Excited about the gender

Hi, mummies when is the best weeks to know about the gender of the baby? And what is the first sign of having a baby girl? I heard they can predict by the heart rate? Is it true? And they said If it's over 140 bpm, you're having a baby girl. Below 140 bpm, you're carrying a boy. I was on 12 weeks pregnant that time they check the heartbeat of my baby was 137bpm.