You can typically hear a baby's heartbeat during pregnancy at these stages:

• By ultrasound, embryonic cardiac activity (heartbeats) can usually be detected around 6 to 7 weeks of pregnancy, often via a transvaginal ultrasound. Around 10 weeks, the heart is more fully formed and called fetal cardiac activity. Early on, the ultrasound detects electrical impulses that represent the heartbeat rather than an actual fully formed heart beating. (6-7 weeks by ultrasound)
• Using a handheld Doppler fetal monitor at a doctor's office, the heartbeat can often be heard around 10 to 12 weeks, but sometimes as early as 8 to 12 weeks.
• With a standard stethoscope, typically you can hear the fetal heartbeat at home starting around 18 to 20 weeks of pregnancy.

In summary, the heartbeat can be seen on ultrasound as early as 6 weeks, heard with Doppler devices in early second trimester, and with a stethoscope from around 18 weeks onwards.