Gaining too much weight while pregnant can have consequences that last well after the pregnancy is over, new research shows.

The findings are probably no surprise to new moms who have struggled to lose that weight, but they are the first to show that these pounds may stick around for decades.

There’s evidence that excess weight gain in pregnancy ups the risk of obesity after delivery, but few studies have looked at the effects of weight gain on obesity risk for more than a couple of years after delivery.

To investigate, Dr. Abdullah A. Mamun of the University of Queensland in Herston, Australia and colleagues looked at body mass index (BMI) in 2,055 women who had delivered babies between 1981 and 1983. BMI is a standard measure of weight in relation to height used to determine if a person is overweight or obese. The researchers used 1990 Institute of Medicine recommendations on pregnancy weight gain, which are based on a woman’s pre-pregnancy BMI. (The IOM updated these guidelines in 2009.)

Women who had gained too much weight during their pregnancy based on the IOM’s older guidelines had gained an average of around 20 kilograms (44 pounds) 21 years later, while women who gained a healthy amount of weight put on 14 kilograms (31 pounds); those who gained too little put on about 9 kilograms (20 pounds).

The relationship between pregnancy weight gain and future BMI actually got stronger once the researchers accounted for a woman’s BMI before she got pregnant, the team reports in the American Journal of Clinical Nutrition.

Women who gained too much weight during their pregnancy, based on the IOM’s guidelines, were twice as likely to be overweight later on, and had a more than four-fold increased risk of being obese. The relationship didn’t change even after the researchers accounted for factors such as whether a woman exercised during pregnancy or how long she breastfed her baby.

It’s possible, the researchers note, that women with metabolisms that predispose them to obesity may put on too much weight in pregnancy and continue to gain too much weight throughout their lives. “However, because we did not have adequate dietary data, we could not exclude the possibility that poor dietary behavioral patterns followed by mothers may continue (after delivery), thereby contributing to weight retention,” they add.

More research is needed to understand the mechanisms that contribute to excess weight gain in pregnancy, Mamun and colleagues say, as well as to investigate strategies for limiting women’s weight gain in pregnancy.

“It remains controversial whether energy restriction should be advised to pregnant overweight women to control the potential risk of gaining excessive weight during pregnancy,” they add, “because such a restrictive diet can affect the growth of the fetus.”

SOURCE: American Journal of Clinical Nutrition, online March 17, 2010.