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The communication between neurons is what helps the foetus learn to move, although during
the first trimester mother probably won’t feel any movement.
In the second trimester, the foetal brain begins to direct the compression of the chest muscles
and movement of the diaphragm. These are kind of like practice breaths and are controlled by
the brain stem. Sucking and swallowing begin around week 16, and by week 21, the foetus can
swallow amniotic fluid.

During the second trimester, the foetus is still testing out movements, kicking and stretching. At
some point between week 16 and week 20, the fetus should be large enough that mom can
feel the baby kick. These movements are directed by the cerebellum. At this point, the foetus
develops the full range of specific foetal movements. In the second trimester, the foetal brain
begins to direct the compression of the chest muscles and movement of the diaphragm. These
are kind of like practice breaths and are controlled by the brain stem.

The foetal brain stem is almost entirely developed by the end of the second trimester. This part of
the brain is located just above the spinal cord. The nervous system has developed enough to
detect loud noises from outside — mom may feel the baby startle when there’s a loud clap of
thunder or a car honking. The baby will start to identify the sound of mom’s voice and may turn
its head up toward the sound. Finally, the brainwaves that occur during sleep, controlled by the
hypothalamus, begin occurring around week 28. Baby will begin experiencing sleep cycles,
including REM sleep, the stage where dreaming occurs.

The third trimester

Brain development in the third trimester is marked by the rapid development of neurons in the
brain and explosive growth. The baby’s brain will triple in size during this time, growing from a
little over 3 ounces to almost 11 ounces at birth. The cerebrum will begin to develop grooves
and ridges and separate into the left brain and right brain. The cerebellum is the fastest-growing
part of the brain in the third trimester. This is the part responsible for motor control, so baby will
begin to move more, wiggling fingers and toes, stretching, and kicking.
Brain development in the third trimester is marked by the rapid development of neurons in the
brain and explosive growth. Your baby’s brain will triple in size during this time, growing from a
little over 3 ounces to almost 11 ounces at birth. The baby is large enough that mother will notice
quite a bit of movement, some of which may be a little uncomfortable. In addition, the foetal
sensory system shows integration and functionality during this period.

The baby’s brain will grow five main parts, each responsible for a different aspect of directing
the body and (eventually) the mind and decision making:

1. The cerebrum is responsible for thinking, feeling, and memory. It’s the largest part of the brain
and contains the cerebral cortex and the frontal and temporal lobes of the brain.

2. The cerebellum is responsible for directing the body’s motor control, from moving arms and
legs to the fine motor skills of pinching and grasping.

3. The brain stem controls the vital functions that keep the body alive. These are mainly involun-
tary systems like heartbeat, blood pressure, and breathing. It also controls the digestive process,
although hunger cues come from elsewhere.