The researchers found that most of the obese women gained too much weight during pregnancy, then, on average, kept on 40 percent of the weight one year after they gave birth.
The study defined excess weight as more than 15 pounds, which until recently was the minimum amount of weight gain recommended for obese pregnant women by the Institute of Medicine.
"We found that nearly three quarters of obese women gain too much weight during pregnancy, and the more weight they gain, the harder it is to lose," said study lead author Dr Kim Vesco, MPH, a practicing OB/GYN and an investigator at the Kaiser Permanente Center for Health Research.
"This extra weight also increases the risk of pregnancy and delivery complications like diabetes, preeclampsia, bigger babies, C-sections, and birthing injuries," Vesco added.
The findings revealed that, on average, women retained 40 percent of the weight they gained during pregnancy one year after delivery. So if a woman gained 15 pounds during pregnancy, she could be expected to retain 6 of those pounds at one year; if she gained 30 pounds, she would retain 12 pounds.
"We need to do a better job of helping obese women control weight gain during their pregnancies. Once the baby is born, it's much harder to change eating habits and start an exercise program," said Dr Victor Stevens, co-author and senior investigator at the Kaiser Permanente Center for Health Research.
"If we can prevent the weight gain in the first place, moms and babies will be healthier in the long run," Stevens added.
The study is published in journal Obstetrics and Gynecology.