The traditional "old wives' tale" is that where the measured heart rate is above 140, the baby is a girl is a girl, but if the measured heart rate is below 140, the baby is a boy. Unfortunately, the old theory doesn't take into account that heart rate varies over the duration of pregnancy.
Heart rate starts off low, and keeps getting faster and faster until about 9 weeks, then starts getting slower for the duration of pregnancy. BabyBPM is different than the traditional old wives' tale because it tries to compare your baby's heart rate to the average heart rate for your baby's gestational age.
Using the old "above or below 140 bpm" theory, every baby would be a girl if you measured at 9 weeks!