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.