Fetuses grow differently in obese women

Obese pregnant women who don’t have serious medical problems are still more likely to have unusually large babies than mothers who aren’t obese, a recent study suggests.

While maternal obesity has long been linked to an increased risk of pregnancy complications for mothers and infants, the current study offers fresh insight into the ways that fetal development may be different when mothers are obese. Among other things, the study found that obese mothers tend to have babies with larger thigh and arm bones and bigger head circumference.

“In general, even among women without major chronic diseases before pregnancy, we observed significant differences in fetal growth between obese and non-obese pregnant women,” said lead study author Dr. Cuilin Zhang of the Eunice Kennedy Shriver National Institute of Child Health and Human Development at the National Institutes of Health in Bethesda, Maryland.

Mothers with larger babies are more likely to have infants that get stuck in the birth canal or require equipment like a vacuum or forceps to aid vaginal deliveries, and they are also more likely to have surgical, or cesarean section, deliveries.

The current study, involving 443 obese and 2,320 non-obese mothers and their babies, showed differences in the infants as early as the second trimester.

All of the women in the study were free of chronic diseases when they got pregnant, and none of them were carrying multiples.

To assess fetal development, researchers examined data from each ultrasound, including measurements for the length of the babies’ humerus, or upper arm bones, femur, or thighbone, head circumference and abdominal circumference.

Starting at 21 weeks’ gestation, femur and humerus length were significantly longer for babies with obese mothers than for infants with non-obese mothers, researchers report in JAMA Pediatrics.

Differences persisted through the end of pregnancy, or 38 weeks’ gestation. At that point in time, half of the babies with obese mothers had a femur length of at least 71 millimeters, compared with 70.2 millimeters for babies with non-obese mothers.

At the end of pregnancy, half of the babies with obese mothers had humerus bones at least 62.2 millimeters long, compared with 61.6 millimeters for infants with non-obese mothers.

Throughout pregnancy, obese women had babies with significantly larger head circumference than non-obese mothers, the study also found.

There wasn’t a meaningful difference in abdominal circumference between babies with obese and non-obese mothers. However, when researchers only looked at obese and normal-weight mothers, excluding overweight women, they did find obesity associated with babies who had larger bellies.

Obese women also had significantly heavier babies than other women, a difference that emerged starting at 30 weeks’ gestation and persisted throughout the rest of the pregnancy.

The study also excluded women with many health problems that can accompany obesity, such as diabetes and high blood pressure, and the results might be different for women with chronic medical issues.

It also wasn’t a controlled experiment designed to prove whether or how maternal obesity influences fetal development.

“What is tricky is that we don’t know exactly why obese women have babies with greater birth weight,” said Dr. Aaron Caughey, a researcher at Oregon Health & Science University in Portland who wasn’t involved in the study.

Lifestyle and behavioral factors like diet and exercise habits might play a role, Caughey said by email. So might underlying differences in metabolism or levels of sugar or fat in the blood – even among women without diabetes.

Genetics or environmental factors might also contribute, Caughey added.

But there is still a lot that obese pregnant women can do to minimize the odds of having an unusually large baby and the birth complications that can follow.

“If a woman who is obese finds herself pregnant, there is an opportunity to start improving her diet and beginning exercise then,” Caughey said. “It has been shown that those obese women who gain less weight in pregnancy are less likely to have big babies.”