Is HIIT safe during pregnancy?

What does the evidence say about HIIT workouts during pregnancy?

1. A study that put pregnant women through an 8-week HIIT program found no adverse effects on pregnancy development, delivery, or neonatal conditions, with some maternal and fetal health parameters improving or staying unchanged. 1
2. That same HIIT study used heart rate monitors, the Borg Rating of Perceived Exertion scale, and the talk test, alongside each participant's individually determined anaerobic threshold, to keep intensity appropriate and safe. 1
3. General guidance for higher-risk pregnancies still centers on moderate-intensity aerobic activity per WHO guidelines, with duration and intensity built around pregnancy-related symptoms, reflecting how much less vigorous-intensity activity has been studied compared with moderate activity. 2
4. Absolute contraindications to any vigorous exercise in pregnancy include gestational hypertension, preeclampsia, ruptured membranes, placenta previa, and preterm labor, and warning signs to stop mid-workout include vaginal bleeding, dizziness, calf pain or swelling, chest pain, and decreased fetal movement. 3
5. In a randomized trial, pregnant women who did three weekly 60-minute HIIT sessions, including 15 to 20 minutes at an average of 87 percent of their max heart rate, for 8 weeks had no negative impact on their pregnancy or delivery, with no cases of high blood pressure in the HIIT group. 4

Is HIIT workouts safe in each trimester?

• First trimester. HIIT studies have generally enrolled participants starting around 13 weeks, into the second trimester, rather than the first, so if you were already doing HIIT before pregnancy, ask your provider about continuing versus scaling back while you're both adjusting to early pregnancy.
• Second trimester. Most of the available HIIT research, including a trial where women trained at about 87 percent of their max heart rate three times a week for 8 weeks, was conducted in the second trimester and found no negative impact on the pregnancy or delivery.
• Third trimester. Absolute contraindications to any vigorous exercise, including preeclampsia, placenta previa, and preterm labor risk, become more relevant to screen for as pregnancy progresses, so a third-trimester HIIT session should still start with your provider confirming none of those apply to you.