New IOC Paper Weighs in on Training During Pregnancy
The International Olympic Committee has issued a detailed statement on the effects of training—both intensely and moderately—on specific aspects of fetal development and natal outcomes. The evidence statement was based on systematic research of the existing literature on pregnancy and exercise.. The group was able to draw conclusions on the effects of exercise on fetal heart rate, the risk of miscarriage, fetal growth and risk of preterm birth, among other outcomes.
For each section of the statement, available databases were searched for terms in various combinations, including pregnancy, exercise, leisure, recreational activity, postpartum, miscarriage, cesarean section, etc. The researchers looked for studies on PubMed, Embase, Cochrane, PEDro, Web of Science and SPORTDiscus.
Fetal heart rate (FHR)
Maternal exercise, regardless of intensity, triggers an increase in FHR; they report that on average this increase is by about 10 to 15 bpm. Prolonged high-intensity exercise, they note, could “compromise fetal well-being,” though this seems to be largely theoretical for now because in all of the studies they looked at, FHR returned to normal once the exercise ceased. They also found that HR decelerations were sometimes reported during exercise, these were “transitory” and only “rare and sporadic.” No newborn abnormalities related to these FHR changes were reported.
Incidents of miscarriage (with “early miscarriage” usually defined as occurring before week 22 of pregnancy), is greater than 10% in the general population, so not uncommon. Early miscarriages account for about 80% of miscarriages. Prevalence is tied to maternal age, with just 5 to 7% of women under age 35 and 22% of women over 40 experiencing miscarriage at any time during pregnancy.
While fertility treatment raises risk, the major cause of miscarriage is thought to be chromosomal abnormalities of the fetus. Female athletes may sustain a miscarriage, then, just as any woman. But does strenuous exercise increase the risk?
In a cohort of over 92,000 women, of whom 3,187 had experienced a miscarriage, 2,551 were interviewed about their exercise habits. The data were obtained either during pregnancy or after an early miscarriage. The risk of early miscarriage increased as the amount of exercise increased, in particular for women in their first trimester who exercised more than 7 hours per week compared to non-exercisers. High-impact exercise such as jogging and racquet sports was also associated with an increased risk of early miscarriage. However, there was no association between exercise and the risk of miscarriage after week 18.
High-impact or highly strenuous physical activity may then be associated with a higher risk of miscarriage during the “fetal implantation phase,” during which the blastocyst can still be flushed out of the uterus. This phase usually ends by day 24. Repetitive heavy lifting during the first trimester also might increase the risk for miscarriage. Accordingly, the IOC says elite athletes "may consider" limiting such activity.
A systematic review concluded that light to moderate intensity physical activity does not increase the risk of miscarriage and may perhaps decrease it.