Women who enter pregnancy with a BMI in the obese range may have less weight to gain to stay within normal limits, according to a study presented at The Pregnancy Meeting in San Francisco.

The new weight gain guidelines would only affect women falling into the severely and morbidly obese categories during pregnancy. Researchers found that underweight, normal weight and obese patients who gained less weight during the last two trimesters of pregnancy were more likely to give birth to infants that were lower than desired birth weight; small for gestational age. Women falling into class II and class III of obesity, severe and morbid, or gaining less weight during the final two trimesters of pregnancy resulted in no ill effects on the fetus or the pregnant woman.

Setting appropriate weight gain guidelines is extremely important for pregnant women. Gaining too much weight and the infant can be born larger than average. Gaining too little weight and the infant can be born smaller than average. In both cases, infants are at increased risk of complications during pregnancy, delivery and after birth. Larger babies are also typically delivered via C-section which comes with additional complications, especially for severely and morbidly obese pregnant women.

Women falling above the normal weight range were also at increased risk of suffering from pregnancy complications, including gestational diabetes, C-section delivery, and labor induction. This was true of women who started the pregnancy above normal weight and those who increase BMI to overweight and obese ranges during pregnancy.

There were about 74,000 women included in the study. The study included 4% underweight, 48% normal weight, 24% overweight, 24% obese. The obese group was broken into three classes based upon BMI: class I = 13%, class II = 6% and class III = 5% of the total obese group.

In addition to the potential need for weight gain guideline changes for class II and class III obese women, researchers also found that women falling into the overweight and obese categories may give birth vaginally (more often) if they are given higher doses of oxytocin more frequently.

Source: University of Rochester Medical Center. Eva Pressman, M.D., Danielle Durie, M.D., David Hackney, M.D., and Nigel Campbell, M.D., Christopher Glantz, M.D., M.P.H, and Loralei Thornburg, M.D. 14 February, 2011.