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The beginning of it all— reproduction.

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The journey begins with a zygote, a fertilized egg that contains chromosomes from both a sperm and an egg. This marks the two-week stage that begins at conception. The germinal stage, during which the zygote begins to divide— one into two, two into four, and so on. By the time this baby is born, it will have divided into trillions of cells.

And then after two weeks, it travels down the Fallopian tube, implants in the uterine wall. That's called implantation.

Half of all zygotes don't make it this far, either because they didn't develop properly or because the spot they implanted was inhospitable.

Once the zygote implants, we begin the embryonic stage, lasting from about the second week through the eighth. Differences in gender development begin during this stage. If it's a male, the embryo starts generating testosterone to fuel further masculine development. If it's a female, development continues without the increase in testosterone production.

So by the time the eighth week has happened— and the embryo is about an inch and a half long and only weighs about five grams— 95% of all the structures and organs in the human body are done. They're completely formed.

Starting at the ninth week, we call that the period of the fetus— the fetal period. And that ninth week goes all the way through full term.

The fetus Increases in size rapidly over the last three months of pregnancy. Digestive and respiratory systems mature. A layer of insulating fat is added below the skin and along the axons of neurons, known as myelination, which speeds the communication between neurons in the brain, which is also rapidly growing in size.

So in the fetal stage, the fetus gets bigger. It goes from five grams to normal size at delivery, somewhere around 7 and 1/2 pounds. And the bulk of that time in terms of development— although all the systems and organs continue to develop— is development of the brain.

So the fetal brain during that period undergoes enormous expansion and development. And the estimate is that for every minute of the fetal period, 250,000 neurons, which are the building blocks of the nervous system, are laid down every minute.

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Genetics is one factor that influences development, and environment is another. As the fetus grows inside the womb, this environment can affect development in many ways. We call this epigenetics, the interaction between environment and genes.

You have to think about the environment of the fetus. So here you are suspended, most of the time upside down, in liquid, and very tightly flexed.

There are no direct connections between the nerves of mothers and fetuses. Everything that the fetus gets goes through the mother's placenta. The placenta is a very unique organ— it's only in people's bodies during pregnancy— and it handles everything between the mom and the fetus. There are no direct blood connections between the mother and fetus.

So anything that the mother has— both good or bad, nutrition or also potentially harmful factors— go from the mom's bloodstream into her blood in the placenta. There is then diffusion across some of the placental tissues into the fetus's vessels from their umbilical cord. We used to think that the placenta was a barrier, and it only let beneficial substances like nutrients pass, but now we know it lets virtually everything pass very quickly.

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It isn't just the balance of nutrition that affects the fetus. Other substances can affect it, and any agent that damages the process of development is called a teratogen.

"Teratogen" is based on the Greek root word "teratos," which means monster. And "gens," which means producing. So teratogen is literally something that is monster producing.

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The clearest negative factor in the United States that has an effect before birth on later development is cigarette smoking. So we know that women who smoke cigarettes during pregnancy have smaller babies and that their babies tend to be born earlier. It's the number one.

Another teratogen that has been very well established is alcohol.

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But it has to be high levels of alcohol— significantly high levels of alcohol.

The bond between the developing fetus and its mother goes beyond what comes through the placenta.

The fetus can hear all sorts of maternal sounds in the mom's body. They can hear the maternal heartbeat very, very prominently. They can hear sort of whooshing and gastric sounds of the mom, and they can also hear the maternal voice.

Because the maternal voice is transmitted both from the mom's voice out into the air back to the abdomen. But also it vibrates through the mom's body, and so the fetus can hear it then.

In addition to auditory cues, research suggests that infants are primed for facial recognition as well.

Their maximal focus is about 12 inches at birth. So it's right at the point where the mother holds the baby and can look at the baby. And the baby's focus is perfectly attuned to that. And the features of the mom's face are very perceptually salient to a newborn infant.

Rather than arriving in the world with a fully developed brain, human beings arrive with brains that do much of their developing within the very environment in which they will function. This feature of development allows us to be one of the most adaptable species on the planet.

The period of time that humans can't walk and can't follow their parents around and have to rely on their parents to take care of them. And then the additional time necessary for children to learn all they need to learn is very long. And the period of gestation is so lengthy, primarily for our brains to develop. The human brain has to absorb a lot of culture, a lot of information, and that takes a long time.

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