Hit or Miss: HIIT During Pregnancy

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Pregnancy brings about remarkable transformations in a woman’s body to support and sustain the growth of a developing baby. Achieving a delicate balance between maintaining health, staying active, and ensuring adequate rest is essential during this time. These physiological changes, which are particularly relevant to physical activity, include an increased heart rate, a relative drop in hemoglobin levels, a subjective sense of breathlessness, lower blood pressure, increased lumbar lordosis, greater joint laxity, elevated cortisol levels, and an overall rise in metabolism.

While exercise has long been regarded as a cornerstone of a healthy pregnancy, the growing popularity of high-intensity interval training (HIIT) raises an important question: is this fast-paced workout suitable for expectant mothers? 

What is HIIT?

High-intensity interval training (HIIT) is a workout format that alternates short bursts of high-intensity activity with low-intensity recovery periods. Sessions typically last 15 to 30 minutes and include activities such as running, cycling, swimming, or resistance exercises⁽¹⁾. Its versatility and efficiency make HIIT appealing for those with limited time.

A standout benefit of HIIT is its ability to produce significant cardiovascular and muscular improvements in a short period. This efficiency helps pregnant women overcome barriers like time constraints and fatigue. Additionally, its adaptable structure accommodates varying fitness levels, ensuring accessibility⁽²⁾.

Why Exercise Matters

Staying active during pregnancy is more than a lifestyle choice - it’s a proven strategy for reducing maternal and fetal morbidity, including excessive weight gain, gestational diabetes mellitus, hypertensive disorders, preterm birth, and cesarean delivery^(3,4). Exercise also boosts mood, combats postpartum depression, and aids recovery by improving physical strength and pelvic health⁽⁵⁾.

Exercise is particularly vital for women with gestational diabetes mellitus. While moderate-intensity workouts are often recommended, recent research suggests that adding high-intensity elements can improve glycemic control and overall fitness⁽⁶⁾. The World Health Organization and American College of Obstetricians and Gynecologists advocate for at least 150 minutes of moderate-intensity exercise weekly, underscoring its importance during pregnancy⁽⁷⁾.

“Staying active during pregnancy is more than a lifestyle choice...”

HIIT Benefits

High-intensity interval training’s biggest draw is its efficiency. A six-week cycling-based HIIT program for women with gestational diabetes mellitus showed improved glucose levels, fitness, and attitudes toward exercise. Participants also reported a 96% compliance rate, emphasizing the practicality of home-based HIIT regimens^(6,8).

Therefore, HIIT increases workout enjoyment, a critical factor in sustaining long-term exercise habits. Researchers at The University of Western Australia compared continuous moderate-intensity cycling with interval-based cycling. They found that participants burned more calories and enjoyed the HIIT session more without experiencing additional physical discomfort⁽²⁾.

Beyond physical health, HIIT also offers psychological benefits. Regular physical activity, including HIIT, reduces stress, improves sleep quality, and enhances overall well-being. These factors are particularly valuable during pregnancy, often marked by emotional fluctuations and physical discomfort⁽⁹⁾.

Fetal Safety

Safety is paramount when considering exercise during pregnancy, and HIIT can be safe for the fetus with proper precautions:

• Staying within limits: Exercising below 90% of maternal heart rate maximum (HRmax) has no adverse effects on fetal heart rate or blood flow^(10,11). As a general guideline, pregnant women should be able to maintain a conversation during exercise to ensure they are not overexerting.
• Monitoring overexertion: Exceeding 90% HRmax may temporarily reduce uterine blood flow, potentially causing reversible fetal bradycardia. While these episodes of bradycardia appear transient, the long-term effects of repeated occurrences are not yet fully understood.  Careful monitoring is especially important for elite athletes⁽¹²⁾.
• Consideration for fetal growth restriction: In pregnancies where the baby shows signs of growth restriction, expert opinion advises a more cautious approach to high-intensity training. If placental blood flow is temporarily reduced during exercise, these babies may have fewer reserves to cope with the changes. 

For example, researchers at Queen’s University in Canada conducted a study to evaluate fetal response to a HIIT resistance circuit in the late second and early third trimesters of pregnancy. Fourteen healthy pregnant women completed a HIIT resistance circuit at 80-90% HRmax with no adverse effects on fetal well-being, supporting the safety of HIIT for women with uncomplicated pregnancies⁽¹⁰⁾. However, individualized assessment and careful monitoring remain essential for ensuring the safety of both mother and baby.

Breaking Barriers

Pregnant women often face barriers to regular exercise, including fatigue, time constraints, and limited access to facilities. HIIT addresses these challenges with short, flexible, and home-based options^(7,9). For example, a home-based cycling program for women with gestational diabetes mellitus improves fitness and glucose control while integrating seamlessly into daily routines. This adaptability underscores HIIT’s potential to empower women during pregnancy⁽⁷⁾.

“Pregnant women often face barriers to regular exercise...”

Safety Guidelines 

To ensure safety and effectiveness, pregnant mothers interested in HIIT should follow these guidelines:

1. Healthcare provider consultation: Discuss any exercise plans with the healthcare professional overseeing their pregnancy, particularly if they have underlying health conditions or (a history of) pregnancy complications. 
2. Monitor effort: Use tools like the Borg Rating of Perceived Exertion (RPE) scale, aiming for a range of 12-14 (moderate) to 15-17 (somewhat hard). Avoid exceeding 90% HRmax (see figure 1)^(10,13).
3. Stay cool and hydrated: Exercise in a well-ventilated environment, drink plenty of water, and avoid overheating. Pregnant women can safely engage in up to 35 minutes of exercise at 80%-90% of their maximum heart rate at 25°C without raising their body temperature excessively⁽¹⁴⁾. 
4. Avoid lying flat: After the first trimester, avoid exercises that require lying flat on your back to maintain optimal uterine blood flow⁽⁷⁾.
5. Special considerations for gestational diabetes mellitus: HIIT has proven effective in improving glucose levels and insulin sensitivity, reducing reliance on medication^(7,9).