Regular exercise during pregnancy can help women and their newborns, but the benefit may depend on the intensity and timing of the exercise regimen.
Previous studies have shown that women who exercise throughout their pregnancies have larger placentas than their more sedentary peers, and that a more intense exercise regimen may result in babies who weigh less at birth. The volume of the placenta is a general marker of the structure's ability to transport oxygen and nutrients to the fetus.
Researchers from Case Western Reserve University in Cleveland, Ohio found that women who exercised at the same rate throughout their pregnancy, or boosted the intensity of their exercise regimen later in pregnancy, gave birth to infants who were lighter and had less body fat than women who slowed down their exercise in the final trimester.
However, none of the babies born to women who exercised moderately or intensely through the ninth month were considered low birth weight. (Low birth weight is associated with developmental problems in childhood and may be linked to heart disease and other health problems later in life.)
The study assigned 75 women who exercised regularly to one of three aerobic exercise programs during their eighth week of pregnancy. Women worked out on a treadmill, took step aerobics or used a stair-stepping machine. Women in the "Lo-Hi" group did 20 minutes of aerobic exercise 5 days a week and increased the duration to 60 minutes by the 24th week of pregnancy until their delivery.
Women in the "Hi-Lo" group exercised for 60 minutes 5 days a week through week 20 and then decreased the duration to 20 minutes from week 24 until the end of the pregnancy.
In the third group, women exercised for 40 minutes 5 days a week throughout their pregnancy.
Infants of women who exercised intensely in the middle or late stages of pregnancy were significantly lighter and had less body fat than infants born to the moderate exercisers, although none of the infants were small enough to be at risk for medical or developmental problems.
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