Does the baby’s heart rate predict boy or girl?

The tale: over 140 means a girl

The rule goes like this: a heart rate over 140 beats a minute means a girl, and under 140 means a boy. Some say it works as early as the first trimester (Healthline). Other versions put the line at 140 or more.

It’s an old tale, and a busy one. Cleveland Clinic puts it with the “old stories passed down from one generation to another” (Cleveland Clinic). In 1999, two Vancouver doctors wrote that many family physicians have been asked “time and time again” about tales like this. The heart-rate test was among the more popular ones, they said (Ostler and Sun, CMAJ, 1999). A New Jersey OB/GYN told WebMD her patients ask her about it daily (WebMD).

Where the 140 line came from is unclear. The Vancouver doctors tried to trace it, and were “unsuccessful in obtaining any information on the origin” of the tales they tested.

What the early-pregnancy studies found

These studies measured heart rates on early scans, then compared them with the baby’s sex, found later.

• Dayton, Ohio, 2006: 477 babies before 14 weeks. Researchers at Wright State University went through 966 first-trimester scans done from November 1997 to February 2003, for reasons such as bleeding, uncertain dates or screening. Multiple pregnancies, miscarriages and unclear sex were left out, leaving 477 single babies, scanned at 9 weeks on average. One sonographer measured each heart rate (by M-mode). The sex was recorded at a scan at 18 to 24 weeks and confirmed at delivery. Girls averaged 151.7 beats a minute and boys 154.9, and the team found “no significant differences” (McKenna and colleagues, Fetal Diagnosis and Therapy, 2006).
• Charleston, West Virginia, 2016: 655 babies at 8 to 13 weeks. Researchers reviewed records and scans from 8 to 13 weeks: 332 girls and 323 boys. Girls averaged 167.0 beats a minute and boys 167.3. The authors found no significant difference, “in contrast to the prevailing lay view of females having a faster FHR” (fetal heart rate). The abstract doesn’t say how the rates were measured or how the sex was confirmed (Bracero and colleagues, Journal of Maternal-Fetal & Neonatal Medicine, 2016).
• Ogun State, Nigeria, 2022: 2,437 pregnancies at 11 to 13 weeks. Researchers at Olabisi Onabanjo University Teaching Hospital looked back at heart rates of single babies, measured at 11 to 13 weeks. The sex was read from a scan at 18 to 23 weeks; the abstract doesn’t mention a check at birth. There were 1,398 boys and 1,039 girls. Girls averaged 165.4 beats a minute and boys 163.2, with “no statistically significant difference” (Oloyede and Lamina, Sudan Journal of Medical Sciences, 2022).
• A 2023 review: 5 studies, 4,308 babies. Researchers at Arak University of Medical Sciences in Iran searched two databases up to December 2022. They pooled five first-trimester studies: three from the U.S., one from Iran and one from Nigeria, including all three above. The oldest, from 1989, included pregnancies at any week. In four, boys’ average was a little faster, and in the Nigerian one, girls’. None of the gaps was statistically significant. The review concluded that “first-trimester FHR is not a reliable predictive test for fetal sex determination” (Nouri and colleagues, BMC Pregnancy and Childbirth, 2023).

The review calls the results “inconsistent.” It cites a 1993 report in a Kentucky medical journal that found girls’ heart rates above 140, while most other studies it cites found no significant difference.

Look at the boys’ averages, too. In all five studies in the review, boys averaged above 140, at 151 to 167 beats a minute. By the tale’s rule, the average boy in these studies would have been called a girl.

What studies found later in pregnancy

After the first trimester, a few studies did find girls’ average higher.

• Vancouver, 1999: 20 women at 26 to 34 weeks. Two doctors timed each baby’s heart rate for 1 minute with a Doppler at regular prenatal visits, and applied the rule: 140 or more meant a girl. Ten boys and 10 girls were born. The rule picked out 1 of the 10 boys and 6 of the 10 girls. The doctors concluded that it “has no value in predicting fetal sex” (Ostler and Sun).
• Ogun State, Nigeria, at 18 to 23 weeks. In the same 2,437 pregnancies, girls averaged 150.9 beats a minute and boys 141.9 at the later scan. The abstract gives no test of whether that gap was statistically significant. Each group’s spread (the standard deviation) was about 23 beats a minute.
• 9,259 cases monitored before labor, 2018. Two researchers based in London, Norway and Sweden analyzed heart-rate tracings recorded before labor by computerized cardiotocography (CTG), with the sex checked after birth. Girls’ baseline rate was significantly higher, even after allowing for the week of pregnancy and the time of day. In absolute terms, the gap was 0.9 beats a minute, which the authors called small (Bhide and Acharya, Acta Obstetricia et Gynecologica Scandinavica, 2018). The abstract doesn’t give the weeks of pregnancy, the place or the years. For context, one Scottish hospital’s guidance says computerized CTG can be used from 26 weeks (University Hospital Wishaw), but that’s its own rule, not this study’s range.
• Florence, Italy, 2019: 689 pregnancies at 37 to 40 weeks. A team at Careggi University Hospital compared the last CTG before labor in uncomplicated single pregnancies, seen in 2014: 335 boys and 354 girls. The median baseline was 135 beats a minute for boys and 136 for girls. That gap was significant in a simple comparison, but not once the mothers’ characteristics were taken into account. The team wrote that its results “suggest that fetal sex did not affect” the CTG measures in these pregnancies (Magro-Malosso and colleagues, Medical Sciences, 2019).

So the research comes near the tale only after the first trimester, and only as a gap between group averages, in some studies. “The fetal heart rate does not predict the sex of the baby,” says Justin Lappen, an obstetrician at Cleveland Clinic.

Why a small gap between averages can’t predict your baby

An average describes a group, not a baby. To sort one baby, the girls’ and boys’ heart rates would need to sit apart. In these studies, they overlapped.

• Near term, in Florence. The middle half of boys had baseline rates between 128 and 141 beats a minute, and the middle half of girls between 130 and 142. A reading of 135 sits inside both.
• Early on, in Ohio. The averages were 3 beats apart. But the standard deviation, a measure of how widely rates spread around the average, was about 23 beats in each group.
• “Significant” isn’t the same as big. Statistical significance “does not measure the size of an effect or the importance of a result,” the American Statistical Association says (ASA, 2016). The 2018 study’s authors made a similar point about their 0.9-beat gap: it “may not be of major clinical significance.”

What a baby’s heart rate does as pregnancy goes on

On average, the heart rate starts below 140 beats a minute at about 5 to 6 weeks and climbs well above it by 9 to 10 weeks. Then it eases off: to about 140 by 20 weeks, and about 130 by term (Radiopaedia). It also changes when the baby is active, Cleveland Clinic says. When can you see the heartbeat on an ultrasound? has the early weeks in detail.

Run the average baby through the tale’s rule, and the verdict flips twice:

Which side of 140 a baby lands on depends a lot on the week.

A heart rate isn’t a health check

These are averages, not a health check. A heart rate heard at a keepsake scan, ours included, doesn’t check on your baby’s health. What a reading means is a question for your doctor or midwife.

If you’re worried about your baby’s heartbeat or movements, call your doctor or midwife now. The NHS in England says to call your midwife or maternity unit immediately if your baby’s movements slow down, stop or change. “Do not wait until the next day,” it says (NHS).

The FDA says home heartbeat monitors, or fetal Dopplers, “should only be used by trained health care providers when medically necessary” (FDA). The NHS advises against them: “Even if you hear a heartbeat, this does not mean your baby is well” (NHS). Is an early ultrasound safe for the baby? has more on home devices.

Why the tale hangs on

If the studies don’t back it up, why does it survive? WebMD offers a few reasons.

• It’s right some of the time. Sex prediction myths persist partly because “sometimes they can appear to be right,” WebMD says. “When you’re looking at 50/50 odds, predictions are bound to come true half the time.”
• It travels online. WebMD describes myths “bouncing from one post to another,” and blames the internet and social media, at least partly, for “the ongoing pregnancy rumor mill.”
• A post can outweigh a doctor. “Sometimes people take what’s said online at higher value than what their physician says,” Sharon Mass, an OB/GYN in Morristown, New Jersey, told WebMD.

Want to play anyway? Our boy or girl quiz puts the heart-rate tale beside six others, just for fun. It reaches the same verdict: studies comparing boys and girls in the first trimester found no meaningful difference. And a baby’s heart rate changes through pregnancy, and with activity.

What does tell you boy or girl

A scan or a blood test detects the baby’s sex, from anatomy or chromosomes. No test or scan is 100% sure. In week order:

• From about 10 weeks: a blood test. In the U.S., cell-free DNA screening (NIPT) is a screening test for chromosomal conditions that your doctor or midwife can order. It “can also tell whether XX or XY chromosomes are present,” and Cleveland Clinic calls that result “highly accurate, but not 100% accurate” (Cleveland Clinic).
• On an ultrasound, as the weeks pass. A scan reads the baby’s genitals, and how sure a call can be depends on the week. How accurate is a gender ultrasound at 13, 14, 15 and 16 weeks? has the study figures. Theories that read an early scan picture have their own guides: nub theory, skull theory, and Ramzi theory, which goes by the placenta’s position.
• At about 18 to 22 weeks: the anatomy scan. In the U.S., this medical scan checks how the baby is developing, and in most cases it can show the sex too, depending on the baby’s position (Cleveland Clinic). In England, whether you’re told depends on your hospital’s policy, and the NHS says a sonographer can’t be 100% certain of the sex (NHS).

When can you find out your baby’s gender? puts every option on one timeline, from blood tests to birth.

Hearing the heartbeat at our Glendale studio

At our studio in Glendale, Los Angeles County, a Gender Confirmation is a 2D scan, “the classic black-and-white picture,” with 8 black-and-white prints. You hear your baby’s heartbeat, and we tell you the heart rate.

We book from 13 weeks, and we’re surest from 15. If you can wait until 15, do: we’re more confident, and a second look is less likely. Once we have a clear view it can be over in 2 minutes. If your baby is being shy, we take longer. When we can’t be sure, we say so, and you come back for a free re-check. No ultrasound can promise 100%.

It’s a keepsake, not a medical exam: it doesn’t check your baby’s health, and it doesn’t replace the anatomy scan or any appointment with your doctor or midwife. The FDA and the AIUM discourage scans done only for keepsakes (FDA, AIUM); Safety, in plain words has their statements. See finding out from 13 weeks, the session for families who’ve been told and prices.

Wondering when you can hear the heartbeat or find out? Add your due date, and the planner marks which sessions are ready for you now and which unlock next. What you type stays on your device.

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