Did you ever do this when you were young: Dive to the bottom of a swimming pool with a friend, then have your friend yell a phrase into the water?
You listen to the phrase all low and garbled, then try to guess what it was. This low, muffled, underwater noise is how the world would sound if you were inside a womb.
Researchers have placed tiny microphones inside a pregnant woman's uterus. They found that outside noises became much lower and muffled, and about thirty decibels quieter. Do these noises just wash around the fetus like so much amniotic fluid, or can the fetus actually hear them?
Actually, after twenty eight weeks a fetus begins to hear the internal sounds of its mother's body-the rush of blood and the rumble of the digestive system. By the third trimester, the fetus can respond to external noises too. As many expectant mothers know, a sudden car horn can start a fetus kicking.
Not only can fetuses hear outside noises, they sometimes remember what they've heard. One study found that infants who repeatedly heard a soap opera theme song in the womb reacted positively when they heard that same music after birth.
Perhaps this ability to remember helps a newborn recognize its mother's voice. A fetus can't hear specific words or intonation-these are too muffled. Instead, they remember the rhythm of their mothers' voices. To check this out, researchers gave newborns a special nipple rigged to a tape recorder. By changing the way they sucked, these infants could modify the cadence of a prerecorded voice. Eighty percent of these newborns sucked in the way that produced a voice with their mother's specific rhythm.