The fetal heart

The embryonic heart starts beating 22 days after conception, or about five weeks after the last menstrual period, which by convention we call the fifth week of pregnancy. The heart at this stage is too small to hear, even with amplification, but it can sometimes be seen as a flickering in the chest if an ultrasound is done as early as four weeks after conception.

The Doppler instrument

After the 9th or 10th week after your last menstrual period, you might be able to hear your baby's heartbeat at your prenatal appointment. Your obstetrical practitioner probably uses a Doppler instrument for this purpose, which bounces harmless sound waves off the fetal heart. The way the sound comes back is affected by motion, so a beating heart creates a change in the sound that can be picked up by the receiver in the Doppler. Whether you actually hear the heartbeat at 9 or 10 weeks depends partly on luck-the instrument must be placed at just the right angle. It also depends on the position of your uterus, and if you're slim or heavy. By the 12th week, the heartbeat can usually be heard consistently, using the Doppler instrument for amplification.