A fetal heart rate of 158 bpm does not reliably show whether the baby is a boy or a girl. Heart rate alone is not a dependable way to predict fetal sex, even when the number sounds high or low.
Some studies have looked at whether boys and girls have different fetal heart rates, especially in the first trimester. Overall, the average heart rates between male and female fetuses were very similar, and any small differences were not useful enough to predict sex.
Fetal heart rate can vary depending on the stage of pregnancy and normal fetal activity. A number like 158 bpm may be normal in many pregnancies, but it should be interpreted along with gestational age and the rest of the ultrasound or prenatal exam.
The most reliable way to find out the baby’s sex is through appropriate prenatal testing or ultrasound when recommended by the pregnancy care team. If there are concerns about fetal heart rate or pregnancy symptoms, an OB-GYN should review the findings rather than relying on home prediction methods.