During gestation, even before the brain and ear are fully formed, babies are highly attuned to the prenatal sounds in their environment. The mother's heartbeat and voice lay the foundation for learning. The other sounds during this time may have a positive or negative influence following birth. Studies show that babies who had prenatal stimulation are more content following birth, but constant noise during pregnancy can be damaging. Some expectant parents are seeking out opportunities, such as prenatal training, to give their babies an optimal prenatal period, while others simply begin actively engaging the baby before birth.
The ear begins to differentiate by the third week of gestation. It is functional at the 16th week, even though the cochlea is not fully formed until four weeks later. Active listening begins at about 24 weeks of gestation; by 26 weeks, babies have already developed certain characteristics of what will be their native language.
The womb is a relatively quiet place; the baby hears prenatal sounds at about 50 or 60 decibels. The mother's heartbeat and voice are the most pronounced sounds and easy for the baby to recognize. Listening to her mother stimulates the baby's brain to higher levels of organization and helps the baby develop important language skills. Studies have shown that a baby prefers to listen to people who are speaking the same language as the mother and does not like to listen to those speaking a different language. Spectral analysis of a baby's first cry after birth shows intonations, rhythms, and other features of speech that match the spectral analysis of the mother's speech patterns.
Scientific studies suggest that listening to prenatal sounds will not make the baby into a prodigy; the time spent in the womb can, however, positively or negatively influence babies for the rest of their lives. Babies who are spoken to and interacted with in utero show a stronger ability to self-soothe after birth. They are born more relaxed, with their eyes and hands open, and are more responsive to their environment as compared to babies who did not receive any prenatal stimulation. Prenatal sounds may also work to the babies' detriment. According to a study of pregnant Japanese women who lived near the Osaka airport, babies who experience constant noise during development have a greater incidence of prematurity and the likelihood of being smaller if they are born at term.
More and more people are beginning to realize that the time before birth is just as filled with learning opportunities and chances for interpersonal interaction as there are after the baby is born. Some pregnant women purchase prenatal training kits to wear for an hour or two each day during the last trimester. These kits contain a belt and speaker that the mother wears over her belly to broadcast preprogrammed soothing prenatal sounds into the womb. Other parents are starting to actively parent their unborn babies without technology by reading stories, reciting nursery rhymes, and playing music with a heartbeat-like rhythm.