It is a myth but in my case it was true my daughters heart rate was 162
I think that the myth is if it's a higher heart-rate, it's a girl...when I had my first ultrasound the heartbeat was around 165bpm....everyone was telling me it was a girl because of the myth about the heart-rate...then I found out she is a girl, and the heart-rate has been around 144bpm my last few appointments...so the myth worked for me :)
Myth, my first was a boy and his heart beat was always in the 170's, now I'm prego with baby boy #2 and his heart beat is in the 140-150 range.
It's an old wives tale, but over like 150 is a girl. Under 150 is a boy. with my daughter, her hr was 179 in the first trimester and dropped to 160 in the second trimester. With my son, his hr was 155 in the first trimester, and 145 in the second. So, it worked with me, but it's just one of those things.