There is a study somewhere on the net that shows that the heartrate varies ONLY with length of gestation and that it is predictive of miscarriages - if it falls a week behind the expected level at any stage. This is most accurate at 8-12 weeks or thereabouts. They plotted heartbeat in pink and blue (pink for girls, blue for boys) and there was total overlap showing that heartbeat has nothing to do with gender.
Sorry can't find the study right now. The wives tale is that less than 140 is a boy, more than 140 is a girl.