By Sarina Gomez
This article is general information, not medical advice. Ask your midwife, OB-GYN or sonographer about your own pregnancy and scans.
Your baby’s heart rate doesn’t tell you the sex. The old rule says over 140 beats per minute means a girl and under 140 means a boy, but a 2023 meta-analysis of five studies and 4,308 fetuses found no meaningful difference between boys and girls in the first trimester. Heart rate changes with the week of pregnancy and with what the baby is doing, so a reading like 158 bpm is normal for both.
What the studies found
Every study that has compared first-trimester heart rates by sex found the averages almost identical. A 2023 systematic review in BMC Pregnancy and Childbirth pooled them and concluded that “first-trimester FHR is not a reliable predictive test for fetal sex determination” (Nouri et al., 2023). Here are the five studies it included:
| Study | Country | Babies | Boys’ average | Girls’ average |
|---|---|---|---|---|
| DuBose, 1989 | USA | 365 | 151.1 bpm | 150.2 bpm |
| McKenna, 2006 | USA | 477 | 154.9 bpm | 151.7 bpm |
| Bracero, 2016 | USA | 655 | 167.3 bpm | 167.0 bpm |
| Keshavarz, 2018 | Iran | 374 | 156.2 bpm | 154.9 bpm |
| Oloyede, 2022 | Nigeria | 2,437 | 163.2 bpm | 165.4 bpm |
Figures as reported in the 2023 review.
Three things stand out:
- Both sexes average well above 140. In every study, boys and girls alike had average heart rates over 150 bpm. By the old rule, nearly everyone would be having a girl.
- The difference flips. Boys were a touch faster in four studies, girls in one. None of the gaps was large enough to use.
- The studies differ more from each other than boys differ from girls. The averages range from about 150 bpm in the 1989 study to about 167 bpm in the 2016 study, a gap of 16 to 17 bpm. Within any one study, boys and girls are 0.3 to 3.2 bpm apart. Timing and method move the number far more than sex does.
The two largest US studies say it plainly. McKenna’s team, looking at 477 scans before 14 weeks, found “no significant differences between male and female FHR during the first trimester.” Bracero’s team, at 8 to 13 weeks, found no difference “in contrast to the prevailing lay view of females having a faster FHR.”
Why one reading says nothing about sex
A heart rate at one appointment mostly tells you how far along you are. The embryo’s heart speeds up through the first trimester and then gradually slows. Radiopaedia, a radiology reference built on published studies, summarizes the typical curve:
| Gestational age | Typical heart rate |
|---|---|
| 5–6 weeks | about 110 bpm |
| 9–10 weeks | about 170 bpm (the peak) |
| 14 weeks | about 150 bpm |
| 20 weeks | about 140 bpm |
| Term | about 130 bpm |
On top of that, the 2023 review notes that heart rate moves with fetal breathing and movement, uterine contractions and some medications. A baby who’s wriggling during the scan will read faster than the same baby asleep a minute later.
Take 158 bpm. At 8 weeks, that’s on the way up to the peak. At 12 weeks, it’s right in the usual range. At 20 weeks, it’s a little above the typical 140 but still inside the 110 to 160 bpm range that Cleveland Clinic describes as a normal baseline. In none of those cases does it point to boy or girl.
The 140 rule was always going to fail for a simple reason: most first-trimester heartbeats are above 140 whatever the sex. That makes it a coin toss that lands “girl” far more often than girls are born.
What heart rate is really checked for
Your provider measures the heart rate to check on your baby’s health and development, not to guess the sex. If you have questions about a number on your scan report, ask the person who did the scan or your OB-GYN or midwife; they know your dates and the context. Our guide on how to read an ultrasound picture explains where FHR and the other labels appear.
About home heartbeat monitors. The FDA says fetal heartbeat monitors (Dopplers) “should only be used by trained health care providers when medically necessary,” and warns that untrained users may interpret the information incorrectly. Counting beats at home to guess the sex adds exposure and answers nothing.
Ways that actually show the sex
If you want a real answer, the options are medical:
- A cell-free DNA blood test from 10 weeks looks at the sex chromosomes (ACOG). In a 2011 meta-analysis, blood-based testing detected boys 95.4% of the time and girls 98.6% of the time (Devaney et al., JAMA).
- The anatomy ultrasound at 18 to 22 weeks, where, according to ACOG, “if the fetus is in a good position, it may be possible to tell the sex.”
Our timeline of when you can find out the gender covers every option, and our rating of whether any gender predictor is 100 percent accurate puts heart rate next to home kits, charts and scans.
Keeping the guessing fun
Heart-rate guessing is harmless as a game. Write down everyone’s call at the scan, then compare at the reveal. The other folk tests, from bump shape to the ring test, are rated in boy or girl old wives’ tales.
If you’d like a reveal moment while you wait, SeeMyBaby’s gender reveal party is made for parents who don’t know or are keeping it a surprise. You pull the ribbon on a gift box, and after you buy the gender add-on on the paywall, it opens on a drumroll with pink or blue balloons and confetti. What it shows is a playful AI guess, just for fun, not a medical result, and the app says so. If you already know the sex, you tell the app and no guess is made.
Sources
- Nouri S, Kalantar MH, Safi F, Almasi-Hashiani A. The role of fetal heart rate in first trimester sonograms in prediction of fetal sex: a systematic review and meta-analysis. BMC Pregnancy and Childbirth, 2023.
- McKenna DS, et al. Gender-related differences in fetal heart rate during first trimester. Fetal Diagnosis and Therapy, 2006.
- Bracero LA, et al. First trimester fetal heart rate as a predictor of newborn sex. Journal of Maternal-Fetal & Neonatal Medicine, 2016.
- Radiopaedia: Fetal heart rate in the first and second trimester
- Cleveland Clinic: Fetal heart rate monitoring
- U.S. Food and Drug Administration: Ultrasound Imaging
- American College of Obstetricians and Gynecologists: Prenatal Genetic Screening Tests and Ultrasound Exams
- Devaney SA, et al. Noninvasive fetal sex determination using cell-free fetal DNA. JAMA, 2011.
More gender guides are on the SeeMyBaby blog.
Questions parents ask
Is a heart rate of 158 bpm a boy or a girl?
It could be either. In the studies above, boys and girls averaged between about 150 and 167 bpm, so 158 sits right in the middle for both.
Does a fast heartbeat mean a girl?
No. A 2023 review of five studies found no meaningful difference in first-trimester heart rate between boys and girls. Boys were slightly faster in four of the five studies.
Do boys have a lower heart rate in the womb?
Not in the first trimester. The averages for boys and girls differed by 3 bpm or less in every study, in both directions.
What is a normal baby heart rate during pregnancy?
It depends on the week: roughly 110 bpm at 5 to 6 weeks, a peak around 170 bpm at 9 to 10 weeks, then a slow decline. Cleveland Clinic gives 110 to 160 bpm as the normal baseline range. Your provider can tell you what’s expected for your dates.
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