I asked Doctor about that myth just yesterday she said that's not even true. She said the baby's heart rate changes because sometimes the baby will be moving more and more awake in the belly. And sometimes they are resting so calm. And i have gotten different heart rates, 155, 153, 144..
There is an old wives tale about how if the heartbeat is 140 or below it's a boy not if it's higher it's a girl. But my baby still isn't under 140 bpm and it's a little boy.
Where I live they don't even tell you the heart rate... so I have nothing to compare with. Are you going to find out?