Final answer:
A regular fetal heartbeat can usually be detected by a Doppler device at around 10 to 12 weeks of gestation.
Step-by-step explanation:
The fetal heart rate (FHR) can be auscultated by Doppler early in pregnancy. The heart and blood vessels are among the earliest organs to develop and function in a fetus. A regular heartbeat can usually be detected by a Doppler device at around 10 to 12 weeks of gestation. This early detection is possible because the fetus has fetal hemoglobin, which allows for efficient oxygen binding at lower pressures, enabling the fetus to obtain oxygen from the mother's blood via the placenta. Thus, the Doppler ultrasound can pick up the sound of the fetal heartbeat during this stage of development.