Can I do HIIT while pregnant?

The verdict

If you were already doing it, usually yes. If you were not, start with something else and build.

ACOG's Committee Opinion 804 is the clearest statement available: women who habitually engaged in vigorous-intensity aerobic activity or who were physically active before pregnancy can continue these activities during pregnancy and the postpartum period. It adds that physical activity and exercise in pregnancy are associated with minimal risks and have been shown to benefit most women, although some modification may be necessary.

The HSE in Ireland says the same thing in plainer words: it is usually safe to continue regular highly vigorous activity as long as you have an uncomplicated pregnancy, and you can adjust and slow down as you need. It adds that you should talk to your doctor about what is recommended for you.

The NHS approaches it from the other direction: if you were not active before you got pregnant, do not suddenly take up strenuous exercise, and if you start an aerobic exercise programme such as running, swimming, cycling or aerobics classes, tell the instructor that you are pregnant.

How hard is too hard

ACOG's Committee Opinion 804 addresses this directly and honestly, including where the evidence runs out.

It states that vigorous-intensity exercise completed into the third trimester appears to be safe for most healthy pregnancies. It then says further research is needed on the effects of vigorous-intensity exercise in the first and second trimesters and of exercise intensity exceeding 90 per cent of maximum heart rate.

It goes further for athletes: more data are needed from athletes who may exert beyond the accepted vigorous definition of up to 85 per cent of capacity, and it is possible that there is an absolute level of intensity or duration which, if exceeded, could place the fetus at risk. Individualised exercise prescriptions may be warranted in pregnant athletes.

That is a guideline saying it does not know where the ceiling is. It is the most useful sentence on this page, because it means an honest answer for someone training near their limit is a conversation with a clinician, not a web page.

The practical intensity test

For everyone else, the tests the guidance gives are simple and require no equipment.

The NHS says that as a general rule you should be able to hold a conversation as you exercise when pregnant, and if you become breathless as you talk you are probably exercising too strenuously. Tommy's calls it the talk test: you should be able to have a conversation while exercising, and if talking makes you out of breath, slow down.

ACOG defines moderate intensity as moving enough to raise your heart rate and start sweating, where you can still talk normally but cannot sing.

Applied to HIIT, that test does something interesting. It does not rule out intervals, but it does mean the recovery periods have to be real recoveries, and it rules out the specific pattern of stacking intervals so densely that you never return to conversational breathing.

What actually needs modifying

The changes ACOG asks for are structural rather than about effort.

• Supine work. ACOG states that maintaining a supine position during exercise after 20 weeks may reduce venous return because of aortocaval compression from the growing uterus, leading to low blood pressure. The NHS advises against lying flat on your back for long periods particularly after 16 weeks. That affects bench work, floor work and any interval spent lying down.
• Joints. Pregnancy hormones relax the ligaments supporting your joints, making them more mobile and at risk of injury, so jerky, bouncy and high-impact motions carry more risk. Box jumps, burpees and heavy plyometrics are the obvious candidates for substitution.
• Balance. Your shifted centre of gravity makes you less stable and more likely to fall, which is worth thinking about for overhead work, rope climbs and anything performed at height or on one leg.
• Heat. ACOG advises avoiding becoming overheated, especially in the first trimester: drink plenty of water, wear loose-fitting clothing and exercise in a temperature-controlled room, and do not exercise outside when it is very hot or humid.
• Abdominal contact. ACOG says contact activities with a high risk of abdominal trauma or imbalance should be avoided, which is the relevant rule for partner work and crowded classes.

What the evidence says about the baby

This is the part people worry about and the part where Committee Opinion 804 has real data rather than inference.

It reports that studies have shown minimum to moderate increases in fetal heart rate of 10 to 30 beats per minute over baseline during or after exercise, and that three meta-analyses concluded differences in birth weight were minimal to none in women who exercised during pregnancy compared with controls.

It notes one qualification: women who continued to exercise vigorously during the third trimester were more likely to deliver infants weighing 200 to 400 grams less than comparable controls, although there was not an increased risk of fetal growth restriction. And it cites a cohort study in which 30 minutes of strenuous exercise in the second trimester was well tolerated by women and fetuses in both active and inactive pregnancies.

Fuel and the thing athletes get wrong

ACOG makes a point about competitive athletes that applies to anyone training hard: such athletes should pay particular attention to avoiding hyperthermia, maintaining proper hydration, and sustaining adequate caloric intake to prevent weight loss, which may adversely affect fetal growth.

HSE Ireland adds the simpler version: do not exercise on an empty stomach, especially for long or intense sessions. It also advises rolling to your side and sitting up if you get dizzy or lightheaded while lying on your back.

When to stop

The HSE in Ireland publishes the clearest stop list of any body. Phone your GP, midwife or obstetrician immediately if you have shortness of breath before or during exercise, chest pain or palpitations, have been feeling faint or dizzy, have regular contractions or early labour, have severe pain in the abdominal, back or pubic areas, or notice reduced movement of your baby.

Tell your coach you are pregnant and how many weeks. The NHS advises making sure any instructor is properly qualified, knows you are pregnant and knows how many weeks pregnant you are.

The short version

Continuing is supported. Starting from scratch is not. Keep the recovery intervals long enough to talk, get off your back after 20 weeks, swap the plyometrics, keep the room cool and eat properly. If you train above 85 per cent of capacity, ACOG's own position is that nobody knows where the ceiling is, and that is a conversation to have with your midwife rather than with a spreadsheet.