Your Questions Answered
Exercise in Pregnancy: Can it affect the growth of my baby?
One of the questions we are commonly asked is whether or to what degree is it safe to exercise during pregnancy. Women often feel conflicted regarding their desire to exercise and remain fit versus their concern regarding possible adverse effects on fetal growth. Concern arises because, during exercise, blood is directed to muscles where oxygen is in more demand. Theoretically this could result in less blood flow to the placenta and impair fetal growth. Early medical studies done in animals seemed to support this and hard medical evidence in humans was lacking.
Recent research has helped to clarify the picture. While the evidence seems to support the idea that increasing levels of exercise can diminish fetal growth, the timing of the exercise, not just the intensity, seems to be important. A recent study done at Case Western Reserve University in Ohio compared 3 exercise regimens. One group exercised 40 minutes per day, 5 days per week until delivery. A second group started at 20 minutes per day then gradually increased to 60 minutes per day by 24 weeks of their pregnancy and continued at this level to term. The third group reversed this pattern, starting at 60 minutes per day and continuing at this level to 20 weeks; this was followed by a gradual decrease to 20 minutes per day by 24 weeks and continuing at this level to term. When the researchers looked at the weight of the newborns, the largest average weight was in the third group (high intensity early, low intensity later). While the weight differential was small (60-70 grams more than in the other groups), it does support this type of exercise regimen as the best one to optimize fetal growth.
Interestingly, they also looked at the estimated placental volumes determined by ultrasound. The largest placentas were seen in the third group as well. Other researchers have shown that vigorous exercise early in pregnancy maximizes an important early stage of placental growth (the larger, basic branches of the placental vascular tree). The later development of the tiny, fine branches that maximize the placental surface area is inhibited by the same levels of exercise. That is, decreased levels of exercise later in pregnancy seem to enhance fetal growth by allowing maximal placental growth.
It is important to note that these effects pertain only to weight-bearing exercise. For example, the women in the Case Western study used a treadmill, Stairmaster, or step aerobics. Other research suggests that non-weight bearing exercise such as swimming can be continued at higher levels to term without adversely affecting fetal growth.
So, yes, exercise in pregnancy can have effects on your baby’s growth. While the effect is small overall, if you are going to exercise regularly, and we very much wish you would, then attention should be paid to how it is done. If you do not presently exercise, it is safe to start at a low level with gradual increases. If you exercise avidly, it is prudent to decrease the intensity and duration during the last trimester. If you still have doubts, a basic guideline that would be safe in all aspects to follow would be to aim for 20-30 minutes of low-intensity exercise (e.g., walking 1-2 miles) 3-5 days per week throughout your pregnancy. It is good for both you and your baby too.