Pregnant women who are obese or overweight are at an increased risk of complications, such as needing caesarean sections, developing gestational diabetes, and even 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 1.8kg 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 behaviours 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 Dr Alan Peaceman, lead author and chief of maternal foetal medicine at Northwestern University Feinberg School of Medicine in the United States.

Seven US clinical centres, 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 behaviour changes such as self-monitoring.

The trial, funded by the US National Institutes of Health, followed participants between nine and 15 weeks pregnant through to the birth of their child.

The study was published 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 the children of thinner women.

The US National Academy of Medicine recommends that women who are not overweight limit their pregnancy weight gain to 11.3kg to 15.9kg (25lb to 35lb).

Women who are overweight should gain no more than 6.8kg to 11.3kg (15lb to 25lb) and obese women should not gain more than 5kg to 9kg (11lb to 20lb).

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 6.8kg. 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 caesarean 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,” Dr Peaceman said. – The Philadelphia Inquirer/Tribune News Service