This has actually been a pretty popular theory in gender prediction for a long time.
The heart rate of your baby fluctuates as they grow and as they move. Heart rates start out slower, and then by 8-10 weeks run in the range of 170-200 BPM (beats per minute).
As your approach mid-pregnancy the average heart rates run between 120 - 160 BPM.
If your baby moves, his or her heart rate goes up, just as your heart rate does with movement. However, none of these is related to the gender of your baby.
A study was actually done to test this theory. The study actually shows that there is no correlation between gender and fetal heart rate. Although they did find a correlation between heart rate and gestational age prior to 9.2 weeks. -- Robin Weiss
