During pregnancy, heart rate variability (HRV) usually falls while resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate rises, and in the last weeks before birth both start to swing back. These are normal changes, so a lower HRV than before you were pregnant is expected. HRV cannot tell you whether you or your baby are well, and warning signs always matter more than the number.
Why pregnancy changes HRV
Pregnancy changes the whole circulation. Blood volume grows and resting heart rate goes up. With a faster heartbeat, the gaps between beats have less room to vary, so HRV tends to fall.
Studies that measured HRV in the clinic point the same way. A 2022 systematic review found eight studies of HRV in healthy pregnancies (Sharifiheris and colleagues, 2022). In five of the seven that compared trimesters, time-domain measures such as SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN and RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD fell from the first trimester to the third. The balance of the frequency measures shifted in a way the authors read as more sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system (“alert” branch) activity toward the end of pregnancy. The studies used very different methods, with recordings from 5 minutes to 24 hours, so the authors could not yet say what a normal pattern looks like.
What wearable studies show
Wearables can follow the same person every night, from before conception to after the birth. Three studies did this with a strap or a ring.
18 active women with a WHOOP strap. Researchers in West Virginia followed women who exercised at least three times a week, from before conception to after birth, for an average of 406 days (Rowan and colleagues, 2022). Nightly HRV fell and resting heart rate rose steadily through pregnancy. About 7 weeks before birth this reversed: HRV climbed until about a week after delivery and then levelled off. Resting heart rate fell and was back to pre-pregnancy levels about three weeks after birth. More daily activity went with higher HRV. Many of the women were recruited from a fertility clinic, so they may not be typical.
Over 5,000 term births with an Oura ring. A study by Oura employees looked at 10,318 pregnancies in Oura users, including 5,039 that ended in a birth at term (Adaimi and colleagues, 2025). Heart rate rose through pregnancy and peaked about 10 beats a minute above the pre-pregnancy level around week 32. HRV mirrored it, falling more than 15 ms below the pre-pregnancy level at its lowest. A first dip showed up as early as week 5. Around birth, heart rate dropped and HRV rose, getting close to pre-pregnancy levels. The study had no medical records, and key details such as the date of birth were self-reported.
241 pregnancies with a WHOOP strap. A study by WHOOP staff and the West Virginia team looked at nightly HRV in 220 term and 21 preterm births (Jasinski and colleagues, 2024). In both groups, the HRV trend lined up better with the weeks left until birth than with the week of pregnancy. The late rise seemed to track the coming birth, even when the birth came early. The authors said the data were too few to predict preterm birth for any one person.
The pattern is not the same for everyone. A small study of 13 healthy pregnant Latina women wearing an Oura ring from about week 22 to a week after birth found HRV rose over that time (Sharifi-Heris and colleagues, 2023). Because it started in mid-pregnancy, it could not show the earlier fall seen in other studies.
Go deeper How to read these studies
- Small or self-selected groups. The strap studies had 18 and 241 people, and the larger studies drew on people who already owned the device. Two were written by people who work for the device maker.
- Different devices. WHOOP and Oura both report RMSSD from a light sensor during sleep, but their numbers are not interchangeable.
- Averages, not rules. “More than 15 ms” is an average change across thousands of people. Your own drop could be bigger or smaller.
- Not a test. None of these studies shows that HRV can screen for pregnancy complications in one person.
After birth
In the first weeks after delivery, HRV and resting heart rate tend to move back toward where they were before pregnancy. In the Oura study, sleep was badly broken after birth: time awake at night rose by about 90 minutes and sleep fell by about 70 minutes compared with before pregnancy (Adaimi and colleagues, 2025). Short, broken nights make nightly HRV noisier, so expect a jumpy trend for a while.
How to read your own data
- Expect a lower number. A gradual fall during pregnancy is the usual pattern. It does not mean poor recovery or that something is wrong.
- Use a new baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline . Your pre-pregnancy average is the wrong yardstick. Compare this week with the last few weeks of pregnancy. The lesson on building your HRV baseline explains how.
- Watch the trend, not single nights. Night feeds, heartburn and getting up to pee all disturb sleep and move nightly numbers. Weekly averages are steadier.
- Look at heart rate too. A rising resting heart rate is part of the normal picture. See HRV vs resting heart rate.
- Ease off if tracking worries you. You can stop checking for a while. Your care does not depend on the number.
When to call a midwife or doctor
HRV cannot check on you or your baby, and a normal reading never rules out a problem. The US Centers for Disease Control and Prevention lists these urgent warning signs during pregnancy and in the year after birth (CDC). Get medical care straight away if you have:
- a headache that won’t go away or gets worse, dizziness or fainting, or changes in your vision
- a fever of 38 °C (100.4 °F) or higher
- extreme swelling of your hands or face
- trouble breathing, chest pain or a fast-beating heart
- severe nausea and vomiting, or severe belly pain that doesn’t go away
- your baby moving less, or stopping moving, during pregnancy
- vaginal bleeding or leaking fluid during pregnancy, or bleeding or discharge after birth
- severe swelling, redness or pain in a leg or arm
- overwhelming tiredness
- thoughts of harming yourself or your baby
If something feels wrong and you are not sure whether it is serious, contact your midwife or doctor anyway, and tell them you are pregnant or recently gave birth.