During the first trimester of pregnancy, there are some important steps in how the human nervous system develops:

1. Neural Tube Formation (Weeks 3-4)

   • The neural tube starts to form around the third week after fertilization.
   • This tube will eventually become the central nervous system (CNS), which includes the brain and spinal cord.
   • About half of the problems with the neural tube happen in the first month. This shows how important it is for mothers to get enough folic acid.

2. Neurogenesis (Weeks 4-12)

   • Neurogenesis, which is the creation of new nerve cells, begins around Week 4. It continues until Week 20.
   • By Week 6, about 30,000 new neurons are produced every day.

3. Neural Crest Formation (Week 4)

   • Neural crest cells come from the ectoderm, which is the outer layer of cells in the embryo.
   • These cells move to form various parts of the nervous system, such as sensory neurons and glial cells, which support the neurons.
   • This step is very important because if something goes wrong, it can lead to disorders in about 1 in every 1,000 babies born.

4. Patterning of the Brain (Weeks 5-12)

   • By Week 7, the main parts of the brain, like the forebrain, midbrain, and hindbrain, begin to take shape.
   • By the end of Week 12, the basic layout of the CNS is set. This is important for the brain and spinal cord to develop further.