But some of the athletes he studied seemed to know no bounds.

"I had a woman, she was huge, and she was running seven-minute miles up to Week 39," Pivarnik said. "It looked like she had a basketball under her shirt."

It was, he said, close to the pace she had run before her pregnancy.

He was worried, but not about the baby. Pivarnik was concerned about the woman's abdominal muscles and whether they would be pulled with all that jostling. "But she claimed she was not uncomfortable, so who was I to argue?" he said.

Both the baby and the woman's abdominal muscles turned out to be fine. Pivarnik's nonchalance about the effects of strenuous exercise on a fetus might take some women and their doctors aback.

But his was an educated response, he said. He based it on his research and on what he and his colleagues found by surveying published studies and case histories of pregnant athletes who continued to train.

Training patterns
"We looked at training patterns during pregnancy and postpartum," Pivarnik said. "And we asked, 'Was the amount of training related at all to adverse events?' The answer was no."

They found athletes who surprised even exercise physiologists with their feats. In 2001, for example, Regan Schreiber, an all-American swimmer at Penn State in the 1990s, swam the English Channel (48 kilometres) in 9 hours 30 minutes. She was 11 weeks pregnant.

The researchers tell of pregnant competitive athletes who significantly elevated their body temperature with strenuous exercise. Doctors often worry that heating a pregnant woman's body may harm a fetus. But those women had normal babies. And a few small studies of pregnant women who deliberately exercised to raise their body temperature also found no effects on the babies.

The researchers add that questions about exercise and pregnancy remain, making it difficult for doctors to give advice. The problem is deciding what lines to draw, and why. If the risks of exercise were low, but real, case reports and small studies would not find them.

The best way to learn how much exercise is safe is with studies in which pregnant women are randomly assigned to exercise regimens. Such studies, though, "can't be done," said Dr. Mona Shangold, the director of the Centre for Women's Health and Sports Gynaecology in Philadelphia. No researchers or women would want to take a chance that the exercise might injure the fetus.

As a result, said Shangold, doctors tend to play it safe.

But the advice often varies by doctor and can be based more on hunches than science, she said. Shangold says she tells women to limit their exercise time to 30 minutes a session, which she said was arbitrary advice.

"The problem is that we don't know yet what the safe limit is," Shangold said. "We are probably more conservative than we need to be." In fact, as Pivarnik found, advice to pregnant women varies considerably. He and his colleagues asked athletes who were or recently had been pregnant what they had been told.
Some received specific advice, like keep your heart rate below 140 beats a minute. That pretty much guarantees you won't be exerting yourself much. It was in 1985 guidelines set by the American College of Obstetricians and Gynaecologists, but it makes little sense, Shangold said.

"Heart rate is not a useful parameter to monitor during pregnancy," she said. "It varies widely during pregnancy and the heart rate response to exercise also varies widely."

Other women reported vague advice like "listen to your body." Others got very conservative advice, like stopping all but the mildest exercise until six weeks after the baby is born.

Even women doctors are not always sure what to do.
New York Times