During pregnancy in a human female, in which of the following...

In human embryonic development, the heart begins to form and starts beating around the $4^{th}$ week (approximately $21$ to $22$ days) after conception. However, the heartbeat is initially too faint to be detected. It becomes clinically detectable for the first time using a transvaginal ultrasound around the $6^{th}$ week of pregnancy.

Answer: $6^{th}$ Week