One of the first things parents ask when they hear their baby's heart beating is, "Why is it so fast?"

Michelle Collins, a certified nurse-midwife from Vanderbuilt University, says a fetus' heart beats so fast because its central nervous system is immature up until the last few weeks of pregnancy.

"It is during the maturation process that the fetal heart gains the ability to 'respond' to what the fetal activity is," Collins says. "For instance, normal fetal heart rate is 110 to 160 beats per minute. Early in pregnancy, when the fetal heart rate is picked up with the Doppler by the obstetrical provider, it will be very near the top of that range consistently, regardless of the fetal activity. As the central nervous system gains maturity, the fetal heart gains the ability to respond to fetal activity. When the fetus is in a sleep state, its heart rate will usually be near the lower end of the range I quoted. When the fetus is awake and active and moving, its heart rate will be higher, and near the top of that range."

And, by the way, there is no correlation between fetal heart rate and fetal sex, Collins says. Sorry!