If you’ve ever sat in one of our ultrasound rooms and asked, “Can you tell if it’s a boy or a girl from the heartbeat?” — you’re far from alone. It’s one of the most commonly asked questions our sonographers hear during early pregnancy scans, and it’s rooted in a piece of pregnancy folklore that has circulated for generations: the fetal heartbeat myth.
The myth goes like this — a fetal heart rate of 140 beats per minute (bpm) or higher means you’re expecting a girl, while a heart rate below 140 bpm points to a boy. It’s a fun, low-stakes bit of pregnancy guesswork that many expectant parents have heard from a friend, a family member, or the internet.
But is there any truth to it?
That’s exactly what our team set out to investigate — and the results have just been published in Sonography, the journal of the Australasian Sonographers Association. The study, Testing the Fetal Heartbeat Myth: A Large Cohort Study Correlation With NIPT, was authored by our very own Senior Sonographer Sarah Dowthwaite and our Medical Director, Dr Karen Mizia.
Why This Study Matters
Fetal sex prediction based on heart rate has been examined before, but earlier studies had a common limitation: they typically confirmed the baby’s sex either through a later ultrasound (18–24 weeks) or at birth — both of which introduce their own margins of error. Ultrasound-based sex determination in the first trimester, for example, is only around 85–87% accurate, since it relies on subtle visual assessment of fetal anatomy.
Our study took a different approach. Instead of waiting for a later scan or the birth itself, we used sex determined by Non-Invasive Prenatal Testing (NIPT) — a blood test that analyses fetal DNA fragments circulating in the mother’s blood. NIPT identifies the baby’s sex chromosomes directly, with sensitivity and specificity both well above 98%, making it one of the most reliable ways to confirm fetal sex early in pregnancy. This makes our study the first to correlate first-trimester fetal heart rate with fetal sex determined specifically by NIPT.
What We Did
Between January and June 2024, our team reviewed the records of women attending for NIPT blood collection at our practices across Sydney. As part of the routine process, each woman had an ultrasound performed immediately beforehand, during which gestational age, crown-rump length, and fetal heart rate were recorded using a standardised protocol.
This study wouldn’t have been possible without the many sonographers across our practices who carefully and consistently captured these measurements as part of their everyday work. Their attention to detail throughout thousands of routine scans is what gave this research its strength.
After excluding pregnancies with multiples, chromosomal abnormalities, or missing data, our final study group included 1,969 pregnancies, all scanned between 10 weeks and 13 weeks 6 days of gestation — precisely the window in which the heartbeat myth is most often discussed with patients.
Of these pregnancies, 963 babies (48.9%) were confirmed female and 1,006 (51.1%) were confirmed male via NIPT.
What We Found
Across the entire study group, the average fetal heart rate for female fetuses was 165 bpm, compared to 166 bpm for male fetuses — a difference of just one beat per minute.
While this tiny difference was technically statistically significant due to the large sample size, it is not clinically meaningful. To put it plainly: knowing a baby’s heart rate tells you essentially nothing useful about whether that baby is a boy or a girl. A one-beat-per-minute average difference falls nowhere near a level that could reliably distinguish one pregnancy from another — particularly when individual fetal heart rates in our study ranged widely, from 104 bpm to 188 bpm, regardless of sex.
We also looked at the data week by week, from 10 to 13 weeks’ gestation, in case the relationship (if any) might be clearer at a specific stage. It wasn’t. At every gestational week studied, the mean difference in heart rate between male and female fetuses remained minimal — between 0 and 2 bpm — with no consistent pattern supporting the 140 bpm threshold used in the popular myth.
Busting the Myth, Backed by Data
Our findings align with the existing body of research on this topic. A 2023 systematic review and meta-analysis had already concluded that fetal heart rate could not reliably predict fetal sex, though it noted a lack of high-quality, large-scale studies to settle the question definitively. Our cohort of nearly 2,000 pregnancies — one of the largest of its kind, and the first to use NIPT-confirmed sex data — adds robust, high-quality evidence to that conclusion.
In short: the fetal heartbeat myth doesn’t hold up. A faster or slower fetal heart rate is not a reliable sign of a baby’s sex, at any stage during the first trimester.
Why It’s Useful for Our Patients
Curiosity about a baby’s sex is completely natural, and it’s one of the most common questions raised during early scans. Having robust, locally generated research means our sonographers can now answer that curiosity with confidence and clarity — explaining not just that the myth isn’t true, but that we now have strong Australian data to back that up.
It’s also a nice example of the kind of research our team is involved in every day, beyond the scan itself — using real clinical data from our own practices to answer the questions our patients are genuinely asking us.
A Note on the Research
As with any study, this one has limitations worth mentioning. It didn’t examine fetal heart rate variability (which changes over time and reflects different physiological processes), and factors like maternal stress, caffeine intake, or fetal movement — all known to influence heart rate — weren’t controlled for. As a retrospective study, there’s also the possibility of some variation in how heart rate was measured between different sonographers. These are noted transparently in the published paper, and point to areas for further research.
What the study does offer, though, is one of the largest and most methodologically considered investigations into this question to date — grounded in real data from real Sydney pregnancies.
The full study, “Testing the Fetal Heartbeat Myth: A Large Cohort Study Correlation With NIPT,” by Sarah Dowthwaite and Dr Karen Mizia, is published in Sonography (2026), the official journal of the Australasian Sonographers Association. You can read the full study here: Testing the Fetal Heartbeat Myth: A Large Cohort Study Correlation With NIPT