(Reuters)
Updated: 2007-07-06 16:20

NEW YORK - Overweight women are known to have a greater chance of giving birth to a larger-than-normal baby. But new research suggests that these odds stay higher even when a woman loses weight before pregnancy.

In a study of more than 146,000 women who'd each given birth twice, researchers found that those who maintained a normal body weight before each pregnancy had the lowest odds of having an abnormally large newborn.

Not surprisingly, women who were overweight or obese before each pregnancy had higher risks of delivering a large baby.

However, overweight women who lost weight before their second pregnancy did not eliminate their increased odds of having an oversized newborn. This, the study authors speculate, could mean that a woman's excess pounds have a lasting effect on subsequent pregnancies, even after she's slimmed down.

Dr. Darios Getahun of the UMDNJ-Robert Wood Johnson Medical School in New Brunswick, New Jersey, led the study. The findings are published in the American Journal of Obstetrics & Gynecology.

Abnormally large newborns are at risk of birth trauma, and often necessitate the use of cesarean delivery. Overweight and obese women are more likely than normal-weight women to have a large newborn, particularly if they have gestational diabetes.

In the current study, Getahun's team looked at whether weight changes between pregnancies affected these odds. The researchers based their findings on the pre-pregnancy weights of 146,227 women who had their first and second pregnancies between 1989 and 1997.

In general, they found, women who were overweight before both pregnancies were 70 percent more likely than women who maintained a normal weight to have an overly large baby in their second pregnancy. The odds were even higher among women who remained obese over time.

Women who trimmed down to a normal weight before their second pregnancy also trimmed their risk of having a large baby. However, the risk was not on par with that of women who'd been consistently thin, the researchers found.

It's possible, according to Getahun's team, that extra weight, even after it's shed, has a long-term effect on future pregnancies, possibly through effects on mothers' metabolism.

Ideally, the researchers conclude, women would lower their odds of having an overly large baby by preventing their own excess weight gain in the first place.