The More Obese a Woman Is, the Greater Her Risk of Having a Stillbirth
Obese women are more likely than their normal-weight counterparts to have a stillbirth, and the risk appears to rise with the degree of a woman’s obesity.1 Overall, Missouri women who gave birth between 1978 and 1997 were about 40% more likely to have a stillbirth if they were obese than if they were normal-weight; the risk of stillbirth was elevated by 30% for women at the low end of the obese range, but was nearly doubled for the extremely obese. The general pattern of increasing risk with more severe obesity held for both black and white women, but the differentials were greater for blacks in every category.
Given a “persistent surge” in extreme obesity among women, and well-established connections between obesity in general and poor birth outcomes, researchers set out to explore the relationships between stillbirth risk and degrees of obesity. They used linked data files from Missouri’s vital statistics system to study maternal characteristics and birth outcomes associated with singleton pregnancies of 20–44 weeks’ gestation. After calculating women’s prepregnancy body mass index (or BMI, defined as weight in kilograms divided by the square of height in meters), they categorized women according to the following weight categories: normal (BMI, 18.5–24.9), class 1 obesity (30.0–34.9), class 2 obesity (35.0–39.9) and extreme obesity (40.0 or higher). (Women who were underweight or who were overweight but not obese were excluded.) Stillbirth was defined as in utero fetal death at 20 or more weeks’ gestation; the researchers computed stillbirth rates, compared them across maternal characteristics by using chi-square tests and used Cox hazards regression to assess risk factors.
Approximately 1.4 million mother-fetus pairs were included in the analyses. Nearly 10% of women were obese: Six percent were at the low end of the obesity range, 2% were in the middle and 1% were at the high end. The proportion who were obese was higher among black women than among whites—13% vs. 9%. Obese women were more likely than those of normal weight to be at least 35 years old, multiparous and black; they had had more schooling than normal-weight women and were more likely to have received adequate prenatal care. Normal-weight women were more likely than their obese counterparts to be married and to smoke.
The frequency of common medical and obstetric complications differed between obese and normal-weight women. Diabetes, chronic hypertension, preeclampsia and eclampsia occurred more often among obese than among normal-weight women; anemia and placental disorders were more frequent among normal-weight women than among those who were obese.
In all, 8,240 stillbirths occurred during the study period, most of them (86%) to normal-weight women. The stillbirth rate was significantly higher among obese women than among those with a normal weight—8.5 vs. 5.5 per 1,000 live births plus stillbirths. Rates also varied by category of obesity, rising from 7.8 per 1,000 for women in the class 1 group to 8.7 per 1,000 for those in class 2 and 11.7 per 1,000 women who were extremely obese.
Obese women overall had a 40% greater risk than normal-weight women of having a stillbirth (hazard ratio, 1.4). Moreover, the differential climbed steadily and significantly with BMI: The increase in risk was 30% among women at the low end of the obese range, 40% among those in the middle and 90% among those at the high end.
Separate analyses by race revealed that the stillbirth rate was higher among black obese women than among white obese women (11.4 vs. 7.8 per 1,000 live births plus stillbirths). Stillbirth risk was directly associated with BMI in both groups, but the magnitude of risk was greater for blacks than for whites. Obese black women overall were almost twice as likely as their normal-weight counterparts to have a stillbirth (hazard ratio, 1.9); the increase was 60% among the least obese women, 90% among the moderately obese and 130% among the extremely obese. For whites, by contrast, the overall increase in risk for obese women was 40%, and the differential rose from 30% to 80% with increasing BMI. The investigators speculate that these differences reflect racial disparities in rates of “obesity-related morbidities.”
According to the researchers, although their analyses may have been affected by the aggregation of different cohorts, controls that they included for year of birth should have largely eliminated the potential bias, and the study’s use of population-based data makes the results “reasonably generalizable.” They remark that their findings should be seen “as impetus for more refined studies that will potentially offer answers to many questions emanating from these preliminary results.”
1. Salihu HM et al., Extreme obesity and risk of stillbirth among black and white gravidas, Obstetrics & Gynecology, 2007, 110(3):552–557.