Target Heart Rate During Pregnancy

You are on a treadmill, and your wrist display reads 155. Your friend told you 140 bpm is the pregnancy limit. You slow to a walk, annoyed. Here is the truth: the 140 bpm rule is outdated, and both ACOG and the 2019 Canadian Guideline say you should ignore a specific number and focus on how you feel. The honest answer to what is a safe target heart rate pregnancy is that there is no single number. This explains what replaced the old rule and how to exercise safely.

For decades, pregnant women were told to keep their heart rate below 140 beats per minute. That number came from a 1990s guideline that has no evidence behind it. Both ACOG Committee Opinion No. 804 (2020) and the 2019 Canadian Guideline for Physical Activity throughout Pregnancy (Mottola et al., BJSM 2018) have eliminated it entirely.

The 2019 Canadian Guideline uses the 220, age formula to estimate maximum heart rate and defines moderate intensity as 64-76% of that number, vigorous intensity as 77-93%. For a 30-year-old, that gives a range of 122 to 175 bpm. ACOG 804 defines moderate intensity as 50-69% and vigorous as 70-90% of the same formula. These ranges overlap, but both are far wider than a flat 140 bpm limit.

Why a Single Number Never Worked

Resting heart rate increases by 10-20 beats per minute by the third trimester (ACOG Committee Opinion No. 804). A target of 140 bpm may be moderate for one woman at 20 weeks and nearly maximal for another at 36 weeks. The old rule did not account for this shift.

Both ACOG and the 2019 Canadian Guideline now recommend using the Rating of Perceived Exertion (RPE) scale rather than heart rate targets during pregnancy. The key instruction: you should be able to hold a conversation while exercising. If you can talk but not sing, you are at moderate intensity. If you cannot say more than a few words without gasping, you have crossed into vigorous intensity, which is safe only if you were active before pregnancy.

The 2019 Canadian Guideline defines moderate intensity as 13-14 on the Borg RPE scale (6-20). That corresponds to an effort level where breathing is heavier but not labored. The failure case: if you rely on a heart rate monitor alone, you might push into an intensity your body cannot sustain because your heart rate is artificially elevated by pregnancy hormones.

How to Apply the Talk Test

Say a sentence aloud during your workout. If you can recite it without stopping for air, increase the intensity. If you cannot finish it, back off. This works for walking, jogging, cycling, swimming, and strength training.

Resting heart rate increases by 10-20 bpm by the third trimester, according to ACOG Committee Opinion No. 804. This change alone makes any pre-pregnancy target heart rate zone invalid. A woman whose resting heart rate was 65 bpm before pregnancy may have a resting heart rate of 80 bpm at 36 weeks. The Karvonen formula, which calculates target heart rate from resting heart rate and max heart rate, would produce a different result for her than for a woman whose resting rate stayed at 65.

The practical implication: do not compare your heart rate during exercise to a number you used before pregnancy. That number is obsolete. Use the talk test instead.

Stop immediately and contact your provider if you experience any of these: vaginal bleeding, amniotic fluid leakage, chest pain, dizziness, headache that persists, calf pain or swelling, decreased fetal movement, or uterine contractions that persist after stopping. These apply regardless of your heart rate.

The failure mode: many women push through mild dizziness because they think their heart rate number is safe. Dizziness is not a sign to slow down; it is a sign to stop. ACOG Committee Opinion No. 804 states that exercise is safe for uncomplicated pregnancies, but it does not override your body's warning signals.

For an uncomplicated pregnancy, neither ACOG 804 nor the 2019 Canadian Guideline restricts exercise. But certain conditions require medical clearance before you continue: placenta previa, preeclampsia, preterm labor risk, cervical insufficiency, or multiple gestation (twins or more) with other complications. Your provider may also restrict exercise if you have a history of miscarriage or a condition that limits blood flow to the uterus.

The 2019 Canadian Guideline recommends 150 minutes of moderate-intensity physical activity per week for uncomplicated pregnancies. That is 30 minutes, five days a week. If your provider restricts activity, ask for an RPE-based alternative, such as walking at a pace where you can still talk.

Both ACOG 804 and the 2019 Canadian Guideline use the percent of maximum heart rate method with the 220, age formula, but only as a secondary reference. The primary method is RPE. The Canadian Guideline cites Mottola et al. in BJSM 2018 for its recommendation. ACOG 804 cites its own committee opinion.

How to Use Heart Rate Numbers If You Still Want Them

If you want a number, calculate your age-predicted max using 220, age. For moderate intensity, aim for 64-76% (Canadian Guideline) or 50-69% (ACOG). The two ranges differ because ACOG's definition of moderate is more conservative. The Canadian Guideline's vigorous range (77-93%) overlaps with ACOG's (70-90%). Do not exceed 90% of your age-predicted max without your provider's approval.

Remember that the 220, age formula has a standard deviation of 12 bpm. Your true max may be 12 bpm higher or lower than the formula predicts, so the number is an estimate, not a prescription.