HRV and Resting Heart Rate During Pregnancy: What Wearables Actually Show

Somewhere in the first trimester, your watch starts saying strange things. Heart rate variability, which you spent two years nudging upward, begins a slow slide. Resting heart rate climbs. Sleep scores wobble. Your recovery score looks like you have been overtraining, when some weeks you have barely trained at all.

For anyone who tracks their health closely, this is genuinely unsettling. It looks like your body is deteriorating. It is not. It is doing something remarkable, and the same shifts you are seeing on your wrist show up consistently in the wearable research on pregnancy, in the same direction, with the same magnitude, in study after study.

Here is what actually changes, why, how to read those numbers without panicking, and the one thing that happens before birth that almost nobody expects.

TL;DR: In healthy pregnancies, heart rate variability generally falls and resting heart rate generally rises as pregnancy progresses. In a 2022 study that tracked 18 women with a WHOOP strap from before conception through the postpartum period, daily HRV declined by an estimated 0.10 points per day and daily resting heart rate rose by an estimated 0.05 bpm per day — around 14 bpm across a full pregnancy — with physical activity moderating both effects. Then roughly 49 days before birth, both trends reversed: HRV began climbing and resting heart rate began falling, and that continued after delivery. A separate study using a sensor bracelet found the same broad patterns in heart rate and HRV, and a 2025 analysis of smartwatch data found HRV declining with elevated night-time stress linked to sleep disturbance. The practical takeaway: your pre-pregnancy baseline no longer applies. Track trends relative to your current pregnancy baseline, not to the numbers you had last year, and treat wearable data as context for how you feel rather than as a diagnostic.

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Why HRV falls and resting heart rate climbs in pregnancy

Pregnancy is a nine-month cardiovascular remodelling project. The heart is not just supporting one body any more, and the adaptations start early.

The first haemodynamic change is a rise in heart rate, beginning somewhere between two and five weeks of pregnancy and continuing well into the third trimester. Stroke volume — the amount of blood the heart pushes out per beat — increases a little later, once venous return rises and systemic vascular resistance falls, and it keeps climbing through the second trimester. By the third trimester, those indices change relatively little: the system has largely finished its adjustment. After delivery, volume loading drops rapidly and the heart works its way back toward its pre-pregnancy output, while some of the structural changes take considerably longer to resolve. Hunter and Robson, writing in the British Heart Journal, noted that the whole picture closely resembles the cardiovascular adaptations seen with endurance training. That is a useful framing: what you are watching is not decline, it is a specific, directed adaptation.

Two more things are happening that feed directly into your HRV number.

Autonomic balance shifts. HRV is a readout of the interplay between your parasympathetic (rest and digest) and sympathetic (fight or flight) branches. Pregnancy tilts that balance toward the sympathetic side, which lowers beat-to-beat variability even when nothing is wrong. Your nervous system is not becoming unhealthy; it is being deliberately reorganised to support a pregnancy.

Sleep architecture changes. Disrupted sleep, more night waking, more time in lighter stages, and rising night-time physiological stress all pull HRV down and heart rate up. A 2025 study that analysed smartwatch photoplethysmography data across 16 weeks of pregnancy found a decline in HRV and an increase in the rhythm-adjusted mean of heart rate, with elevated night-time stress linked specifically to sleep disturbance and daytime fatigue. Some of what looks like a recovery problem on your dashboard is a sleep problem, and some of that sleep problem is simply being pregnant.

What the wearable research actually shows

Three studies are worth knowing about, because together they describe the shape of the curve.

The WHOOP pregnancy study (2022). Eighteen healthy women wore a WHOOP strap before conception, throughout pregnancy and into postpartum, with data collected over an average of 406 days per participant. The findings: a steady decline in daily HRV and a steady rise in daily resting heart rate as pregnancy progressed, both highly statistically significant, and both moderated by how many minutes of activity the women logged. Activity minutes themselves fell from around 28 minutes a day pre-pregnancy to 14 minutes by the third trimester, which is itself worth knowing — the drop in training is often partly reactive to how the body feels, not just caution.

The sensor bracelet study (2023). Researchers at University Hospital Zurich followed 23 pregnant women using a wrist-worn sensor that recorded heart rate, HRV, breathing rate and skin temperature. They produced standard curves for each parameter and found several of them differed from the older literature — a reminder that much of what we think we know about pregnancy vitals comes from clinic measurements rather than continuous home data. They also found that reported emotions, including feeling stressed, tired or happy, were significantly associated with how the physiological parameters moved. Your subjective state and your wearable data are not independent variables, especially in pregnancy.

The circadian rhythm study (2025). Using smartwatch PPG data from free-living participants, researchers tracked heart rate and HRV circadian rhythms across pregnancy and found significant patterns over 16 weeks: HRV declining, heart rate's rhythm-adjusted mean rising, and night-time stress up. The authors framed HR and HRV rhythm parameters as potential biomarkers for pregnancy complications — work that is promising but early, and not something you should try to self-diagnose from.

The consistent summary across all three: HRV down, resting heart rate up, sleep more disrupted, and the pattern is normal rather than pathological in uncomplicated pregnancies.

The reversal that happens about seven weeks before birth

This is the detail from the WHOOP study that most people have never heard, and it is the reason a low HRV in the third trimester is not a reason to worry.

At approximately 49 days before birth — roughly seven weeks out — the trends flipped. Daily HRV began to increase steadily, and resting heart rate began to decrease steadily, regardless of activity level, and those changes continued into the postpartum period. The researchers described it as a rapid improvement in cardiovascular health before birth that had not previously been documented.

In practical terms: if you are tracking through a pregnancy, the last stretch is often when your numbers start to look better, not worse. Do not assume a rising HRV late in the third trimester means anything is wrong. It appears to be part of the normal trajectory. And if your numbers keep dropping right up to delivery, that is worth mentioning to your midwife or obstetrician rather than dismissing — it may be completely normal for you, but it is the kind of pattern a clinician should be aware of.

Stop comparing yourself to your pre-pregnancy baseline

The single most useful thing you can do with your wearable during pregnancy is change the comparison.

Your HRV from before pregnancy describes a different physiological system. Judging a third-trimester HRV against a pre-conception baseline is like comparing your heart rate during a hard interval session to your heart rate on the sofa — the number is real, the comparison is meaningless.

What to do instead:

• Build a new baseline. Give the app a few weeks of pregnancy data and track change relative to that. Most wearable platforms, including the health data your watch feeds into Century, work on rolling baselines rather than fixed lifetime values, which is exactly what you want here.
• Watch the trend, not the level. A gradual, smooth decline across weeks is the expected picture. A sudden drop from one week to the next, especially alongside feeling unwell, is the kind of thing worth attention.
• Treat absolute numbers as informative, not diagnostic. A resting heart rate in the mid-to-high 80s can be entirely normal late in pregnancy for someone whose pre-pregnancy rate was in the 50s. It is a big change and it looks alarming on a graph, but it is inside the expected range for the situation.
• Note the context. Activity minutes, sleep, stress and illness all shift these numbers, both in pregnancy and outside it. The same baseline-versus-daily-fluctuation logic that applies to your normal training applies here, with a pregnancy-sized multiplier.

What is actually useful to track while pregnant

The pattern here is that trends matter and single days do not. If you take one thing from the wearable literature on pregnancy, it is that the absolute values are expected to move a lot and the direction of travel is expected to be predictable.

Exercising in pregnancy without overreaching

Physical activity is the one input in this equation you can influence, and the evidence supports it. ACOG's committee opinion on physical activity and exercise during pregnancy and the postpartum period states that, in the absence of obstetric or medical complications, activity in pregnancy is safe and desirable, and that women who were physically active before pregnancy can generally continue, with some modification as the body changes. The benefits listed in the guidance include lower rates of gestational diabetes, fewer caesarean births and operative vaginal deliveries, faster postpartum recovery, and a role in preventing postpartum depressive disorders. WHO physical activity guidance similarly recommends at least 150 minutes of moderate-intensity activity per week for pregnant and postpartum women, in the absence of contraindications.

How to apply that with your watch on:

• Use the talk test, not a heart rate ceiling. Being able to hold a conversation is the simplest practical marker of moderate intensity, and it sidesteps the fact that your heart rate response to exercise changes across pregnancy. Our Zone 2 talk test guide covers the mechanics in detail.
• Expect higher heart rates for the same effort. If you were running at 140 bpm before pregnancy, the same pace may now put you at 155. That is not automatically a problem, but the percentage-based zones you set last year no longer line up with your physiology. Recalibrate, or go by feel.
• Prioritise consistency over intensity. Twenty-five to thirty minutes of moderate movement most days beats two heroic sessions that leave you wrecked for three days.
• Modify as you go. As pregnancy progresses, most people need to reduce impact, avoid prolonged lying flat on the back after the first trimester, avoid breath-holding during heavy lifts, skip activities with a fall or abdominal-trauma risk, and pay more attention to hydration.
• Let recovery data inform the next day. If your HRV is falling faster than your recent baseline and your resting heart rate is up sharply, that is a reasonable signal to take an easy day. The menstrual cycle and recovery guide covers the same principle in a different hormonal context: your physiology has phases, and training load should respect them.

When a number is worth a phone call

Wearable data is not a diagnostic tool, and in pregnancy some symptoms need a clinician rather than an app. Obstetric guidance lists warning signs that mean stopping exercise and contacting your provider, including:

• Vaginal bleeding, or fluid leaking from the vagina
• Regular painful contractions
• Shortness of breath before you have started exerting yourself
• Dizziness, fainting, or a persistent headache
• Chest pain, or a racing or irregular heartbeat that comes with feeling unwell
• Calf pain or swelling, or muscle weakness that affects your balance

Separately, call your provider about any change in fetal movement, or anything else that feels wrong to you, whether or not your watch flagged it. Continuous monitoring is a helpful backdrop; it does not replace the midwife who knows your history.

And, as with anything in this category, what follows on your wrist is educational information, not medical advice. Talk to your own obstetric provider about what applies to your pregnancy.

Postpartum: what to expect from your numbers

The reversal that starts before birth continues after it. In the WHOOP cohort, the improvement in HRV and the fall in resting heart rate carried on through the postpartum period, but the recovery arc is long and interrupted by the single biggest confounder there is: sleep deprivation. Night feeds, broken sleep and the general chaos of early parenthood will pull HRV down and heart rate up regardless of how fit you are.

The reasonable expectations:

• Do not assess your "true" postpartum baseline in the first weeks. You are measuring sleep deprivation as much as physiology.
• Once sleep stabilises, HRV and resting heart rate usually trend back toward pre-pregnancy values over months, not weeks.
• Postpartum cardiovascular and musculoskeletal recovery takes longer than most people expect. Return-to-exercise guidance generally starts with walking and breathing work and progresses over weeks. Your recovery tracking guide applies here with a much longer timeline.

Using the data without spiralling

One more caution, because it is a real phenomenon: watching a metric you cannot influence, every single day, is a fast route to anxiety. There is a name for the sleep version of it — orthosomnia, where preoccupation with sleep data worsens sleep — and the same trap exists for HRV in pregnancy. A number you check five times a day and interpret as a score for how your pregnancy is going is not informative. It is noise with emotional consequences.

The healthier pattern is a weekly review rather than a daily verdict. Once a week, look at the trend for resting heart rate, HRV, sleep and activity together. Ask whether the direction of travel makes sense for how far along you are. Then close the app. Century AI is built around exactly this idea — a daily health score and recovery score fed by the watch you already wear, with trends as the headline rather than single readings — because a smooth multi-week line is far more informative than today's number, especially during a nine-month physiological project.

Bottom line

Pregnancy moves your HRV down and your resting heart rate up, gradually and predictably, because your cardiovascular system is being rebuilt to support two bodies. The research is consistent on this, including a clear reversal in the final seven weeks before birth that almost nobody is told to expect. The numbers are not a verdict on your health, and they are certainly not comparable to the ones you had before conception.

Track trends against a fresh pregnancy baseline. Use the talk test rather than a heart rate ceiling to judge exercise intensity. Keep moving, sleep as well as the situation allows, and let your provider — not your watch — make the calls that need a clinician. What you are watching on your wrist is one of the more fascinating adaptations in human physiology, not a warning light.

Century AI helps you understand your body with a daily health score, recovery score, and sleep insights — using the watch you already wear.