heart beat gender theory?

Hanna
So the ultra sound below is my rainbow baby. I have been told by yet another doctor not to get my hopes up with this pregnancy. But im not letting that or any medical issues stop me from giving my little rainbow all of my heart. 
How ever when i was in the doctors to make sure everything was fine after some bleeding it was brought to my attention by a positive and uplifting nurse that there a BPM theory. That if the heart rate is lowwer its a boy and if its higher its a girl..? What is everyones oppinion on this? Does it really work this way or is it just a hit or miss kind of theory?