Today’s conventional wisdom is that healthy pregnant women can benefit from exercise. In fact, the new exercise guidelines from the U.S. Department of Health and Human Services recommend that expectant mothers should try to exercise at a moderate intensity for at least two and a half hours throughout the week, and after having their baby. Women who already exercise and are active can maintain that during and after pregnancy, as long as they stay healthy and check in with their doctor to see if their regimen needs tweaking.

PregnantBut a new study suggests that high levels of activity may increase the risk of pre-eclampsia, a condition affecting pregnant women that’s marked by high blood pressure and protein in the urine and can ultimately cause death to mother or baby. It can also cause symptoms such as headaches, swelling and weight gain. The study looked at the leisure-time physical activity of 85,139 pregnant women in Denmark, assessing the time spent exercising, what type of exercises the women did, and at what intensity.

Among the findings, women who did moderate-level activities for long durations had an elevated risk of severe pre-eclampsia, but there was no correlation between low levels of vigorous activity only (from one to 269 minutes per week) and having the condition.

Severe pre-eclampsia risk was also higher for women whose vigorous activities lasted more than 270 minutes per week, compared with women who did no vigorous activity.

Exercise intensity levels were measured using metabolic equivalents, or the ratio of someone’s working metabolic rate compared with their resting metabolic rate. Researchers noticed a greater severe pre-eclampsia risk among women who came in at more than 40 Metabolic Equivalent Task hours per week, but saw a reduced risk among those in the range of 30 to 40 MET hours per week. Researchers estimated that about 40 MET hours per week was the equivalent of one hour of bicycling or 50 minutes of jogging per day. In addition, specific exercises were examined, with high levels of bicycling or brisk walking — more than 270 minutes per week — linked with a greater risk of severe pre-eclampsia.

In the study, published online this month in the British Journal of Obstetrics and Gynaecology, researchers wrote, "We were not able to confirm that increasing leisure time physical activity in early pregnancy protects against pre-eclampsia. In fact, our data even suggest that leisure time physical activity exceeding 270 minutes/week in first trimester may increase the risk of severe pre-eclampsia."