how do we know that babies can hear before birth?
Perhaps the most significant one a baby hears in utero is his mother's voice. Around the seventh and eighth month, a fetus's heart rate slows down slightly whenever his mother is speaking, indicating that mom's voice has a calming effect. By the time they're born, babies can actually recognize their mother's voice. In one study, doctors gave day-old infants pacifiers that were connected to tape recorders. Depending on the babies' sucking patterns, the pacifiers either turned on a tape of their mother's voice or that of an unfamiliar woman's voice.