviews during pregnancy and early motherhood. Approximately 60% of all general practitioners in Denmark chose to take part in the recruitment. Pregnant women were approached at the first antenatal care visit, which usually takes place in weeks 6–10 of gestation, and approximately 60% of the eligible women accepted the invitation and signed an informed consent form. Computer-assisted telephone interviews were conducted by trained female interviewers.

Of all pregnant women recruited to the Danish National Birth Cohort from May 1, 1998, to April 30, 2001, (n = 63,908), we included women who had participated in the first telephone interview (n = 55,432), which took place at approximately 16 weeks of gestation (interquartile range 13–19 weeks). The main exposure variable was prepregnancy BMI (weight in kilograms/height in meters squared), based on self-reported information on prepregnancy weight and height from the first telephone interview. Body mass index was categorized according to the definitions of the World Health Organization as underweight (BMI < 18.5), normal weight (18.5 ≤ BMI < 25), overweight (25 ≤ BMI < 30), and obese (BMI 30+).2 Women with missing information on BMI were excluded from the study (n = 886, 1.6%).

The Danish person identification system was used to link the Civil Registration Register and the Birth Register to identify both live births and stillbirths. The National Discharge Register was used to identify spontaneous abortions, induced abortions, ectopic pregnancies, and hydatidiform mole. In less than 1% of the pregnancies, we used information on outcome from the pregnancy interviews because of inconsistencies in the National Discharge Register or because the outcome of pregnancy was not otherwise available. We were able to retrieve outcome information on all pregnancies in the study.

The definition of a stillbirth was a fetal death with a gestational age of 28 completed weeks or more. Medical records of all stillbirths were collected nationwide to categorize cause of death according to the classification of Andersen et al.17 This classification is a hybrid of 2 British categorizations, focusing on obstetric factors18 and fetal and neonatal factors.19 Thus, Andersen's category “intrapartum events” includes both deaths from mechanical causes during labor and unexplained intrapartum asphyxia. Because we wanted to study stillbirth related to placental causes, we decided a priori to modify Andersen's category “feto-placental dysfunction,” which also includes umbilical cord complications. These cases were placed in a separate category, whereas “fetal growth restriction” (FGR), “infarction of the placenta without FGR,” and “abruptio placenta” were combined into a single category, “placental dysfunction.” Fetal growth restriction was defined as birth weight more than 2 standard deviations below the mean for gestational age on Marsal et al's reference curve.20 All records were coded independently and blindly by 2 of the authors (E.A.N. and B.H.B.).

Gestational age as recorded in the National Discharge Register was used in the main analyses. It is a clinical evaluation of gestational age at time of birth or pregnancy termination and is widely based on ultrasound measures. We excluded women with missing information on gestational age (n = 30), and women who participated in the first interview after termination of pregnancy, or at an estimated gestational age less than 28 days (n = 11).

Weight gain was defined as the average weekly increase between the self-reported weights in the first and second pregnancy interviews for the women who provided a first interview between 9 and 24 completed weeks of gestation, those who provided a second interview between 26 and 38 completed weeks of gestation, and those who had at least 6 weeks between the 2 interviews.

Maternal characteristics and life style exposures through the first part of pregnancy included coffee consumption, smoking, alcohol intake, physical exercise, and socio-occupational status. Physical exercise was measured in minutes per week by combining all kind of reported exercise. Socio-occupational status was based on the current or most recent job within 6 months, or, if the woman was attending school, on the type of education. The category “high” included women in management or in jobs requiring higher education, generally more than 4 years beyond high school. Office workers, service workers, skilled manual workers, and women in the military constituted the “middle” category, whereas unskilled workers and unemployed women were classified in the “low” category. We were not able to categorize 2,357 women (4.3%) who were then categorized according to their husband's socio-occupational status. Reproductive history included numbers and types of previous pregnancies, infertility treatment, and waiting time to pregnancy. The categorization of these variables is displayed in Table 1.

Table 1

Table 1

Table 1

Table 1

Obesity-related diseases in pregnancy were identified in the National Discharge Register. Preeclampsia included eclampsia and was defined from the presence of the diagnoses O14 or O15 (International Classification of Diseases, 10th Revision). The codes I10 through I15 and O10, O11, and O13 were used to identify chronic and gestational hypertension. Diabetes included both pregestational and gestational diabetes and was identified by the codes E10 before or in pregnancy and O24 in pregnancy. We suspected underreporting of type 2 diabetes in the National Discharge Register, so we also relied on self-reported information on diabetes from the telephone interviews. The study was approved by the Scientific Ethics Committee of the Cities of Copenhagen and Frederiksberg on behalf of all the committees in Denmark. The study was also approved by the Danish Data Protection Board.

The main outcome in the study was fetal death, defined as either a spontaneous abortion (< 28 completed weeks of gestation) or a stillbirth (28+ completed weeks of gestation). Intrapartum events were not considered a fetal death because the etiology may be different and highly dependent on the quality of clinical care during labor.4,21 Multiple pregnancies (n = 1,174) were coded as live births if both (n = 1,151) or just one (n = 10) baby was liveborn.

The relative risk of fetal death according to prepregnancy BMI was estimated as a hazard ratio (HR) by using a Cox regression with gestational days as the underlying time scale and the day of the first pregnancy interview as entry time.22 To improve comparability we stratified the analysis by gestational week of the first pregnancy interview. Follow-up ended at the time of fetal death, live birth, intrapartum event, induced abortion, hydatidiform mole, or emigration. We used a 2-sided significance level of .05 in all statistical tests.

In the Cox regression analyses, a number of potential confounders was used to adjust the effect of BMI on fetal death. These confounders were chosen a priori and included age, parity, height, socio-occupational status, smoking, coffee consumption, and alcohol consumption, because these covariates have been considered in previous studies of stillbirth.3,4,6,23,24 Finally, physical exercise was included because it has been suggested that the association between obesity and stillbirth may be confounded by this variable.6

In a separate analysis of stillbirth, the relative risk according to prepregnancy BMI was estimated as an odds ratio (OR) using multiple logistic regression. Only ongoing singleton pregnancies at 28 completed weeks of gestation and with a completed first interview before this point were included in these analyses. The effect of BMI on the risk of stillbirth was estimated with adjustments for the same variables as in the Cox regression. This analysis was repeated after exclusion of women with obesity-related diseases in pregnancy, including preeclampsia, other hypertensive disorders, and diabetes. Finally, the effect of weight gain, included in the model as a continuous variable, was assessed in this subsample. A likelihood ratio test of the hypothesis of no interaction was used to assess whether the effect of BMI on fetal death/stillbirth was modified by categories of height, parity, smoking, or weight gain.

Because 1,436 women contributed more than one pregnancy to the study, we used robust methods to obtain standard errors of the estimates that allowed for within-cluster correlation.25 This methodology is implemented in STATA 8.0 Special Edition (Stata Corporation, College Station, TX), which was used for all statistical analyses.

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RESULTS

The study population consisted of 54,505 pregnancies and 679 fetal deaths. Eight percent of the women was categorized as obese, 19% as overweight, 68% as normal weight, and 5% as underweight. The overall crude risk of stillbirth was 2.8/1,000 (n = 155, 13 intrapartum events excluded) and of spontaneous abortions during follow-up was 9.6/1,000 (n = 524). Compared with normal-weight women, obese and overweight women were slightly younger, smoked more, drank less alcohol, exercised less, and had lower socio-occupational status (Table 1). Preeclampsia, other hypertensive disorders, and gestational diabetes were more common among overweight and obese women. Moreover, overweight and obese women reported longer waiting times to pregnancy and had more frequently received infertility treatment.

The models used to describe the association between BMI and fetal death included an interaction term between BMI group and gestational age, which was categorized into 6 periods as shown in Table 2. This approach was used because an initial assessment of the assumption of proportional hazards across levels of BMI showed a statistically significant increase of the relative risk of fetal death with gestational age for both overweight and obese women (P = .02 and P = .001, respectively).

Table 2

Table 2

When stratified by gestational age in this manner, the crude rates of fetal death before 14 completed gestational weeks were comparable among the different groups of BMI (Table 2; Fig. 1). Subsequent to week 14, the expected decline in the absolute rate of fetal death across gestation was more pronounced in normal-weight women than in either overweight or obese women. This pattern of divergence resulted in an adjusted relative risk among obese women, which steadily increased with gestational age and ended with a 360% higher risk of stillbirth in pregnancies exceeding 280 days (Table 2). The higher relative risks were statistically significant from week 14 and onwards. A similar, but less pronounced, trend from early to late pregnancy was observed among overweight women, ending with nearly a 190% higher risk of stillbirth in the last gestational age category. The adjustment for a variety of maternal characteristics and life style factors had virtually no effect on the crude estimates (Table 2). Moreover, the effect of BMI on fetal death was not modified by height (P = .83), parity (P = .92), or smoking (P = .24).

Fig

Fig

A number of additional analyses were performed to assess the robustness of these results to changes in specific aspects of the analytic approach (data available on request). We repeated the analyses after including intrapartum events as fetal deaths. This resulted in a less pronounced trend with gestation in both overweight and obese women. To evaluate the impact of the method for calculation of gestational age, we repeated the analyses with gestational age based on the last menstrual period. A slightly lower excess risk of fetal death in obese women was observed, but results remained otherwise unchanged. We also explored whether the association reflected factors related to subfertility among overweight and obese women by restricting the analysis to primi gravidae with a waiting time to pregnancy of 6 months or less and no reported infertility treatment. This restriction led to an even higher excess risk among obese women compared with women of normal weight.

Next, we focused on the overall risk of stillbirth in 51,655 singleton pregnancies (151 stillbirths), using the same adjustments as in the analysis of fetal death (Table 3). To evaluate whether obesity-related diseases mediated the risk of stillbirth, we excluded women with diabetes (n = 735), preeclampsia (n = 1,122), and other hypertensive disorders (n = 790) from the sample (some had several of these diseases). This resulted in only minimal changes of the adjusted risk estimates. Compared with women of normal weight, overweight women still faced twice the risk of stillbirth, whereas obese women had a 240% increase in risk. Also, the effect of BMI on stillbirth was not modified by height (P = .09), parity (P = .57), or smoking (P = .96).

Table 3

Table 3

Within the subset of women without obesity-related diseases, and with weight gain data, we examined the possible effect of maternal weight gain on the risk of stillbirth (n = 39,187). The adjusted odds ratio related to an increased weight of 100 g per week was estimated to be 0.94 (95% CI 0.87–1.03), and those related to overweight and obesity were only slightly reduced. Interaction between BMI and weight gain was not found (P = .37).

Finally, we considered recorded causes of stillbirth. The distribution of causes according to BMI in women without obesity-related diseases is displayed in Table 4. Intrapartum events are included as a separate category. The crude odds ratios showed that unexplained intrauterine deaths and, especially, deaths due to outcomes categorized as placental dysfunction were increased among overweight and obese women. Adjustment for confounders did not change the results, but the limited number of cases may not justify such detailed analysis.

Table 4

Table 4

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DISCUSSION

The risk of fetal death was elevated among obese women and, to a lesser degree, also among overweight women. A temporal trend of the risk appeared after 14 weeks of gestation and continued to increase across gestation, with the highest risk of stillbirth after week 40. Weight gain in pregnancy was not significantly associated with the risk of stillbirth for any of the BMI groups.

Hypertensive disorders, diabetes, systemic lupus erythematosus, chronic renal disease, thyroid disorders, and cholestasis in pregnancy have been associated with stillbirth.26 A review of the medical records of all stillbirths in the study showed that these women were without medical diseases, except those 11 women who suffered from obesity-related diseases and had already been excluded from the analysis. We therefore believe that the presence of these medical diseases cannot account for the observed association between obesity and stillbirth.

We found that obesity was associated with a 5-fold increase in risk of stillbirth with placental dysfunction, but the confidence interval was wide. Biologic mechanisms behind this excess risk remain unknown. Obese pregnant women without clinical disease may present with metabolic and vascular abnormalities similar to those seen in preeclamptic women,9,10,27 where failure of normal placentation may lead to both infarction and abruptio of the placenta in later pregnancy.28–30 A recent study suggested that placental dysfunction causing stillbirth may be an endpoint for impaired placental function in the first 10 weeks of pregnancy.31 One may speculate that endothelial dysfunction in obese women is associated with such impaired early placental function.

It has been suggested that rapid fetal growth due to fetal hyperglycemia may place the fetus at risk of death by hypoxia if the placenta cannot transfer sufficient oxygen for metabolic requirements.5 We would expect such stillbirths to appear as unexplained intrauterine fetal deaths in late pregnancy, with a particular excess risk among obese women, as found in our study. However, even when gestational age was taken into account, the birth weights of unexplained intrauterine deaths among obese women were typically lower than the median birth weights of all live births (data available on request). This suggests the presence of some intrauterine growth restriction rather than excess fetal growth.

Excess risk of fetal death in obese women may in part be explained by deranged endocrinology and impaired fertility in a subgroup of obese women.32,33 Restricting our analysis to fertile women with their first pregnancy did not support this hypothesis.

We found no association between BMI and fetal death in the gestation period before week 14, and it cannot be ruled out that obesity acts as a pregnancy-preserving factor in very early pregnancy, leading to delayed fetal loss of less viable fetuses. A recent study found a slightly elevated risk of first-trimester abortions in obese women,14 which may suggest that obesity is at least not protective of early fetal loss.

There are potential criticisms of this study. Self-reported weight is likely to be underreported, whereas height is likely to be overreported, resulting in an underestimation of the true BMI.34 We assumed that the large majority of the participants would report prepregnancy weight and height as obtained during routine antenatal care. This would reduce underreporting because all women are weighed in early pregnancy. For women who experienced a stillbirth, the self-reported information on weight and height was consistent with the information obtained from the medical records. All information was collected at the time of entry into the study and misclassification is therefore likely to be nondifferential. Information bias, if present, would most likely lead to an underestimation of the true effect of obesity.

Information about obesity-related diseases in pregnancy was based on the National Discharge Register, and we found relatively low frequencies of these diseases. A validation study of the register has recently been published, and for categorizations of diseases at the level we used, a high validity was found.35

Recruitment to the Danish National Birth Cohort took place in 2 steps, and each of these steps involve a selection.16 We found, however, that the prevalence of prepregnancy obesity in the study population was very close to the national estimates in the general female population aged 25–44 years, where 9.1% were found to be obese and 20.6% to be overweight.36 Also, the absolute risk of stillbirth in singleton births in the study was 0.32% (intrapartum events included) and only slightly lower than the similar national estimate of 0.37% for the years 1999–2001.37 All comparisons are made within the cohort. Selection bias requires selection on both the exposures and outcomes, and the outcome was not known at the time of recruitment. We therefore believe that the comparisons between normal-weight and obese women in our study were not subject to selection bias. The association between obesity and fetal death may, however, be different in other populations with higher overall risks of fetal death.

Three studies have reported an increased risk of stillbirth in both normal-weight and overweight and obese women when compared with lean women with a BMI less than 18.2 or 20, respectively.3,4,6 However, in the most recent study, the results became less consistent when women with obesity-related diseases in pregnancy were excluded.6 In a large British register-based study, a modestly increased risk of stillbirth was found only in obese women relative to women of normal weight (BMI 20–25) after adjustment for obesity-related diseases in pregnancy.5 The interpretation of the findings in these studies is complicated by the choice of reference groups. We found that the temporal pattern of fetal death among underweight women indicates a high risk of late spontaneous abortions, which may explain in part the subsequent, lower risk of stillbirth. Previous studies have relied on retrospectively collected data on exposure, register data, or medical records, and information on some important confounding factors was missing. The present study is based on prospectively collected data on a large number of potential risk factors from more than 54,000 women. The consistency between various sources of information was high, and we were able to obtain complete ascertainment of all pregnancy outcomes. Furthermore, we had a complete and cause-specific ascertainment of all stillbirths. Although the study was large, the number of stillbirths was limited and reduced our ability to investigate more specific etiological hypotheses.

We found that prepregnancy obesity correlates with an increased risk of both late spontaneous abortion and stillbirth. The present evidence cannot preclude the possibility that obesity and stillbirth share common causes, and that the association we see is not causally related to obesity per se. Advocating weight reduction for obese women who want to become pregnant may be advisable, taking into account other benefits of such a weight reduction. Nevertheless, such advice is offered in a context where causal inference is still speculative.

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© 2005 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.