Being overweight or obese increases a woman’s chances of having an extra-big baby, even after the effects of pregnancy-related, or “gestational,” diabetes are taken into account, new research shows.

Excess weight in and of itself also sharply increased a woman’s risk of pre-eclampsia, a potentially deadly pregnancy complication, Dr. Boyd E. Metzger of Northwestern University Feinberg School of Medicine in Chicago and his colleagues found.

Women have more difficulty delivering very large babies, while these newborns are also at risk of suffering injury during birth, including shoulder dislocation. While women who are overweight or obese are known to run a greater risk of having very large babies and experiencing other pregnancy complications, it has been difficult to separate out the effects of a mother’s weight from those of gestational diabetes, Metzger and his colleagues note in the British Journal of Obstetrics and Gynecology.

This led them to investigate whether body mass index (BMI) - a standard measure of weight in relation to height used to gauge how fat or thin a person is - might influence pregnancy risks and fetal and newborn health, independently of a woman’s blood sugar levels.

The study involved 23,316 women from 15 different medical centers in nine different countries. All had undergone an oral glucose tolerance test, which is used to identify women with, or at risk for, pregnancy-related diabetes; at that time, their height and weight were measured, too.

The researchers then used statistical techniques to control for women’s oral glucose tolerance test results. Even after this adjustment, they found that the women with BMIs of 42 or greater, denoting severe obesity (for example, a 5-foot-5-inch tall woman weighing at least 250 pounds), were at more than triple the risk of having an excessively large baby, compared to the thinnest women in the study, who had BMIs of 22.6 or less (a 5’5” woman weighing less than 138 pounds).

The heaviest women’s risks of having a C-section were more than doubled, while their likelihood of pre-eclampsia was 14-fold greater than for the leanest women. However, the heaviest women’s risk for delivering a preterm baby was actually cut in half.

These findings help sort out the role BMI and gestational diabetes each play in the risk of complications of pregnancy and delivery, Metzger told Reuters Health in an interview.

He noted that recent studies have shown that dietary changes can effectively treat gestational diabetes for more than 90 percent of women with the condition.

“We’re pretty confident that treating gestational diabetes going forward is going to continue to be beneficial,” the researcher said. “We have much less evidence at this point as to how to neutralize or reduce the impact of overweight on pregnancy outcome.”

What is becoming clear, he added, is that it’s probably a woman’s weight before she gets pregnant, rather than how much she gains during pregnancy, that’s important in determining risk.

SOURCE: BJOG, online January 20, 2010.