WASHINGTON -- Pregnancy and birth make big changes in a woman's life, and doctors want women to consider adding one more.

They are encouraging women to exercise, figuring soon-to-be or new mothers may be more responsive to the message.

"The desire to return to prepregnancy size and shape, the need for increased energy, and the need for stress reduction can be effective motivators to start or resume exercise," said Dr. Erika N. Ringdahl.

Ringdahl's analysis of the benefits and the risks of exercise appears in the February issue of a medical journal, The Physician and Sportsmedicine.

And a doctor's group says pregnant women generally can do the minimum that federal guidelines recommend for adults -- 30 minutes of moderate activity, such as a brisk walk, on most days.

Unless there are medical reasons to avoid it, "pregnant women should be encouraged to engage in regular, moderate intensity physical activity," the American College of Obstetricians and Gynecologists said. Their policy statement was published in January in ACOG's journal, Obstetrics and Gynecology.

Being active can give pregnant women the same health benefits that women who are not pregnant get, the medical group said. In addition, it cited evidence that exercise can prevent gestational diabetes, a form of diabetes that can begin during pregnancy and disappear following delivery.

Pregnancy spurs some women to make big changes in the way they live, said Ringdahl, of the University of Missouri School of Medicine. However, the window in which doctors can encourage those changes passes quickly. "By six weeks postpartum, many women have established a routine that excludes regular exercise," her article said.

Women who recently gave birth have some barriers to exercise. During pregnancy, they may have lost conditioning and gained weight. The demands of new motherhood may cut into their available time. And joints may still be loose as an aftereffect of pregnancy's higher levels of the hormone relaxin, which lets ligaments stretch to facilitate birth.

Exactly when to start or return to exercise is up to the mother and her doctor, the article said. And some women -- such as those who had troubled pregnancies or other exercise risk factors -- may bear more cautious watching, it said.

New mothers who exercise sometimes worry about how their workouts could affect the production and quality of their milk.

Studies have found that regular aerobic exercise does not affect milk flow or nutritional content, the article said. But other studies indicate babies prefer to nurse before the mother exercises, because post-exercise milk can contain higher levels of lactic acid, a normal byproduct of muscle activity, it said.

Some new mothers may also be concerned about stress incontinence. The article recommended urinating before exercise, doing isometric contraction exercises to strengthen muscles of the pelvic floor, and not doing high-impact activities such as running.

Most women can start or continue exercise during pregnancy, the doctors' group said.

Pregnant women should talk with their doctor about exercise, the statement said. Exercise can be dangerous for some women, and some forms of exercise are risky for all women, it said.

Doctors don't want women to exercise if they have such problems as heart or lung disease or preeclampsia, a condition marked by high blood pressure, and which could lead to maternal or fetal death. Exercise also is prohibited for women who are at risk for premature labor because they are carrying more than one baby.

And the doctors say women who appear capable of exercise should stop if they develop such problems as vaginal bleeding, dizziness, headache, chest pain, preterm labor or decreased fetal movement.

Scuba diving is off-limits because it may create gas bubbles in the fetus' circulatory system. The doctors want their patients to avoid sports that can lead to collisions, such as ice hockey, soccer and basketball.

The recommendations note that competitive athletes will want to keep training and competing during pregnancy. It says this is for the pregnant woman and her doctor to decide.

Prolonged strenuous exercise raises the temperature. And, while the report by the medical group's Committee on Obstetric Practice said there was no evidence in humans that this could lead to problems in the fetus, the guidelines' lead author had some doubts.

Some studies in lab animals have linked severe exertion to a higher risk of neural tube defects such as spina bifida, in which a portion of the spine is left exposed, said Dr. Raul Artal of St. Louis University School of Medicine. Less than 30 minutes of exercise is acceptable but, just to be safe, marathon running should be avoided even in the first two months of pregnancy, he said.