NEW YORK (Reuters Health) – The Institute of Medicine recommends that normal weight women gain 25 to 35 pounds during pregnancy, but now new research indicates that this amount of weight gain increases the risks of macrosomia and large for gestational age (LGA).

The results also indicate that while prepregnancy body mass index modifies the association between weight gain and LGA, it does not affect the association with macrosomia.

Prior research has tied high pregnancy weight gain with excessive fetal growth, note Dr. Patricia M. Dietz and colleagues from the Centers for Disease Control and Prevention, Atlanta. Whether prepregnancy body mass index, which is considered in the Institute’s recommendations, affected the association was unclear.

To investigate, the researchers analyzed data from 104,980 singleton term births that occurred from 2000 to 2005 and were included in the Pregnancy Risk Assessment Monitoring System, a population-based registry. Large for gestational age was defined as greater than 90% of birthweight for gestational age and macrosomia as a birthweight of at least 4500 grams.

The researchers report their findings in the American Journal of Obstetrics and Gynecology for July.

As pregnancy weight gain increased, so did the risk of both LGA and macrosomia. In the case of LGA, however, the prepregnancy body mass index had a modifying effect.

For the same amount of weight gain, lean women had a greater increase in the LGA risk than did obese women. Relative to weight gain of 15 to 25 pounds, weight gain of 46 pounds or more increased the odds of LGA in lean women by 6.0-fold, but by only 2.4-fold in obese women.

Weight gain of 26 to 35, 36 to 45, and 46 or more pounds increased the odds of macrosomia by 1.5-, 2.1-, and 3.9-fold, respectively, relative to weight gain of 15 to 25 pounds, the report shows.

“Healthy women have been found to gain an average of 27.5 pounds,” the authors note. “The results of our study suggest that weight above this amount may not be beneficial for the mother or for the fetus, regardless of the mother’s prepregnancy body mass index.”

Reference:
Am J Obstet Gynecol 2009;51:e1-e6.