Final answer:
A fetal heart rate can be first heard at around the tenth week of pregnancy, when the developing heart starts beating clearly and regularly.
Step-by-step explanation:
The fetal heart rate can be first heard at a clear and regular pace at the beginning of the tenth week of pregnancy. The development of the heart starts early in the embryonic stage as a tube-like structure, which begins beating around the fourth week. However, it does not pump blood until approximately the fifth week, when the embryonic liver starts to produce red blood cells. By the end of the 12th week, the fetus has grown significantly and the organs have developed to a stage that makes the heartbeat detectable via medical instruments such as an ultrasound.