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By the time you reach your third trimester, your little one is eavesdropping on your conversations, hearing the sound of your heartbeat, and even tuning in to the music you listen to. But it took a while for this to happen, and there were many steps along the way.
That's because the ear is a remarkably complex organ, with three separate parts (the inner ear, middle ear, and outer ear), and each of these structures develops on its own timetable.
Early development of the ears
By 5 weeks of pregnancy, two small spots appear on either side of your baby's head – these eventually become the inner ears. Soon they'll start to fold inward and form sacs just beneath the surface of your baby's skin. The sacs then lengthen into a tube, and the main organ of hearing (cochlea) takes shape at the end.
Around 8 weeks of pregnancy, the ear bones – tiny structures that vibrate and help process sounds – begin as small clumps of tissue that slowly harden over the coming weeks. A tube-like cavity that will become the middle ear begins to form around these structures.
How your baby's hearing develops
Around 12 weeks of pregnancy, specialized sound transmitters called hair cells spring up inside the cochlea and eventually connect to a nerve that sends sound impulses to the brain.
Once this connection is made at about 22 weeks of pregnancy, your baby may be able to hear sounds faintly from inside your body, such as your breathing, your heartbeat, and your digestion. These sounds will grow louder as your baby's hearing improves.
By 23 weeks, your baby can hear sounds from the outside world. At first, your baby's ears can hear only low-pitched sounds, meaning he can hear male voices more clearly than female voices. But as your baby's ears and brain mature, he'll gradually perceive a wider range of sounds, including voices in the higher ranges (like yours).
Keep in mind that outside sounds have to pass through a few barriers, including your skin and the wall of your uterus as well as the amniotic fluid. And because they're coming through these layers, all sounds are muffled to your baby.
Outside sounds also must be heard over the background noises that your body makes – experts think is now about as loud to your baby as the sound of a washing machine.
At around 26 weeks, your baby may begin to respond to the sounds he hears with changes in heartbeat, breathing, and movement. (If the noise is particularly loud, your baby may startle – a movement you might feel.) Prenatal sonograms have also caught changes in babies' facial expressions when they hear music.
By 32 to 35 weeks of pregnancy, the middle ear cavity, outer ear canal, and outside part of the ear are fully formed. The opening of the middle and outer ears enables your baby to hear sounds that reach the ears through air once she leaves the watery environment of the uterus. At this point in your pregnancy, your baby is ready to listen and respond to the sounds she'll hear as soon as she's born.
Interestingly, researchers have shown that a newborn baby can remember some of what she heard inside the womb. For one thing, she'll show a clear preference for your voice over others. There's even some evidence that newborns recognize music heard repeatedly while inside the uterus. So if you serenade your bump with your favorite lullaby over the course of your third trimester, she might find it familiar once she's in your arms.
What you can do during pregnancy
You can take a few steps during pregnancy to protect your baby's hearing:
- Be careful with medications. Taking certain drugs during pregnancy, including some antibiotics, has been linked to hearing loss in babies. Tell all your health care providers (even your dentist) that you're pregnant, and talk to your pregnancy care provider about any medications you're taking.
- Protect your baby's hearing by protecting your own. There's some evidence that long-term exposure (like 8 hours a day, every day) to very loud noise can damage your baby's hearing. (But an hour or so at a loud concert should be okay.)
- Eat fish low in mercury. Fish is chock-full of healthy omega-3 fatty acids that help your baby's brain grow and develop, but some kinds also contain high levels of mercury – which has been linked to certain birth defects, including hearing loss. To balance these risks and benefits, the U.S. Food and Drug Administration advises pregnant women to eat 8 to 12 ounces a week of a variety of low-mercury fish. It's best to choose a variety, such as salmon, cod, shrimp, and tilapia, and avoid the four types of fish that have the highest amounts of the metal: swordfish, tilefish, shark, and king mackerel.
- Don't drink alcohol. Drinking alcohol during pregnancy can cause a group of birth defects known as fetal alcohol spectrum disorders (FASDs). Children with an FASD can have hearing loss and trouble with speech. No one knows how much alcohol it takes to cause the condition, so it's best to be on the safe side and avoid alcohol altogether.
Key milestones in fetal hearing development
|Weeks pregnant||What's happening|
|5 weeks||Inner ear starts to develop.|
|12 weeks||The cochlea and middle ear are forming, and hair cells begin to sprout.|
|22 weeks||Your baby can hear sounds inside your body, like your heart beating.|
|23 weeks||Your baby can hear sounds from the outside world, like a dog barking.|
|26 weeks||Your baby can hear a broader range of sound and can respond with changes in heartbeat, breathing, and movement.|
|32-35 weeks||All parts of the ear are completely formed, and your baby’s hearing continues to be fine-tuned.|