A Doppler device is a small, portable machine that uses ultrasound waves to detect and magnify the baby's heartbeat. Doctors use this test during most office visits to verify that the baby is alive.

"We use Doppler in the clinic to detect the heartbeat because it's more readily available than ultrasound machines," says Layan Alrahmani, M.D., a board-certified ob-gyn, maternal-fetal medicine ...