Regular exercise before pregnancy may protect against so-called pelvic girdle pain as the fetus grows, according to a new study from Norway.

Between 20 and 40 percent of pregnant women experience some pelvic girdle pain late in pregnancy, associated with joint and ligament changes in their body caused by the growing baby, said lead author Katrine Mari Owe of the Norwegian Institute of Public Health in Oslo.

Other studies have found that exercise can lessen chronic pain for people who are not pregnant, likely due to endorphin release, she told Reuters Health by email.

"Though the long-term effect of exercise on pain remains unclear, women who exercise regularly pre-pregnancy are more likely to continue throughout pregnancy," she and her colleagues write in the British Journal of Sports Medicine.

Pelvic girdle pain is defined as pain across the back and front of the pelvis, Owe's team explains. It has been linked to depression and higher amounts of sick leave during pregnancy, they note.

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Past research has already shown that women who experience pelvic girdle pain are less likely to exercise during pregnancy. The researchers wanted to see if exercise levels before pregnancy were linked to a likelihood of having pelvic girdle pain.

They analyzed data on more than 39,000 women in Norway who became pregnant with their first child between 2000 and 2009 and were part of a long term study that followed the health of both mothers and children.

At clinic visits when they were about four months pregnant, the women answered questions about the type and frequency of exercise they did in the three months before becoming pregnant.

More than half of the women said they exercised at least three times a week before pregnancy, while only seven percent said they were not exercising at all. Almost all of those who had exercised before pregnancy were still doing so at about four months gestation.

When they were 7.5 months pregnant, about 1 in 10 women reported pelvic girdle pain, including 1 in 8 of those who had reported no exercise.

Those who reported pain tended to be less healthy in several ways, including more reports of smoking, being overweight and having a history of depression or lower back pain before pregnancy.

Those who did not report pelvic girdle pain tended to be healthier and to report exercising three to five times per week before becoming pregnant.

Those who did high impact exercise - like running, jogging, playing ballgames/netball, high impact aerobics or orienteering - were the least likely to report pelvic pain.

Overall, exercising more than five times weekly pre-pregnancy did not appear to add to the benefit.

"What was surprising to us was that even women who exercised only once or twice a week (before pregnancy) had lower risk of developing pelvic girdle pain compared to those who did not exercise at all," Owe said.

Few women are severely disabled by pelvic girdle pain, according to Hilde Stendal Robinson of the University of Olso, who was not part of the new study.

Exercise in general is beneficial, and pregnancy is no exception, Robinson told Reuters Health by email.

"It is important that women with pelvic girdle pain seek help to deal with the pain throughout pregnancy," Owe said. "We have no evidence to recommend complete rest for nine months, but she must take into account the pain and find alternative ways to stay physically active."

There are few effective treatments for pelvic pain during pregnancy, but it almost always resolves after delivery, she said.