From the second month, a fetus may show reflexes that seem to involve the senses. While most nerve cells are produced during the first few months of prenatal development the sense cannot work until these cells make synapses.

  1. Nerve growth begins when a sheet of cells on the back of the embryo fold in the middle to form the future spinal cord. At one end, the tube enlarges to form the brain’s major sections.
  1. 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 ½ weeks. By month’s end the ear begins to take shape.
  1. 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.
  1. At 15 weeks the fetus can grasp, frown, squint, and grimace. It may suck its thumb and shallow. These movements correspond to the development of synapses in the brain.
  1. At 20 weeks, nerve-cell production slows as the existing cells grow larger and make more complex connections. The sense of taste and smell are now formed. The nerve cells serving each of senses are developing into specialized areas of the brain.
  1. The fetus can fell movement and may respond to sound as early as 24 weeks.
  1. 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.
  1. 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.
  1. 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.