I always thought this was true as well, although my ob told me that he knows a lot of babies who would have to change their gender if it was! The radiographer at the ultrasound clinic also said it doesn't seem to work from what they see.

At our 12 wk appt, my baby's hr was 155 bpm, then 150 bpm at the u/s and then last week at our 19 week scan it was 139 bpm. And she's definitely a girl!! So I've thrown the theory out of the window. The only thing I have been told is scientifically true is that at the start the hr is faster than as the pg progresses.