I asked my ob this as well and she said there was no truth to it at all!! She would just laugh whenever I talked about it and said the heart rate was a moment in time and was dependent on wha tth ebaby was doing eg rest/moving/sleep.

The heart rate for my DS1 and DS 2 and then DD were all fairly similar (on average around 140ish).

A friend of mine swears by the heart rate being a predictor of the sex and she used to stress me out by saying you are having a boy even though I knew I was having a girl (she didnt know that I knew) but her comments were enough to make me think the sonographer may have got it wrong - but in the end they didnt and we had a girl.