By 24 to 26 weeks, the fetus is fully-formed, in the sense that all of the main organs are there and working, although not terribly efficiently. Babies born at this gestational age now have a good chance of survival (which wasn't the case just a few years ago!), but it takes tremendous effort and skill on the part of neonatologists, nurses, and other specialists to make that happen. In the uterus, during the last 12-13 weeks (the third trimester), the main business is growth.

Growth depends on the placenta

The main event for the baby during the last weeks of pregnancy is growth. Her weight triples in the last 12 weeks, and her length doubles. She takes on stores of fat, protein (in the form of muscles and other tissues), calcium (bones), and key nutrients such as iron.

All of this build-up depends on a well-functioning placenta. In the placenta, blood from the mother circulates around a system of tiny fetal blood vessels, allowing nutrients and oxygen to pass from the mother's blood to the baby's.

Because the fetus and mother are genetically different, blood cannot simply flow from the mother into the baby. Instead, nutrients from the mother's blood drift through the walls of the fetal capillaries in the placenta and enter the fetal bloodstream. Waste products from the baby pass in the opposite direction.

The placenta is one of the first structures to form during pregnancy, but it really comes into its own in the last trimester. Illnesses or environmental forces which interfere with the placenta can restrict the fetus' growth.

One such factor is cigarette smoking by the mother, which causes blood vessels to squeeze shut. In the placenta, that means less blood available to the infant--and thus less nutrition. Severe diabetes in the mother also can interfere with the blood vessels in the placenta, and hence with infant growth.

Brain growth

The most dramatic growth takes place in the brain with new brain cells and many new connections between cells forming by the minute. The long root-like extensions called axons carry electrical signals from one part of the brain to another, some over quite long distances. A fatty substance, called myelin, develops around the axons, allowing the electrical signals to move much more rapidly and efficiently.

A lot of the brain growth in the third trimester (and in the first two to three years of life) involves laying down of myelin. Myelin is also important in the spinal cord and the nerves that reach to every part of the body, allowing the quick back and forth flow of information that is required for coordinated, skillful movement. The jerky, poorly coordinated movements of newborns (and particularly in premature infants) are due in large part to their relative lack of myelin.

Can I come out, yet?

The official definition of "full term" is 38 to 42 weeks of gestation. From 36 weeks on, most infants do well, with minimal special care. At younger and younger gestational ages, more medical intervention is necessary, along with greater chances of long-term problems relating to the prematurity. Still, with modern care many very tiny, very early babies thrive. Given a choice, however, the best place for most babies during the third trimester is inside.