When can the heartbeat first be detected?

The human fetal heart begins beating about 22-23 days after conception. (Though the heart is not yet fully formed, cardiac muscle contractions have begun.) That's called the 5th week of pregnancy, since it's 5 weeks after the last period. The fetus at this stage is between 2 and 3 mm in size.

Sometimes it's possible to detect the heartbeat, with a Doppler instrument, almost this early, but usually the first such detection is later, around week 10, or at least by week 12. By week 21, the heart can be heard with a standard stethoscope.