A baby's heart starts developing a regular rhythm by around 5 weeks of pregnancy, which is called embryonic cardiac activity. This early heartbeat can usually be detected by an ultrasound at 6 to 7 weeks of pregnancy and can be heard by Doppler from about 10 to 12 weeks. The heart is not fully formed at this early stage but is developing electrical impulses that generate the heartbeat sounds heard during ultrasounds. The fetal heart becomes fully formed by about week 10 of pregnancy. By mid-second trimester, it is fully functioning to pump blood through the baby's body. The heartbeat typically starts around 22 days after conception, before many mothers realize they are pregnant. Early detection often depends on the type of ultrasound used, such as vaginal ultrasound, which can detect the heartbeat earlier than abdominal ultrasound. The heart rate peaks around 9 weeks gestation at over 170 beats per minute and ranges from about 120 to 160 bpm in the second and third trimesters.
