The ultimate transformation of a fertilized blob into the very person reading this--you--is an amazing process. Of course, one structure, the brain, makes us what we are on the food chain. The superior brain of Homo sapiens has allowed us to dominate and run the world. (Whether our brains are superior enough to do that well is a whole other topic!)
The brain of the developing fetus changes so much over the course of gestation that it is even possible to date the stage of the pregnancy by what the brain looks like. That makes us the thinking beings that we are is the development of neurons (nerve cells). Nature has taken advantage of a property of geometry to cram in as many neurons as possible into a relatively small space--that property being surface area. For the brain is not just a chunk of connected nerve cells, but bundles and tracts that interweave--up and down, over and under, in and out--presenting as the famous sausage-like convolutions that are the crowning glory of our species. Between the convolutions are deep separations, called "sulci." The more convoluted the brain is and the deeper the sulci, the more surface area there is to pack extra neurons.
It is during the second trimester (around 20 weeks) that the sulci begin developing, and they significantly deepen after 28 weeks. By 40 weeks, or term, the brain is a masterpiece of architecture, which because of convolutions and sulci, takes the mental abilities of a brain that should be about 10 feet wide and puts them into a package that will fit comfortably inside your typical head.
The second aspect I would like to touch upon is how does that head, with its precious masterpiece of a brain, come out of the mother intact? The forces of delivery can be pretty rough.
The bony skull protects the brain, but this hard capsule cannot be too rigid or it would have trouble passing through the maternal pelvis at birth. Herein is a major challenge in the delivery of a baby:
1). The brain needs protection on the way out, and 2). the protection can't keep it from getting out.
The skull is not one sealed container, but a collection of plates. In the newborn, these plates have not yet fused together, allowing them to be flexible. The separations between these unfused skull plates are called "sutures," and they have nothing to do with the modern sense of stitching. The sutures of the skull run between the bony plates that together makes up a helmet of protection for our quarterback, the brain. But because of these sutures, the plates can move--there is flexibility, which is so necessary when trying to pass through the mother's pelvis. Sometimes the newborn's skull can look like the shape of his or her mother's pelvis, being "molded" into that shape by passing through it. This misshapen head is usually back to it's cute, round shape withing a couple of days, but when particularly severe can raise the eyebrows of the new parents, for sure.
At birth the brain doesn't stop growing, as is true with the rest of the body. Therefore there's considerable slack that the sutures provide so that the skull can balloon out with the growth of the brain. Just as these separations among the bones of the skull allow the head to mold itself to a shape that can deliver, they also allow spread after delivery when the brain increases in size further.
There are areas where there is considerable distance between the skull plates, where the sutures are widest, and these are called "fontanelles." There are four main fontanelles. The anterior one is the famous "soft spot" of the baby's head. There is also a corresponding one at the back of the head and one on either side (called the temporal fontanelles). At birth, the sutures and the wider fontanelles allow the protective skull around the brain to be somewhat flexible as the baby's head negotiates that passageway to the outside world. Since the skull is somewhat like a globe, the force against the brain during delivery is evenly spread out so no one part of the brain gets too much pressure. This protects the brain from the compression/decompression exerted against it by the maternal pelvis and vaginal walls.
The skull plates continue growing toward each other, ultimately fusing together into the solid bony skull that continues to protect our most cherished organ, the brain. (Did a man just say that?) The sutures, fontanelles, and flexibility are no longer necessary, because the protection that our skulls provided us at birth have allowed us to be smart enough not to stick our heads into anything that tight again. We hope.