When does the fetus's brain begin to work?

Generally speaking, the central nervous system (which is
composed of the brain and the spinal cord) matures in a sequence from "tail" to
head. In just the fifth week after conception, the first synapses begin forming
in a fetus's spinal cord. By the sixth week, these early neural connections
permit the first fetal movements--spontaneous arches and curls of the whole
body--that researchers can detect through ultrasound imaging. Many other
movements soon follow--of the limbs (around eight weeks) and fingers (ten
weeks), as well as some surprisingly coordinated actions (hiccuping, stretching,
yawning, sucking, swallowing, grasping, and thumb-sucking). By the end of the
first trimester, a fetus's movement repertoire is remarkably rich, even though
most pregnant women can feel none of it. (Most women sense the first fetal
movements around eighteen weeks of pregnancy.)

The second trimester marks the onset of other critical
reflexes: continuous breathing movements (that is, rhythmic contractions of the
diaphragm and chest muscles) and coordinated sucking and swallowing reflexes.
These abilities are controlled by the brainstem, which sits above the spinal
cord but below the higher, more recently-evolved cerebral cortex. The brainstem
is responsible for many of our body's most vital functions--heart rate,
breathing, and blood pressure. It is largely mature by the end of the second
trimester, which is when babies first become able to survive outside the
womb.

Last of all to mature is the cerebral cortex, which is
responsible for most of what we think of as mental life--conscious experience,
voluntary actions, thinking, remembering, and feeling. It has only begun to
function around the time gestation comes to an end. Premature babies show very
basic electrical activity in the primary sensory regions of the cerebral
cortex--those areas that perceive touch, vision, and hearing--as well as in
primary motor regions of the cerebral cortex.

In the last trimester, fetuses are
capable of simple forms of learning, like habituating (decreasing their startle
response) to a repeated auditory stimulus, such as a loud clap just outside the
mother's abdomen. Late-term fetuses also seem to learn about the sensory
qualities of the womb, since several studies have shown that newborn babies
respond to familiar odors (such as their own amniotic fluid) and sounds (such as
a maternal heartbeat or their own mother's voice). In spite of these rather
sophisticated abilities, babies enter the world with a still-primitive cerebral
cortex, and it is the gradual maturation of this complex part of the brain that
explains much of their emotional and cognitive maturation in the first few years
of life.