I used to swear by the over 150 a girl under 150 a boy rule and it has been correct more times than not for over 10years including my children. However when my little sister was pregnant and I was looking after her ( I do Antenatal clinic) her baby that turned out to be a girl would always stump us. Each week it would change from 160bpm to the next 140 bpm and even on the CTG it would have a base line that changed from one to the next Very irritating especially when my sister wouldnt find out the sex on scan.
But usually you don't take any notice what the rate is untill around 18 wks and just write heart rate heard in the notes.