saguaro cacti sprouting from either side of your baby’s head. They are called primordial otocysts, and they will form a great deal of your child’s hearing apparatus. Once this territory is established, the next weeks are devoted to setting up house, from internal hairs that look like tiny whiskers to the canals they line, which look just like snail shells.
When do these structures hook up to the rest of the brain, allowing babies to hear? The answer should be familiar by now: not until the beginning of the third trimester. At six months, you can supply a sound to a fetus in the womb (mostly clicks) and listen in astonishment as the brain weakly fires back electrical responses! In another month, this crackling call-and-response increases not only in intensity but in speed of reaction. Give it another month or so, and everything has changed. Now you have a preterm infant who can not only hear and respond but can discriminate between various speech sounds like “ah” and “ee,” or “ba” and “bi.” We once again see this paratrooper pattern of establishing the territory first, then hooking things up to central command.
Babies can hear mom’s voice in the womb by the end of the second trimester, and they prefer it to other voices at birth. They respond especially strongly after birth if mom’s voice is muffled, re-creating the sonic environment of the womb. Babies even respond to television shows their mothers watched while they were pregnant. One funny test exposed preterm infants to the opening jingle of a particular soap opera. When these babies were born, they would stop crying the moment they heard that jingle! Controls had no such distinguishing response.
Newborns have a powerful memory for sounds they encountered while still in the womb.
The point is not to panic over your reading or viewing habits. The point is simply that newborns have a powerful memory for sounds they encountered while still in the womb in the last part of gestation.
Smell
The same thing is true of smells. Just five weeks after fertilization, you can see the brain’s complex wiring for smell. But, as with the other senses, the perception is not available simply because the machinery is there. At first, babies suffer from an acutely stuffy nose. The nasal cavity is filled with material that probably works like protective shrink-wrap, shielding the nose’s delicate interior tissues until they are ready to become operational. Smelling, at least as we know it, is probably impossible.
All of that changes during the third trimester. The protective plug is replaced with snot (mucous membranes)—and lots of neurons hooked directly into the perceptual areas of the brain. Mom’s placenta also becomes less picky, granting permission for more and more smell-mediating molecules (called odorants) to enter the womb. Because of these biological changes, the olfactory world of your baby becomes richer and more complex after the sixth month of gestational life. Smells don’t have to be right under baby’s nose. Your baby can detect the perfume you wear and even the garlic you ate.
As a newborn, your baby will actually prefer these smells. The preference is called “olfactory labeling.” This is the basis for a piece of advice by neuroscientist Lise Eliot, in her book What’s Going On in There?: Don’t wash baby with soap and water immediately after she’s born. The smell of amniotic fluid calms her down, studies show. Why? As with sounds, smells remind babies of the comfortable home they were inhabiting for the past nine months.
Balance
Here’s something you can try at home if you are eight months pregnant or if you have a baby younger than 5 months old. If the infant has already arrived, place him on his back. Then gently lift up both of his legs, or both of his arms, and let them drop back to the bed of their own weight. His arms will usually fling out from the sides of his body, thumbs flexed, palms up, with a startled look on his face. This is called the Moro reflex.
At eight months of pregnancy, you can usually observe the Moro reflex internally. If you are reading this in your soft bed, go ahead and roll over; if you are seated, stand up. Feel anything dramatic? A fetus is capable of executing a full Moro while still in the womb. These actions often incite it.
The Moro reflex is normal and usually occurs if an infant is startled, especially if he senses he is falling. It is believed to be the only unlearned fear response humans possess. It’s important that an infant has these reflexes. The absence of a good solid Moro can be a sign of a neurological disorder. Infants need to be able to do it within five months of birth. It is time limited, though; its persistence beyond five months is also a sign of a neurological disorder.
The Moro demonstrates that a great deal of motor (movement) and vestibular (balance) abilities have already been laid down by eight months. Vestibular abilities allow muscles to be in constant communication with the ears, all coordinated by the brain. You need a fairly sophisticated form of this communication in order to do a Moro.
Babies don’t start off capable of doing full-tilt gymnastics, of course. But they are capable of “quickening,” which is a flutter of embryonic limbs, at about six weeks post-conception, though the mother usually can’t feel anything for another five weeks. This movement is also important. It must occur, or your baby’s joints will not develop properly. By the middle of the third trimester, your baby is fully capable of deliberately commanding her body to perform a coordinated series of movements.
Taste
The tissues that mediate “gustatorial sensations” don’t emerge from your embryo’s tiny tongue until about eight weeks after conception. That doesn’t mean your baby simultaneously acquires the ability to taste something, of course; that doesn’t happen until the third trimester. Once again we see the reception-before-perception pattern of sensory development.
At that point, you can observe some behaviors familiar to all of us. Third-trimester babies change their swallowing patterns when mom eats something sweet: They gulp more. Flavorful compounds from a mother’s diet cross the placenta into the amniotic fluid, which babies in the third trimester swallow at the rate of a quart a day. The effect is so powerful that what you eat during the last stages of pregnancy can influence the food preferences of your baby.
In one study, scientists injected apple juice into the wombs of pregnant rats. When the rat pups were born, they showed a dramatic preference for drinking apple juice. A similar taste preference happens with humans. Mothers who drank lots of carrot juice in the later stages of pregnancy had infants who preferred carrot-flavored cereal. This is called flavor programming, and you can do it soon after your baby is born, too. Lactating mothers who eat green beans and peaches while nursing produce weaned toddlers with the same preferences.
It’s possible that anything that can cross the placenta can incite a preference.
Getting it just right
From touch and smell to hearing and vision, babies have an increasingly active mental life in the womb. What does this mean for parents eager to aid that development? If motor skills are so important, shouldn’t moms-to-be do cartwheels every 10 minutes to induce the Moro reflex in their in utero partners? If food preferences are established in the womb, shouldn’t moms-to-be become vegetarians in the last half of pregnancy if they want their kids to eat fruits and vegetables? And is there an effect, beyond creating potential preferences, of pumping Mozart or Dr. Seuss into your unborn baby’s brain?
It is easy to start making assumptions. So a word of caution. These studies represent the edge of what is known, and it is very easy to over-interpret what the data mean. These are all interesting research questions. But today’s data are not strong enough to solve the mystery of early mental life. They are just enough to reveal it.
The Goldilocks Effect
The biology of infant brain development reminds me of “Goldilocks and
