The researchers say their analysis reinforces the view that exercise is good for a pregnant woman and her baby and does not hold any increased risk of preterm birth.
The study is published in the American Journal of Obstetrics & Gynecology.
Senior author Vincenzo Berghella, professor at Thomas Jefferson University in Philadelphia, PA, and director of its Division of Maternal-Fetal Medicine, says:
"The thinking was that exercise releases norepinephrine in the body, which is a chemical that can stimulate contractions of the uterus, and thus lead to preterm birth."
He and his colleagues pooled and analyzed data from nine randomized controlled studies.
Overall, the analysis included 2,059 women: with about half assigned to an aerobic exercise group and half assigned to a control group.
The women in the exercise group did aerobic exercise for 35-90 minutes, three or four times a week for 10 weeks - or up until their delivery. The controls did no exercise.
No increased risk of preterm delivery
The results showed there was no significant difference between the exercise and control groups in terms of incidence of preterm delivery (before 37 completed weeks of gestation).
However, there was a higher incidence of vaginal delivery (73.6 percent versus 67.5 percent), and a significantly lower incidence of cesarean delivery (17.9 percent versus 22 percent) in the exercise group than in the control group.
Also, the researchers found a lower incidence of gestational diabetes and lower incidence of high blood pressure in the exercise group compared with the control group.
Looking at the babies, the researchers found no differences in low birthweight and average birthweight between the exercise group and the control group.
All the pregnant women in the analysis were carrying a single baby - no twins - and they were of normal weight to start with. There were also no health problems that prevented them from exercising.
Support current guidelines for exercise in pregnancy
Prof. Berghella says the results support the current guidelines from the American Congress of Obstetricians and Gynecologists (ACOG), which follows the Centers for Disease Control and Prevention (CDC) recommendation that pregnant women get at least 150 minutes of moderate-intensity aerobic activity every week.
However, he also acknowledges that "there are many reasons women pull back on exercise during pregnancy - discomfort, an increase in tiredness and feeling winded by low level exertion."
Aerobic exercise is activity that moves the large muscles of the body - such as those of the legs and arms - in a rhythmic way (for example as in swimming and walking).
Moderate intensity means the exercise raises heart rate and the body starts to sweat - "you can talk but you can't sing" is a useful way to think of it.
Examples of moderate-intensity aerobic activity include swimming, brisk walking, and general gardening (such as raking, weeding, or digging).
However, the ACOG guidelines also recommend that even if they are healthy, pregnant women check with their doctor or healthcare team member during early prenatal visits and get their advice on what kinds of exercise are safe and fit their needs.
A person can achieve 150 minutes a week by doing 30-minute aerobic workouts on 5 days of the week. It is just as effective to do 10-minute bursts of equivalent intensity activity two or three times every day.
"This paper reinforces that exercise is good for the mom and the baby and does not hold any increased risk of preterm birth."
Professor Vincenzo Berghella