Around which day after fertilization does the heart begin to beat?

• A. Day 7
• B. Day 15
• C. Day 12
• D. Day 21

Answer: (B)

The key idea here is the timing of when the heart first starts beating during early development. In humans, the heart begins as a simple tube and starts rhythmic contractions a little after the third week of development, roughly around day 21 to day 22 after fertilization. That initial beating marks the onset of cardiac activity and the beginning of circulation, even though the heart is far from fully formed at that stage. By about six weeks gestational age, a detectable heartbeat is usually seen by ultrasound.

Among common day estimates, day 21 best matches this onset of heart activity. The earlier days are well before the heart has formed enough to beat, so they don’t align with when cardiac contractions actually begin.