Prenatal Development Fact Sheet
Please THINK before you PRINT. Go Green
From the second month, a fetus may show reflexes that seem to involve the
senses. However, since the brain is still immature, it feels things quite
differently from the way we do. While most nerve cells are produced
during the first few months of prenatal development, the senses cannot
work until these cells make synapses. Early reflexes and movements seem
to function in making these connections, molding the senses, and training
the fetal brain to perceive.
Keiko Ohnuma has provided a chronological summary of sensory
development:
1. Nerve growth begins when a sheet of cells on the back of the embryo
folds in the middle to form the future spinal cord. At one end, the
tube enlarges to form the brain's major sections.
2. First responses are reflexes, some of which occur even before the
sense of touch is developed. The fetus will flex its head away from
stimulation around the mouth as early as 7 1/2 weeks. By month's end
the ear begins to take shape.
3. Touch receptors around the mouth are developed by the twelfth
week and elsewhere by the fifteenth. Touching the palms makes the
fingers close, touching the soles of the feet makes the toes curl down,
touching the eyelids makes the eye muscles clench. Nerve cells have
multiplied, synapses are being formed.
4. At 15 weeks the fetus can grasp, frown, squint, and grimace. It may
suck its thumb and swallow. These movements correspond to the
development of synapses in the brain.
5. At 20 weeks nerve-cell production slows as the existing cells grow
larger and make more complex connections. The senses of taste and
smell are now formed. The nerve cells serving each of the senses are
developing into specialized areas of the brain.
6. The fetus can feel movement and may respond to sound as early as 24
weeks.
7. At 25 weeks some babies born prematurely can survive. Nerve
supply to the ear is complete. Brain scans show response to touch at 26
weeks and to light at 27 weeks. A light shone on the mother's abdomen
will make the fetus turn its head, indicating some functioning of the
optic nerve. 8. The eyes open in the womb and the fetus may see its hand and
environment. Some researchers put the start of awareness at the 32nd
week, at which time neural circuits are as advanced as a newborn's.
Brain scans show periods of deep sleep.
9. The fetus begins to develop daily activity cycles. At 35 weeks hearing
is mature. At birth the baby can see shapes and colors within 13 inches
of its face, can distinguish loudness, pitch, and tone, and may even show
a preference for sweets and for the scent of its mother's skin.
Ohnuma, K. (1987, July/August)