Final answer:
During the first-trimester obstetric sonogram, an ultrasound is used to demonstrate fetal heart activity due to its non-invasive nature and lack of electromagnetic radiation, making it safe for both mother and fetus.
Step-by-step explanation:
The ultrasound is the imaging technique used in the first-trimester obstetric sonogram to demonstrate fetal heart activity. Ultrasound uses high-frequency sound waves to generate real-time images of the fetus and is particularly valuable for its safety, as it does not use electromagnetic radiation.
This technique is also the least invasive and can safely assess fetal growth and development, including the detection of fetal heartbeat through Doppler-shifted ultrasound.