In women with a history of miscarriage, higher levels of physical activity were linked to a greater risk of very early pregnancy loss, according to a study published in Fertility and Sterility.
Highly active women had roughly twice the risk of very early pregnancy loss than those who were less active.
There was no association between physical activity and miscarriage risk in women with confirmed pregnancies.
Women with a history of miscarriage may benefit from dialing back activity when trying to get pregnant. That’s the take home message from a University of Massachusetts (UMass) Amherst study that found high levels of physical activity can increase the risk of subclinical pregnancy losses—a very early pregnancy loss, which often occurs before a woman even knows she’s pregnant.
In the study, which was published in the journal Fertility and Sterility, the researchers analyzed data from the Effects of Aspirin in Gestation and Reproduction (EAGeR) trial, which recruited women between 18 and 40 years old with one or two pregnancy losses who were trying to conceive from 2007 to 2011. Senior author Brian Whitcomb, Ph.D., associate professor of epidemiology at UMass Amherst’s School of Public Health and Health Sciences, who was working at the National Institutes of Health at the time, was involved in the original four-year trial.
The researchers monitored the study participants using a combination of home pregnancy tests and laboratory screenings to verify when pregnancy occurred based on a rise in the hormone hCG, one of the earliest indicators of pregnancy. Among 785 women who became pregnant, 188 (23.9 percent) experienced pregnancy loss, including 55 subclinical losses that were detected only from hCG testing.
The researchers then compared the pregnancy outcomes with the amount of exercise the women were doing based on physical activity questionnaires. They defined exercise levels in terms of MET (metabolic equivalent) exercise hours per week. METs are a measurement of how much energy you expend during any given activity. For instance, sitting on your couch is a 1 MET activity. Cycling at a moderate effort (about 13 mph) is considered an 8 MET activity. The more exercise you do, especially at higher intensities, the more MET hours you accumulate.
Women in the middle and highest exercise groups—accumulating 27.8 and 95.7 METs per week—had a two-fold risk for very early pregnancy loss compared to women in the lowest exercise group, who exercised an average of 7.7 MET hours per week.
“Among women in the lowest third based on METs, risk of a subclinical pregnancy loss was 4 percent and total risk of pregnancy loss—subclinical and miscarriage—was 23 percent,” said Whitcomb, who is also a cyclist. “The middle and top third based on METs were similar [to each other] with respect to risk—8 or 9 percent for subclinical losses and 25 or 24 percent percent for total risk.”
Doing the math, 28 to 96 MET hours per week is the equivalent of riding 3.5 to 12 hours per week at a moderate clip. An average of about 8 MET hours per week would obviously be dramatically less—only an hour of exercise at that level, or about two and a half hours of very easy pedaling per week.
Importantly, there was no relationship between physical activity at any level and “clinically recognized loss,” a miscarriage once the woman has been confirmed as pregnant by means beyond the hCG test, such as through a very early ultrasound and/or the detection of a fetal heartbeat.
“Our results are consistent with the idea that physical activity may increase risk of pregnancy failure shortly after implantation, which may be a time of greater vulnerability compared to later in pregnancy,” Whitcomb said.
Subclinical pregnancy loss can be difficult to detect because they can only be detected through a home or laboratory test—a woman hasn’t missed a period yet, so unless she’s testing herself regularly, she likely doesn’t even know she’s pregnant. Though exercise is generally good for health, it is a form of stress, and the researchers wanted to see how this stress impacts this early period of implantation and pregnancy.
“We were able to address an interesting gap in the literature, where there has been conflicting evidence of whether physical activity can have a negative, beneficial, or no effect on pregnancy,” Whitcomb said in a press release.
The researchers say their study suggests that women who have lost a pregnancy and/or have been having difficultly conceiving may want to avoid high levels of physical activity in the earliest stage of a subsequent pregnancy, or around the time of trying to become pregnant.
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Otherwise, there is no reason for women with no known history of fertility problems or pregnancy loss to stop or reduce their physical activity. Lead author Lindsey Russo, Ph.D.(c), of UMass Amherst’s School of Public Health and Health Sciences, noted that the American College of Obstetricians and Gynecologists advises that “women with uncomplicated pregnancies should be encouraged to engage in aerobic and strength-conditioning exercises before, during, and after pregnancy.”
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