the birth defects observed in the lab.
A delicate web of cells, crackling with electricity
Fortunately, most babies’ brains form just fine. The brain end of the neural tube continues its construction project by creating bulges of cells that look like complex coral formations. These eventually form the large structures of the brain. Before the first month, the baby’s tiny pre-brain cell has grown into a hefty army, millions of cells strong.
The brain does not develop in isolation, of course. The early embryo temporarily displays gill arches around the fourth week, for example, much like the ones fish have. These soon convert into face muscles and the throat structures that will allow your baby to speak. Your embryo next gets the stub of a tail but soon reverses course and resorbs the structure. There are strong evolutionary roots to our development, and we share this miracle with every other mammal on the planet. Except for one thing.
Those bulges at the end of your embryo’s neural tube will turn into a great big, fat, super-smart brain—about the heaviest brain-per-body mass that exists on the planet. This massive structure is composed of a delicate spider web of cells, crackling with tiny bolts of electricity. Two types of cells are important here. The first type, glial cells, make up 90 percent of the brain cells inside your child’s head. They give the brain its structure and help the neurons correctly process information. It’s a good name; glial is a Greek word for glue. The second type of cell is the familiar neuron. Though they do a lot of your child’s thinking, neurons make up only about 10 percent of the total number of brain cells. That’s probably where we get the myth that you use only 10 percent of your brain.
One neuron, 15,000 connections
So how do cells turn into brains? Embryonic cells are manufactured into neurons in a process called neurogenesis. This is when the baby would like to be left alone, in the first half of pregnancy. Then, in the second half of pregnancy, the neurons migrate to the region they eventually will call home and start wiring together. This is called synaptogenesis.
Cell migration reminds me of when tracking bloodhounds are suddenly loosed from the sheriff’s truck to pick up the scent of a criminal. Neurons bolt out of their ectodermal cages, crawling over one another, sniffing out molecular cues, pausing, trying out different pathways, slithering helter-skelter throughout the developing brain. Eventually they stop, having arrived at a destination that may be pre-programmed into their cellular heads. They look around their new cellular digs and try to hook up with the neighbors. When they do, tiny, lively gaps between neural cells are created, called synapses (hence, the term synaptogenesis). Electrical signals jump between the naked spaces to allow neural communication. This final step is the real business of brain development.
Synaptogenesis is a prolonged process, for an easily understood reason: It is ridiculously complex. A single neuron has to make an average of 15,000 connections with the locals before its wiring job is done. Some neurons have to make more than 100,000 connections. That means your baby’s brain has to lash together an astonishing 1.8 million new connections per second to make a complete brain. Many of the neurons never complete the process. Like post-sex salmon, they simply die off.
Even given this incredible speed, baby brains never make the birth deadline. About 83 percent of synaptogenesis continues after birth. Surprisingly, your baby girl’s brain will not completely finish its wiring until she is in her early 20s. Boys’ brains may take even longer. In humans, the brain is the last organ to finish developing.
When can baby hear you, smell you?
The purpose of that furiously fast (then frustratingly slow) production is to build a functioning brain, one that can receive and respond to inputs. So the questions for prying parents become: What do fetuses know, and when do they know it? When is your baby capable of sensing, say, taps on your belly?
The developmental principle to remember is this: The brain spends the first half of pregnancy setting up its neuroanatomical shop, blissfully ignoring most parental involvement. (I am referring to well-intentioned interference. Drugs, including alcohol and nicotine, clearly can damage a baby’s brain during pregnancy.) The second half of pregnancy is a different story. As brain development moves from mostly neurogenesis to mostly synaptogenesis, the fetus begins to exhibit much greater sensitivity to the outer world. The wiring of cells is much more subject to outside influences—including you—than the act of creating them in the first place.
The senses develop strategically
What is baby’s approach to constructing the brain’s sensory systems? Ask paratroop commanders. They will tell you that successfully fighting a war involves three steps: parachuting into enemy territory, securing hostile real estate, and communicating back to home base. This process gives central command both knowledge about progress and “situational awareness” of what to do next. Something similar happens to sensory systems in the brain as they develop in utero.
Like parachutists securing enemy territory, neurons invade a given region of the brain and establish various sensory bases. Neurons that hook up to the eyes will eventually be used for vision, ears for hearing, nose for smelling. Once their areas are secured, these cells will establish linkages that help them reach out to the perceptual command-and-control structures also growing up within the brain. (In the real world of the brain, there are many central commands.) These CEO-like structures, which give us perceptual abilities, are busily capturing territory just like the paratroopers. And they are some of the last areas in utero to wire up properly. This means neurons hooked up to the eyes or ears or nose might receive a busy signal when they try to report back to their home base. Because of this odd timing, parts of a baby’s brain can respond to sensory stimulation before a baby can actually perceive being stimulated.
But once babies can perceive inputs like sounds and smells, starting around the second half of the pregnancy, they become precisely attuned to them. And they subconsciously remember. Sometimes it’s spooky, as legendary conductor Boris Brott discovered one day.
Babies remember
“It just jumped out at me!” Brott exclaimed to his mother. Brott had been at the podium of a symphony orchestra, conducting a piece of music for the first time, when the cellist began to play. He instantly knew he’d heard this piece before. This was no casual reminder of some similar but forgotten work: Brott could predict exactly what musical phrase was coming next. He could anticipate the flow of the entire work during the course of the rehearsal; he knew how to conduct it even when he lost his place in the score.
Freaking out, he called his mother, a professional cellist. She asked for the name of the piece of music, then burst out laughing. It was the piece she had been rehearsing when she was pregnant with him. The cello was up against her late-pregnancy mid-abdomen, a structure filled with sound-conducting fluids, fully capable of relaying musical information to her unborn son. His developing brain was sensitive enough to record the musical memories. “All the scores I knew by sight were the ones she had played while she was pregnant with me,” Brott later said in an interview. Incredible stuff for an organ not even 0 years old.
This is but one of many examples of how babies in the womb can pick up information from the 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.
Let’s look at when your baby’s senses—touch, sight, hearing, smell, balance, taste—start to function as you transit through pregnancy.
Touch
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, and 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.
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