Final answer:
A fetal heartbeat can usually be detected with a Doppler ultrasound between 10 to 12 weeks of gestation through the detection of Doppler shifts that result in a beat frequency.
Step-by-step explanation:
A fetal heartbeat can typically be detected by a Doppler ultrasound between 10 to 12 weeks of gestation. This medical diagnostic technique utilizes the Doppler effect to measure the tiny changes in frequency caused by the fetal heartbeat. As the fetal heart is a moving source, it reflects the ultrasound, causing the detectable shifts known as Doppler shifts. These shifts, although small in comparison to the original ultrasound frequency, help in producing a beat frequency that can be measured more easily.
During an ultrasound, the Doppler effect results in a double frequency shift, as the fetal heart first acts as a moving observer, receiving the Doppler-shifted frequency, and then as a moving source when it reflects the sound. By assessing this beat frequency, which negates any variations in the source frequency, healthcare providers can successfully monitor the fetal heartbeat.