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outer world. As we’ll see, what you eat and smell can influence your infant’s perceptions, too. For a newborn, these things are the familiar comforts of home.
    Here’s when your baby’s senses—touch, sight, hearing, smell, balance, taste—start to function as you transit through pregnancy:

Touch: 4 weeks
    One of the earliest senses to come on line is touch. Embryos about 1 month old can sense touch to their noses and lips. The ability spreads quickly; nearly the entire surface of the skin is sensitive to touch by 12 weeks of age.
    I swear I could detect this by the time my wife was in the middle of her third trimester with our youngest son. He was quite a mover, and at times I could see what looked like a bulging shark’s fin move across my wife’s belly, swelling, then submerging. Creepy. And cool. Thinking it might be the little guy’s foot, I tried touching the bulge when it appeared one morning. The bulge immediately “kicked” back (!), causing us both to yelp with excitement.
    If you try this in the first half of pregnancy, you won’t get any results. Not until about the fifth month after conception can babies truly experience touch in the way you and I might perceive it. That’s when your baby’s brain develops“body maps”—tiny neurological representations of his entire body.
    By the beginning of the third trimester, a fetus readily displays avoidance behaviors (trying to swim away, for example, when a needle comes near for biopsy). From this we conclude that babies can feel pain, though it is impossible to measure this directly.
    The fetus appears to possess sensitivity to temperature by this time, too. But it’s possible that the wiring diagrams for temperature sensation aren’t fully completed at birth and that they require experience with the outside world to fully develop. In two unrelated child-abuse cases, a French boy and an American girl were kept in isolation for years. Both children had an eerie inability to distinguish between hot and cold. The little girl never dressed appropriately for the weather, even when it was freezing outdoors. The little boy regularly pulled potatoes out of a roaring fire with his bare hands, oblivious to the temperature difference. We don’t know exactly why. We do know that touch remains very important for a baby’s development after birth.

Sight: 4 weeks
    Can babies see in the womb? That’s a tough question to answer, mostly because vision is our most complex sense.
    Vision begins developing about four weeks after conception, the fetus forming little eye-dots on either side of her tiny head. Cup-shaped structures within these dots soon emerge, which will form, in part, the lens of the eye. Retinal nerves then snake out from behind these primitive eyes, trying to reach the back of the head and connect to regions that will eventually form the visual cortex. The cells in this cortex have themselves been busy, getting ready to greet these neural travelers and form partnerships. The second and third trimesters are filled with massive neural meet-and-greets in these regions, a fair bit of cell death, and lots of chattering connectivity. At this point, the brain is forming about 10 billion new synapses per day. You’d think a baby would get a migraine!
    One result of all this activity is that the neural circuitry necessary to control blinking, dilation of pupils, or tracking moving objects is present before birth. Experiments show that infants just entering the third trimester will move or alter their heart rate, or both, in response to a strong light beamed at the womb. But it takes so long to build adequately functioning circuits that the baby needs more than nine months to finish the job. The brain will continue forming 10 billion synapses a day for almost a year after birth. During that interval, the brain uses external visual experiences to help it finish its internal construction projects.
Hearing: 4 weeks
    If you were to tell me that an important