Pregnant women who are obese or overweight are at an increased risk of complications, such as needing cesarean sections, developing gestational diabetes, and even the dangerous high blood pressure known as preeclampsia. Diet and exercise, researchers say, can help them safely control their weight gain during pregnancy.

But there is a catch.

The women in a recent study who lost weight  – about four pounds on average – after they were at least nine weeks into their pregnancies did not have fewer obstetric complications. Researchers are concluding that to lower their risk of complications, women may have to change their behaviors before or immediately after they conceive.

"We think that by the time these women are already in the second trimester, it may already be late to change important outcomes," said Alan Peaceman, lead author and chief of maternal fetal medicine at Northwestern University Feinberg School of Medicine.

Seven clinical centers including Brown and Columbia Universities recruited 1,150 pregnant women for the trial and split them into a control group and an intervention group. The second group focused on reducing calories, increasing physical activity and incorporating behavior changes such as self-monitoring. The trial, funded by the National Institutes of Health, followed participants between nine to 15 weeks pregnant through the birth of their child.

The study was published last week in the journal Obesity.

Most American women of childbearing age are overweight or obese. These women are more likely to gain excess weight in pregnancy, and to retain those pounds after childbirth. Their children, too, are more likely to be obese than are the children of thinner women.

The National Academy of Medicine recommends that women who are not overweight limit their pregnancy weight gain to 25 to 35 pounds. Women who are overweight should gain no more than 15 to 25 pounds and obese women should not gain more than 11 to 20 pounds.

Advice from doctors on weight gain during pregnancy has varied over the years. In the 1950s, the standard advice was not to gain more than 15 pounds. But expectant mothers weren't gaining enough, and that led to low birthweight babies that were at higher risk for developmental problems. By the late 1970s, mothers were told to "eat for two."

When pregnant women began to gain more weight, the thinking was that it was not a medical concern and they would be able to lose it after they gave birth. But, by the early 2000s, doctors found the extra pounds contributed to high blood pressure, gestational diabetes, and more cesarean sections.

The infants also faced more risks.

"Excess maternal weight gain was not just associated with bigger babies, but those babies ended up with an increased risk of obesity and childhood diabetes," Peaceman said.