The third trimester of pregnancy is when the demands of the fetus are greater due to rapid growth and development. Fetal growth is dependent upon blood supply of oxygen and nutrients from the mother via the placenta. Vigorous intensity exercise (defined as at least 70% of maximum heart rate or an exertion level at which conversation cannot be maintained) has been proposed to impact this blood supply thus raising concerns about the impact of more intense exercise on fetal growth. Low birth weight is of significant concern as this is the single most important predictor of neonatal morbidity and mortality. 

Given the popularity of exercise such as running, triathlon, high intensity interval training (HIIT), boot camp and CrossFit in Boulder County, pregnant women may be wondering about the risks and benefits for themselves and their developing child during pregnancy. 

A recent systematic review and meta-analysis (Beetham et al BMC Pregnancy and Childbirth 2019) investigated the effects of vigorous intensity exercise on birth weight. Nearly 8,000 low-risk women participated in various trials included in this analysis with exercise modalities including running, swimming, circuit training, interval training, weight lifting and plyometrics (most were cardiovascular exercise). No significant difference existed in birth weight for babies of mothers who participated in vigorous exercise versus those who did not. The authors concluded that vigorous intensity exercise does not appear to compromise birth outcomes for most low-risk pregnancies. It is important to note that the current body of evidence regarding high intensity resistance training and high intensity cardiovascular exercise (>90% of maximum heart rate) is still evolving.

To learn more about recommended prenatal exercise and the benefits of starting and exercise program during pregnancy, please contact the women’s health experts at MEND.

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