Determining the gender of a baby through fetal heart rate during scanning can provide some indication, but it is not considered a highly reliable method. The theory suggests that if the fetal heart rate is above 140 beats per minute (bpm), it indicates a girl, while below 140 bpm indicates a boy. However, there is limited scientific evidence supporting this theory. Fetal heart rates can vary throughout pregnancy and are influenced by various factors such as gestational age, activity level, and maternal health. It is important to note that ultrasound or genetic testing are more accurate methods for determining the baby's gender. If you are curious about your baby's gender, consult with your healthcare provider for more information on available options.