Materials and Methods
Currently, there are no available national stillbirth databases in the United States that include maternal prepregnancy weight. Therefore, we queried all 50 states individually to identify state-level birth and stillbirth databases that contained this information. We initially identified 17 states with the desired information and excluded those with small delivery numbers, prohibitively high cost for access to data, or missing variables of interest. We used the vital records data from the states of Washington and Texas to examine the association between prepregnancy BMI and risk of stillbirth. Washington and Texas databases originate from birth and death certificates that are completed near the time of birth. To improve data quality, both birth and death certificates records are checked and edited for accuracy. For example, data value restrictions are placed on variables such as maternal age, and warnings are generated if maternal age is <14 or > 49 years. Both states perform routine, periodic data quality checks to identify facilities and providers with large amounts of missing data. In addition to internal quality measures, the National Center for Health Statistics also conducts independent reviews of each state to verify data quality. To ensure future compliance, field visits are conducted when noncompliant sites are identified.
This study was approved by the Drexel University College of Medicine Institutional Review Board, and by the Departments of Public Health of Washington and Texas.
Analysis was limited to singleton pregnancies that delivered between 20 and 42 weeks’ gestation, based on best clinical estimate of gestational age. The best clinical estimate of gestational dating, which incorporates data from other sources in addition to the last menstrual period, is considered superior to the menstrual-based estimate, which also is contained in the vital statistics data. Pregnancies with missing pregestational weight, height, or gestational age and those with maternal height recorded as <48 inches were removed from the cohort. Severe fetal anomalies that included anencephaly, spina bifida, cardiac anomalies, diaphragmatic hernia, gastroschisis, omphalocele, and chromosomal abnormalities were also removed. We excluded underweight women (BMI, <18.5 kg/m 2 ) from analysis, because the primary objective of this study was to determine the effects of obesity on stillbirth risk.
For births in Washington, we abstracted data between 2003 and 2011 (n = 784,861 live births and 4735 stillbirths); for births in Texas, we abstracted data between 2006 and 2011 (n = 2,422,522 live births and 13,939 stillbirths). Of these births, we sequentially excluded 101,827 multiple births, 11,438 births with gestational age at delivery <20 or ≥43 weeks’ gestation, 7937 births with severe anomalies, 86,744 births with missing covariate data, and 149,629 births to women with BMI <18.5 kg/m 2 or height <48 inches. After all exclusions, 2,868,482 singleton births (9030 stillbirths) remained for analysis ( Figure 1 ). Nearly one-third of the stillbirths were excluded because of missing variables. We conducted a separate comparison of this group with those included for analysis to identify any selection bias within the data.
Maternal prepregnancy weight generally was self-reported by the woman and recorded by the prenatal care provider at the first prenatal visit. The care provider also recorded height at the first prenatal visit. BMI was defined as the ratio of maternal prepregnancy weight (in kilograms) over square of height (in meters). After the recommendations of the World Health Organization, we categorized BMI in the following manner: normal (18.5-24.9 kg/m 2 ), overweight (25.0-29.9 kg/m 2 ), obese class I (30.0-34.9 kg/m 2 ), obese class II (35.0-39.9 kg/m 2 ), and obese class III (40.0-49.9 kg/m 2 ). We separately analyzed women with a BMI ≥50 kg/m 2 . Obesity of this magnitude was once considered rare, but its prevalence has been increasing at a rapid rate. A growing body of evidence suggests that this group is at even higher risk of maternal and fetal complications than less severely obese women.
We divided gestational age into 4 periods in the following manner: early preterm (30-33 weeks), late preterm (34-36 weeks), early term (37-39 weeks), and late term (40-42 weeks). We calculated the HR of stillbirth that was associated with obesity for each gestational period using Cox proportional hazards regression analysis. We estimated the confounder-adjusted HR of stillbirth in relation to BMI. Potential confounders considered for adjustment included maternal age (grouped as <20, 20-24, 25-29, 30-34, 35-39, and ≥40 years), primiparity, education (grouped as less than high school, high school diploma, and bachelor’s degree or higher), no prenatal care, race/ethnicity (non-Hispanic white, non-Hispanic black, Hispanic, and other race), and smoking. In addition, we also adjusted the analyses for chronic hypertension and pregestational diabetes mellitus. We calculated the population-attributable risk (PAR) using the adjusted HRs for each BMI class for the 4 gestational periods as described earlier.
Because at any given gestational age the fetus is at risk of stillbirth not only during that week but during the remaining weeks of gestation, we also estimated the risk of stillbirth for ongoing pregnancies based on gestational age with the following fraction:
No . of stillbirths for given week of gestation + all stillbirths that occured thereafter All deliveries during and after the given week of gestation
Results
In this cohort of >2.8 million women that delivered a singleton birth, 51.4% were normal weight; 26.0% were overweight; 13.1% were class I obese; 5.8% class II obese; 3.4% class III obese, and 0.4% had BMI ≥50 kg/m 2 . Subject characteristics for each BMI class are listed in Table 1 . On average, women with higher BMI at the start of pregnancy gained less weight, were older than those with lower BMI, and had achieved lower education levels. A higher proportion was black and multiparous, especially among the highest obesity classes. They were more likely to smoke and more likely to have diabetes mellitus and hypertensive disorders in pregnancy. Contrary to previous reports, obese women demonstrated a trend toward earlier delivery as BMI increased.
| Variable | Normal weight | Overweight | Class | Body mass index ≥50 kg/m 2 | ||
|---|---|---|---|---|---|---|
| I (30.0–34.9 kg/m 2 ) | II (35.0–39.9 kg/m 2 ) | III (≥40 kg/m 2 ) | ||||
| Washington, n | 221,859 (40.4%) | 173,436 (31.6%) | 86,599 (15.8%) | 40,430 (7.4%) | 23,672 (4.3%) | 3123 (0.6%) |
| Texas, n | 1,252,048 (54.0%) | 571,770 (24.7%) | 287,805 (12.4%) | 125,971 (5.4%) | 72,600 (3.1%) | 9166 (0.4%) |
| Total, n | 1,473,907 (51.4%) | 745,206 (26.0%) | 374,404 (13.1%) | 166,401 (5.8%) | 96,272 (3.4%) | 12,289 (0.4%) |
| Characteristics | ||||||
| Average weight gain, lb a | 33.3 ± 32.1 | 31.3 ± 35.2 | 27.9 ± 37.1 | 24.6 ± 38.5 | 20.8 ± 38.6 | 17.7 ± 47.2 |
| Mean maternal age, y a | 26.6 ± 6.2 | 27.3 ± 6.1 | 27.5 ± 5.9 | 27.7 ± 5.8 | 28 ± 5.6 | 28.8 ± 5.4 |
| Height, in a | 63.9 ± 2.8 | 63.7 ± 2.9 | 63.6 ± 2.9 | 63.8 ± 3.0 | 63.9 ± 3.1 | 63.6 ± 3.4 |
| Race, % | ||||||
| White | 44.4 | 40.5 | 39.4 | 42.0 | 42.7 | 40.1 |
| Black | 8.4 | 10.1 | 11.1 | 12.6 | 15.7 | 22.2 |
| Hispanic | 39.4 | 44.3 | 45.6 | 38.7 | 38.7 | 35.1 |
| Other | 7.9 | 5.2 | 3.9 | 2.9 | 2.9 | 2.6 |
| Education, % | ||||||
| <12 y | 24.1 | 27.1 | 26.4 | 23.1 | 20.2 | 19.6 |
| High school diploma | 49.9 | 52.6 | 57.6 | 62.6 | 67.0 | 69.5 |
| ≥Bachelor’s degree | 26.0 | 20.4 | 16.0 | 14.3 | 12.8 | 10.8 |
| Smoking, % | 8.3 | 8.5 | 9.5 | 10.1 | 11.0 | 10.5 |
| Primiparous, % | 42.1 | 33.9 | 30.4 | 29.8 | 29.9 | 29.3 |
| No prenatal care, % | 5.6 | 5.7 | 5.2 | 4.9 | 4.7 | 4.9 |
| Assisted fertility, % | 0.5 | 0.4 | 0.4 | 0.5 | 0.5 | 0.3 |
| Pregestational diabetes mellitus, % | 0.3 | 0.6 | 1.1 | 1.7 | 2.5 | 3.8 |
| Chronic hypertension, % | 0.5 | 0.9 | 1.7 | 2.8 | 4.6 | 8.8 |
| Gestational diabetes mellitus, % | 2.6 | 4.5 | 6.7 | 8.6 | 10.6 | 13.5 |
| Gestational hypertension, % | 3.4 | 4.9 | 6.7 | 8.7 | 10.5 | 13.4 |
| Gestational age at delivery, % (mean birthweight, g) | ||||||
| <30 wk | 0.8 (1095) | 0.9 (1060) | 1.1 (1009) | 1.3 (1025) | 1.6 (984) | 1.8 (1035) |
| 30-33 wk | 1.3 (1963) | 1.3 (1958) | 1.5 (1969) | 1.6 (1959) | 1.8 (1919) | 2.2 (1966) |
| 34-36 wk | 6.3 (2680) | 6.3 (2749) | 6.9 (2792) | 7.4 (2833) | 8.1 (2876) | 9.7 (2933) |
| 37-39 wk | 63.5 (3263) | 64.2 (3336) | 65.1 (3376) | 66.1 (3410) | 67 (3442) | 68.1 (3479) |
| 40-42 wk | 28.2 (3509) | 27.2 (3570) | 25.5 (3594) | 23.5 (3617) | 21.6 (3623) | 18.2 (3616) |
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