Women who are obese when they become pregnant are more likely than other expectant mothers to have a stillborn baby. But most studies of this relationship have included too few people to give detailed information about which obese women are at greatest risk, or which stages of pregnancy are most likely to be affected.
New Stanford research, led by Suzan Carmichael, PhD, and published online this week in PLOS ONE, changes that. The study used a very large California database of vital records on live births and stillbirths, allowing Carmichael's team to compare 4,000 stillbirths - in which the baby was born dead after at least 20 weeks of pregnancy - to a control group of 1.1 million live births that followed full-term pregnancies.
With the large data set, the researchers were able to examine the effect of mothers' race and ethnicity, whether the mothers had previously given birth, and how far along the pregnancies were at the time of the stillbirths. They excluded from analysis the cases in which an obvious fetal factor (such as a chromosomal abnormality) or a known maternal disease (such as diabetes) was probably responsible for the stillbirth.
What emerged is a complicated picture. Overall, greater obesity was linked with greater risk of stillbirth, with a 10-unit increase in body mass index equivalent to a 1.5- to twofold increase in stillbirth risk, a finding echoed by other recent research.
But the increase in risk wasn't equal across all groups of women, or all stages of pregnancy. For instance, among Hispanic women who had never had a child before, the most extreme level of obesity conferred a five- to sixfold increase in the risk of having a stillbirth between 20 and 23 weeks of pregnancy and about a twofold increase in the risk of stillbirth near the baby's due date, but was not linked with any change in the risk of having a stillbirth between 24 and 36 weeks' gestation.
A few themes did emerge, however. Obesity consistently increased the risk for the very earliest stillbirths (between 20 and 23 weeks), regardless of a mother's ethnicity or whether she had had other children. This is similar to another recent Stanford finding that obesity increases the risk for the earliest premature live births.
In the paper's discussion section, the authors write:
Obesity and stillbirth are both complex, and many potential factors may contribute to their association. Stillbirth may stem from a variety of adverse conditions, including placental insufficiency, preterm onset of labor or rupture of membranes, infection and cord abnormalities. Obesity could contribute to any of these problems. In addition, obesity may contribute to lower sensitivity with regard to detection of fetal complications, on the part of monitoring tools or maternal ability to detect changes in fetal movement.
The authors hope their findings will help shed light on what causes stillbirth and how, perhaps, some cases might be prevented.
The research was funded by the March of Dimes Prematurity Research Center at Stanford University and the Stanford Child Health Research Institute.
Previously: Women who have had a stillbirth are more likely to experience long-term depression, study shows, Losing Jules: Breaking the silence around stillbirth and A call to "break the silence" of stillbirth
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