Let’s recount the events you cite:
In October 2011, a 27-year-old woman ran the Chicago Marathon while 39 weeks pregnant. After 6 hours and 25 minutes, she crossed the finish line, grabbed food, then went straight to the hospital where she delivered a healthy baby weighing 7 pounds, 13 ounces.
Then on May 29 this year, a 44-year old woman ran the Tenzing Hillary Everest Marathon while three months pregnant with her fourth child. She was the first woman to cross the finish line, winning the race in 6:02:10. That race starts at 17,598 feet.
Additionally, a week later, a Michigan woman went on a 10-mile training run and ended her day giving birth. She didn’t even know she was pregnant. Her baby girl was born five weeks early at 6 pounds, 6 ounces, and was considered healthy.
Every time one of these stories comes up, people wonder weather or not the mother could’ve harmed her baby by running during pregnancy. The New York Times covered the topic in 2007, concluding that even doctors don’t know the appropriate level of activity to maintain during their own pregnancies.
We caught up with Dr. Jim Pivarnik, Director of the Center for Physical Activity and Health at Michigan State University, to see if knowledge about staying fit during pregnancy has evolved in the past six years. Surprisingly, he says it hasn’t shifted much. The same issue hindering progress a half-decade ago still exists: researchers can’t ethically make pregnant women train hard to see how it affects their babies.
That leaves us with conflicting scientific opinions. For example, one small paper, Pivarnik says, found no link between vigorous exercise and miscarriage. But plenty of other studies have associated “high impact exercise” during the first 18 weeks of pregnancy with miscarriage.
Findings like these make it tough to establish definite rules. “I would be amazed if we ever got to the point in my lifetime where we said it’s OK to run marathons when you’re pregnant,” Pivarnik says.
That said, one popular guideline stating that pregnant women must keep their heart rates under 140 beats per minute has been overturned. “It’s from 1985, and it’s not there anymore,” Pivarnik says. Other than that, there's only one rule currently in place: Women must be evaluated on a case-by-case basis.
“We do tell women if they’ve been doing relatively active things, certainly you can continue it as long as it doesn’t start to be uncomfortable or bother you,” Pivarnik says. “Nausea, dizziness, pain—if none of that comes up, it should be OK.”
Staying active during pregnancy (general guidelines advocate walking for 30 minutes, five times per week) is extremely important. “A lot of times it helps make labor and delivery easier. There’s risk prevention of gestational diabetes, some hypertensive disorders,” Pivarnik says. “There’s some research done that says women who continue to exercise during pregnancy will be more likely to continue it during postpartum, and therefore drop the pregnancy weight gain easier.”
THE BOTTOM LINE: There are no hard and fast guidelines establishing what exercise is and is not appropriate during pregnancy. Generally, active women who don’t experience any adverse symptoms may be able to continue their training through pregnancy without harming their babies. But make sure to run your training and racing plans by your doctor so she can monitor you through your pregnancy.