If the unborn baby’s fetal heart rate is above 140 PM (or) 160 BPM , some people use to think it’s a girl whereas if it’s below 140 BPM they used to say it’s a boy baby. This is a complete myth and there is no scientific evidence to support the theory that if the baby’s heart rate is above 140/160 BPM we can conclude that it’s a girl.
This is one of the old wives tales which has been disproved by research papers. Here is one such research paper which proved with evidence that there is no big difference between male and female fetal heart rates and we won’t be able to decide gender based on the fetal heart rate
Generally the fetal heart rate starts at around 85 beats per minute and increases every day by 3 beats per minute. Then after it reaches around 175 beats per minute it reverses and starts to beat slowly and by the 2nd trimester it averages around 120 to 160 beats per minute.
In the above study researchers were not able to find distinguish male and female fetuses based on their heart rate. There was also another study published in 2018 where the researchers compared fetal heart rate for fetuses in 9000 pregnancies and they were not able to find or distinguish between male and female fetuses
Even though there is a lower baseline FHR with male fetuses, there is no major clinical significance as there absolute differences are very small, so the researchers were not able to decide whether the fetus is male or femlae depending upon their fetal heart rate.