How far along do you need to be to hear a baby's heartbeat during an ultrasound?

According to Baby Center, is possible to hear the baby's heartbeat at about 8 weeks via ultrasound. A fetus's heartbeat begins at about 6 weeks gestation.

If an early ultrasound is not done, the first chance to hear a baby's heartbeat is at about 10 weeks of gestation using a fetal Doppler, which is a handheld ultrasound device. Baby Center points out, however, that it is more common to pick up a heartbeat with a fetal Doppler device at 12 weeks. The ability to hear the heartbeat is affected by the mother's weight, the baby's position within the uterus and the accuracy of the due date.