How HRV trends may help identify preterm births during pregnancy

Originally published on November 10, 2022

Heart rate variability, or HRV, may offer an earlier signal that a pregnancy is tracking toward preterm delivery. In Episode 197 of the WHOOP Podcast, Emily Capodilupo, Senior Vice President of Research, Algorithms, and Data at WHOOP, and Dr. Patrick Carroll, Chief Medical Officer at WHOOP, explain how WHOOP researchers analyzed 1,200 pregnancies and found that HRV and resting heart rate tend to reverse course about seven weeks before delivery. This article breaks down what that inflection point means, why timing matters for follow-up care, and how WHOOP Pregnancy Insights can help members put normal physiological changes into context.

To listen to episode 197 in full, head to the WHOOP Podcast on Spotify.

What did WHOOP find about HRV and preterm birth?

WHOOP researchers found that HRV usually trends downward through most of pregnancy, then turns upward about seven weeks before delivery. In pregnancies that end preterm, that turn appears earlier, which means the timing of the inflection may help flag risk before labor begins.

Capodilupo says the pattern became clear when WHOOP researchers reviewed 1,200 pregnancies and compared people who delivered before 37 weeks with those who delivered at full term. The same overall pattern also showed up in resting heart rate, which tends to rise through pregnancy and then start falling near delivery. WHOOP summarized the finding in its pregnancy digital biomarker research announcement.

As Capodilupo puts it:

“What we were able to see was that this inflection point actually has nothing to do with being 33 weeks, although that’s the most common place to see it, and everything to do with being 7 weeks away from delivery.”

What you should take away

• WHOOP researchers observed an HRV inflection point that appeared about seven weeks before delivery.
• Earlier HRV inflection points were associated with pregnancies that ended preterm.
• Resting heart rate moved in the opposite direction, rising through most of pregnancy and then falling near delivery.

If you want to hear Capodilupo unpack how the seven-week signal emerged from the data, listen to the full episode on Spotify.

How did WHOOP research show that the inflection point tracks time to delivery?

That first finding led to a deeper question: was the signal tied to a specific week of gestation, or to the number of weeks left before birth? WHOOP could answer that because the team had far more pregnancy outcomes than the earlier academic study.

A West Virginia University School of Medicine study led by Dr. Shon Rowan followed 18 women from preconception through postpartum. All 18 delivered full term, so the original inflection appeared around week 33. WHOOP then surveyed members and identified 1,200 pregnancies, which made it possible to separate pregnancies ending at different times and test whether the signal was counting forward from conception or backward from delivery.

Capodilupo explains the value of that larger dataset with one statistic:

“About 8.4% of our dataset was premature deliveries.”

What you should take away

• The original academic signal came from 18 full-term pregnancies, so it could not test preterm timing.
• The WHOOP dataset of 1,200 pregnancies allowed researchers to compare full-term and preterm deliveries directly.
• The inflection point aligned with time remaining until delivery, not simply with reaching week 33.

If you want to hear Capodilupo go deeper on why scale changed the research question, listen to the full episode on Spotify.

Why can an earlier HRV inflection point matter for preterm birth care?

An earlier inflection point does not mean labor can be prevented, but it may create time for extra screening and preparation. Carroll describes the signal as a screener that can help prompt a provider conversation before delivery becomes urgent.

That matters during preterm birth because fetal lungs are among the last systems to develop. Carroll says that if a provider confirms elevated risk, follow-up tools such as cervical length assessment or a cervical swab can help estimate whether preterm labor is likely in the next two to four weeks. He also notes that antenatal corticosteroids given to the mother can support fetal lung maturity before birth.

Carroll frames the value this way:

“So what we look at it as, it’s a screener that can actually lead to some interventions and actions.”

What you should take away

• WHOOP research points to a screening signal, not a diagnosis.
• An earlier inflection point may justify follow-up with an obstetric provider and added screening.
• Carroll says screening can create time for interventions such as antenatal corticosteroids that support fetal lung maturity.

For Carroll’s full take on why earlier screening can change preterm birth care, listen to the full episode on Spotify.

How can WHOOP help pregnant members interpret HRV and resting heart rate?

WHOOP can help show whether HRV and resting heart rate are changing in ways that match expected pregnancy patterns for your baseline. That context can make Recovery and nightly vitals easier to interpret when normal pregnancy physiology would otherwise look unusual.

Capodilupo says that seeing her own data match the larger pregnancy trend reduced uncertainty during her first pregnancy. WHOOP built that research into Pregnancy Insights, which gives pregnant members weekly trend views for HRV and resting heart rate. WHOOP also added reproductive health features across life stages in this overview of reproductive health tracking.

Capodilupo describes the reassurance clearly:

“Seeing my data do exactly what that median of those 1,200 pregnancies was doing, I think just made the whole process a little bit less scary.”

What you should take away

• WHOOP can place pregnancy-related HRV and resting heart rate changes in baseline-based context.
• Pregnancy Insights is designed to show trend direction over time, not just isolated daily readings.
• Members should bring unusual trends to a provider rather than treat app data as a diagnosis.

If you want to hear Capodilupo go deeper on how Pregnancy Insights was shaped by her own experience, listen to the full episode on Spotify.

Why is wearable pregnancy research useful beyond individual pregnancies?

Beyond one pregnancy, the bigger value is scale, continuity, and access. Continuous, noninvasive tracking makes it possible to study physiological changes that are hard to capture in traditional research, especially in a field where pregnancy data has been limited.

WHOOP has already published related work on pregnancy exercise, HRV, and resting heart rate and on a West Virginia University pregnancy study using WHOOP. Carroll also points to a care access problem outside the data itself. He cites a March of Dimes report on maternity care deserts showing that 36% of U.S. counties do not have facilities or providers for deliveries.

Carroll highlights the research model in one line:

“This is a non-invasive study, right? It’s a wearable.”

What you should take away

• Wearable pregnancy research can collect continuous data without invasive testing.
• Large opt-in datasets can answer questions that smaller academic studies often cannot reach.
• Carroll says maternity care deserts make earlier screening signals more useful for planning care and delivery location.

The bottom line

• WHOOP research found that HRV and resting heart rate often reverse course about seven weeks before delivery.
• Earlier HRV inflection points may help flag pregnancies that are tracking toward preterm birth.
• The WHOOP dataset of 1,200 pregnancies made it possible to separate time in pregnancy from time remaining until delivery.
• Carroll describes the HRV signal as a screener that may prompt follow-up testing and provider conversations.
• Pregnancy Insights can help members interpret normal shifts in HRV and resting heart rate against their own baseline.
• The current screening concept is most relevant to singleton pregnancies without certain complicating factors discussed in the episode.
• Continuous, noninvasive data collection gives pregnancy research a way to study physiology at a scale traditional methods rarely reach.

Frequently asked questions about things discussed in this episode

How does WHOOP measure HRV during pregnancy?

WHOOP measures HRV during sleep and compares it with your personal baseline, which makes pregnancy-related changes easier to interpret as trends over time.

What does WHOOP do for spotting possible preterm birth risk?

WHOOP can surface an earlier HRV and resting heart rate inflection pattern that appeared about seven weeks before delivery in this research, which may help prompt an earlier provider conversation.

What does WHOOP show pregnant members inside Pregnancy Insights?

WHOOP shows weekly HRV and resting heart rate trends relative to expected pregnancy patterns and your own baseline.

What does WHOOP recommend if the inflection point appears early?

WHOOP recommends bringing the trend to your obstetric provider, because the signal is intended to support follow-up care rather than replace clinical evaluation.

How does WHOOP research protect privacy in pregnancy studies?

WHOOP uses opt-in, de-identified data for this type of research, which allows large-scale analysis without exposing individual member identities.

What does WHOOP say about diagnosis during pregnancy?

WHOOP presents this pregnancy finding as an informational screening signal, and members should not use it as a substitute for professional medical advice, diagnosis, or treatment.

For pregnancy tracking, the clearest value of WHOOP here is seeing when HRV and resting heart rate are following expected patterns and when those trends deserve a provider conversation.