"They" (whoever that might be!) say that girls' heart rates are faster than boys, and that you can find out the gender of your baby by listening to it with a Doppler or fetoscope, or asking your doctor or midwife what your baby's heart rate is. This theory was apparently quite popular, because there have been studies to research if there was any truth to the idea that boys have slower heart rates than boys. Are you curious what the outcome was? Researchers found that the hear rates of a fetus vary throughout pregnancy. Listening to a baby's heart beat could perhaps reveal something about the gestational age, but said medical trials found absolutely no connection between heart rates and gender.
I guess that's another myth that is well and truly busted. If you want to find out your baby's gender, you will have to have either an ultrasound or an amniocentesis which is actually the most reliable gender determination method. If are having trouble deciding whether you want to know if you are expecting a girl or a boy, you can have a look at our reasons to keep your baby's gender a surprise until birth, as well as the reasons to find out your baby's gender.