My GP and I had a conversation about this the other day.

She said if the heartbeat is measured weekly from around 34 weeks onward you will see a pattern, and if over 140 it's a girl, under is boy, and it is quite accurate in her experience. We will see when I get that far along, I'll already know the sex so should be interesting!!

BTW My baby's HB has been 169 (8 weeks), 167 (13 Weeks), 154 (14 Weeks 5 Days), and last time it was measured it was 133 (16 weeks) resting, and as high as 166 active. So, I'm confoozled lol