The Fertility Signal Hiding Between Your Heartbeats

In a 2018 study published in PLOS One, researchers tracked the heart rate variability of 180 women undergoing 261 cycles of in vitro fertilization. They measured each woman at four points: the start of menstruation, the day of the trigger shot, the day before embryo transfer, and the day after. The women who did not get pregnant had lower HRV at every single timepoint.

That is a striking pattern. And it is part of a much larger conversation happening quietly in reproductive medicine, one that connects the rhythm of your heartbeat to the rhythm of your cycle, the function of your ovaries, and how your body actually handles the physiological cost of trying to conceive.

Heart rate variability, or HRV, is the variation in time between consecutive heartbeats. It is not a measure of how fast your heart is beating; it is a measure of how flexibly it adjusts. Higher HRV generally signals a well-regulated autonomic nervous system, the body’s automatic control center for everything from breathing to digestion to hormone signaling. Lower HRV suggests that system is under strain. Over the last decade, researchers studying fertility have started asking a question that, until recently, was almost impossible to answer with any precision: does the state of that system actually affect whether and how someone gets pregnant? The early evidence is suggestive, sometimes striking, and worth understanding, both for what it can tell you and for what it can’t.

Here is what is known, what is being studied, and what to make of the increasingly common claim that the wearable on your wrist can give you fertility-relevant information.

HRV is one of the most useful windows we have into autonomic nervous system function. That system has two branches: the sympathetic (the “fight or flight” branch, which speeds you up) and the parasympathetic (the “rest and digest” branch, which calms you down). They are not opposed so much as in constant negotiation. When you exhale, your heart rate slows. When you inhale, it speeds up. That moment-to-moment dance is your vagus nerve, the major highway of parasympathetic signaling, doing its job. HRV captures it.

Researchers have been using HRV as a biomarker for autonomic function for decades, in everything from heart disease to PTSD to athletic recovery. What is newer is its application to reproductive medicine, where it is emerging as a sensitive, non-invasive way to detect shifts that hormones, stress, inflammation, and overall physiological load are making to your body before any of them show up in a standard fertility workup.

If you have ever felt like your body has phases, your HRV agrees.

Multiple studies have now mapped a consistent pattern across the menstrual cycle. In the follicular phase, when estrogen is rising, parasympathetic activity is higher and HRV tends to follow. Around ovulation, vagally-mediated HRV drops noticeably. Through the luteal phase, as progesterone rises, HRV stays lower while resting heart rate ticks up. A 2017 paper documented this autonomic shift across cycle phases in detail.

The most provocative finding came in 2020, when researchers publishing in Scientific Reports showed that ultradian (within-day) HRV rhythms anticipated 100% of LH surges in the women they studied. In other words, the cardiovascular system was telegraphing the hormonal event that triggers ovulation, hours before it actually happened.

That is not the same thing as saying HRV is a substitute for an ovulation predictor kit. Some studies, including a widely cited 2003 paper, have found cyclic hormone variation that did not cleanly map to HRV changes. But the broader pattern is real, and it is part of why continuous HRV tracking has moved from sports science into reproductive science.

The IVF study mentioned at the top of this article is part of a small but growing body of evidence that lower HRV correlates with reproductive challenges most standard fertility workups don’t routinely measure. A 2014 study of 100 women with unexplained recurrent pregnancy loss found significantly lower HRV, specifically on standard deviation of normal-to-normal intervals (SDNN) and total power, compared with 61 healthy controls. The authors interpreted the finding as evidence of altered autonomic function in this group. An active clinical trial is now testing whether targeted interventions can shift that pattern.

The endometriosis literature is even more developed. A 2020 Scientific Reports study found that women with endometriosis exhibit reduced vagal activity compared with controls. A 2021 paper went further and showed that women with lower resting vagally-mediated HRV experienced more intense pelvic pain and a higher number of severe pain descriptors. PCOS researchers have started catching up too: a 2024 systematic review and meta-analysis documented altered autonomic drive in women with PCOS, with implications that extend beyond fertility into cardiovascular risk.

A pattern starts to emerge across these studies. Conditions that affect fertility, and the experience of trying to conceive itself, leave fingerprints on the autonomic nervous system. HRV picks them up.

Anyone who has spent time inside fertility care has heard some version of “stress can affect fertility.” Anyone who has actually been the patient has heard it as something closer to try to relax, which is, at best, useless advice when you are three years into TTC and budgeting for another IUI.

Here is the more honest version of what the science says. Chronic stress activates the hypothalamic-pituitary-adrenal axis. That axis shares regulatory territory with the hypothalamic-pituitary-gonadal axis, which governs your reproductive hormones. The mechanisms by which sustained stress can shift cycle length, ovulation quality, and even early pregnancy outcomes are real. But until very recently, the actual stress load a person was carrying was not measurable in any individualized, day-by-day way. Self-report questionnaires were the standard. They miss most of it.

HRV changes that. It picks up the cumulative physiological signature of stress, sleep debt, inflammation, recovery quality, and overall autonomic load. It is why elite athletes, military operators, and now fertility researchers all use it as a single, comparable signal of how the body is doing.

For patients, this matters in a specific way. It moves the conversation from “are you stressed?”, a question with no useful answer, to “is your body in a state where its reproductive systems are likely to perform well?” That is a more honest, more actionable question. And it is one you can now actually monitor.

The catch, and it is a big one, is that the HRV reading you get depends entirely on how it is measured.

Most consumer wearables, your watch and your ring, measure HRV using photoplethysmography, or PPG. A small green or infrared light shines through the skin, and the device infers your heartbeat from changes in blood flow. PPG works well for resting heart rate trends, but for the precise beat-to-beat interval data that HRV depends on, it is noisy. Motion, skin tone, body fat, hydration, sensor placement, and even tendon position all affect the signal. PPG-derived HRV is useful for personal trend-tracking, but it is not the kind of measurement the studies cited in this article are based on.

The research on HRV and fertility is almost universally done with ECG, the same technology used in cardiology. ECG measures the electrical activity of the heart directly through electrodes placed on the chest, producing the gold-standard beat-to-beat data the field relies on. Until recently, ECG-grade HRV monitoring outside of a clinic meant a Holter monitor or a research-only chest strap.

This is where it becomes useful to mention OTO Fertility, a clinical-grade physiological monitoring platform that uses a medical-grade chest strap and sensor, rather than a wrist wearable, to capture ECG-level HRV alongside two other modalities: ECG morphology and DC-EEG (a direct-current brain potential measurement). OTO’s hardware has been validated against gold-standard equipment, with deviations not exceeding 25 milliseconds in 95% of cases compared to Holter ECG and correlation coefficients above 0.96 against the Polar S810I, a research-grade reference monitor. The platform was originally developed for elite athlete performance monitoring and has been adapted for use in reproductive health settings.

The point isn’t that consumer wearables are useless. They are remarkable, and the fact that an Apple Watch or Oura ring can give you any HRV trend at all is genuinely useful for noticing your own patterns. The point is that if you are going to make health decisions in a fertility context based on an HRV signal, the precision of that signal matters. A noisy estimate of a real biomarker is still a noisy estimate.

It would be irresponsible to write all of the above without naming what is still unsettled.

The IVF predictive study is one cohort. It is well-designed and has been cited widely, but it has not been replicated at scale. The menstrual cycle research is more robust but still has contradictory findings around how strongly hormone changes drive HRV shifts. The recurrent pregnancy loss studies are observational; they tell us autonomic differences exist, not that fixing them fixes outcomes. The endometriosis and PCOS findings are correlational and do not yet point to a clear causal mechanism.

HRV is also influenced by an enormous number of things that have nothing to do with fertility: sleep, alcohol, illness, hydration, training load, age, body composition, and even how the strap or watch is positioned. A single low number on a single day doesn’t mean anything. The signal is in the trend, not the point.

What the evidence does support is this: HRV is one of the better non-invasive readouts of how the body is doing, and the body’s state matters for reproduction. The connection is real. The clinical applications are still being worked out.

For a long time, fertility care has been organized around what shows up on a panel: hormones, follicle counts, motility, morphology. Those numbers matter. They have also never been the whole picture. What is becoming clearer, slowly and with the appropriate caveats, is that the state of your nervous system is not separate from the state of your reproductive system. It is part of it.

You are not imagining it when your hardest weeks line up with your longest cycles. You are not making it up when the months you felt the most regulated felt different in your body, too. The signal is real. We are only now building the tools to see it clearly.

That doesn’t mean HRV is a fertility test. It isn’t, and the science doesn’t claim it is. What it is, increasingly, is a window into how your body is doing on a given day, and a way to take seriously what your physiology is telling you, instead of being told to relax.

That is a different conversation than the one fertility patients have been having for decades. It is a more honest one.

Key research cited in this article:

Yu et al., 2018. HRV among women undergoing IVF: predictive ability for pregnancy (PLOS One)

Periovulatory HRV decrease across the menstrual cycle (Biology, 2023)

Ultradian HRV rhythms anticipate LH surge (Scientific Reports, 2020)

Altered autonomic nervous system activity in women with unexplained recurrent pregnancy loss (2014)

Reduced vagal tone in women with endometriosis (Scientific Reports, 2020)

Vagally-mediated HRV and endometriosis pain severity (2021)

HRV in PCOS: systematic review and meta-analysis (2024)

HRV as a translational biomarker of autonomic function (2023)