Fetal Heart Rate Boy or Girl What Studies Show

The story goes like this: if a baby’s heart beats faster than 140 beats per minute, it’s a girl, and if it’s slower, it’s a boy. It’s one of the most repeated pregnancy myths, and unlike a lot of folklore, it makes a claim that can be tested. Researchers have tested it, several times. So, fetal heart rate boy or girl: is there a real link? In the early weeks, which is where most of the careful research has looked, the studies say no. The details of how they reached that answer are more interesting than the myth.

Where the 140 idea comes from

It’s not completely invented. A 2016 study in the Journal of Maternal-Fetal & Neonatal Medicine, by Luis Bracero and colleagues, summarizes the older research. One small 1993 study by Hall and colleagues assigned sex to 100 fetuses using a 140 bpm cutoff and reported that it worked fairly well. It reported positive predictive values of 91% for males and 74% for females. Other studies didn’t back that up. According to the same summary, a test of the 140 cutoff on 20 pregnancies got only 10% sensitivity for predicting boys. A study of 500 fetuses in early labor found no significant difference. Another of 1,884 pregnancies, looking at the final hour before delivery, found female fetuses spent more time above 150 bpm and male fetuses more time below 120 bpm. The Bracero team notes that many of these later-pregnancy studies were small, measured at different stages, and gave mixed results, and that “other variables affect the FHR during labor.”

So the myth rests on a patchy record, mostly from late pregnancy and labor, when many other factors affect heart rate. That’s why researchers went back and looked at the first trimester.

Fetal heart rate boy or girl in the first trimester

McKenna and colleagues, 2006 (Fetal Diagnosis and Therapy). They measured heart rate in 477 single pregnancies at under 14 weeks, then confirmed sex later by ultrasound and at delivery. The average was 151.7 bpm for females and 154.9 bpm for males (p = 0.13), not a significant difference. The authors wrote that “contrary to beliefs commonly held,” there was no significant difference in the first trimester.

Bracero and colleagues, 2016. This team reviewed records of 655 single pregnancies scanned between 8 and 13 weeks at a hospital in Charleston, West Virginia, with sex confirmed at birth. They excluded mothers and fetuses with conditions that might affect heart rate. The averages were almost identical: 167.0 bpm for females and 167.3 bpm for males (p = 0.62). Breaking the data down week by week from 8 to 13 weeks showed no difference either. The study had planned to enroll 1,600 pregnancies but stopped early after an interim look. The gap was only 0.3 bpm, and the authors calculated that more than 12,000 participants would be needed to detect a significant difference of that size. The one clear sex difference they did find was in birth weight: the boys were heavier at birth.

Notice something odd. Both studies looked at roughly the same stage of pregnancy, but Bracero’s averages are about 12 to 15 bpm higher than McKenna’s. Bracero’s team suggests McKenna’s group may have included very early pregnancies, at 6 to 7 weeks, when heart rates are known to be lower. Heart rate changes quickly in early pregnancy, which is one reason a single number is a shaky basis for any prediction.

Illustration of a perfectly level balance, a nod to the “no real difference” result (artistic).

What the meta-analysis found

A meta-analysis pools studies to get a bigger, steadier answer. In 2023, Shadi Nouri and colleagues at Arak University of Medical Sciences in Iran published a systematic review and meta-analysis in BMC Pregnancy and Childbirth. It asked whether first-trimester fetal heart rate differs between boys and girls. They screened 223 records and found five studies with usable data: three from the United States (including the two above), one from Iran and one from Nigeria, covering 4,308 fetuses in total.

The individual studies didn’t even agree on the direction. Three found slightly faster rates in males, the Nigerian study found slightly faster rates in females, and Bracero found essentially none. None of the differences reached statistical significance. Pooled together, the result was a standardized mean difference of 0.04 (95% confidence interval −0.09 to 0.16, p = 0.553). That is statistically indistinguishable from zero. The authors concluded that first-trimester heart rate “is not a reliable predictive test for fetal sex determination.”

The limits of the evidence

A “no” answer here is strong but not airtight, and the meta-analysis is open about its weak points:

• Only five studies. It’s a small base for pooling. The authors also found significant heterogeneity, meaning the studies disagreed with each other more than chance alone would explain. They suggested differences in populations, mothers’ ages, gestational age, sonographer experience and equipment as possible reasons.
• Signs of publication bias. The funnel plot and Egger test suggested some relevant results may be missing. A statistical correction for that (“trim and fill”) still gave no significant difference.
• One study swings the result. When the large Nigerian study was removed, the remaining four showed a small but statistically significant tendency for faster heart rates in males (standardized mean difference 0.091). That’s the opposite of what the myth claims, and still a small effect.
• Mixed wording. The paper’s summary says male fetuses “show faster FHR but such sex-related difference is minimal,” which sits awkwardly beside a pooled result that wasn’t significant. The safest reading is that any real difference, if one exists, is tiny.
• Early pregnancy only. The studies above and the meta-analysis cover the first trimester. The later-pregnancy and labor studies are older, smaller and inconsistent, as summarized above, so this article makes no claim that they settle the question for later stages.

Why an average isn’t a predictor

Even if a small average difference were real, it couldn’t sort individual pregnancies. In the Bracero data, the spread within each group (a standard deviation of about 9 to 10 bpm) was some 30 times larger than the 0.3 bpm gap between boys and girls. The two groups overlap almost completely. This is a common pattern in biology: a difference that shows up only across thousands of cases tells you almost nothing about any single one. The 140 rule asks a heart rate to do something the data says it can’t.

This article summarizes research on a popular myth. It isn’t medical advice, and the heart rates above are study averages, not reference ranges. Questions about a pregnancy are best taken to a clinician.

If you’re curious how heart rate differences between boys and girls show up after birth, Odinity has a student experiment comparing heart rate and blood pressure responses to exercise in boys and girls.