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The first 24 hours after fertilization are spent "detangling" and stretching out the chromosomal material that now makes up the full complement of DNA that will become the human child. The five-day journey through the fallopian tube is devoted to the continued division of cells. By the time the egg finally reaches the uterus, it will be a mass of about 100 cells, called a blastocyst. As the cellular material breaks free of the capsule surrounding the egg, it will seek nourishment and come to rest on the thick blood-rich endometrial layer of the uterus. Exposed now to the mother's immune system, the ball of cells produces chemical signals to inform the mother that there is no threat. Specialized burrowing cells invade the endometrium, tapping into the rich source of nutrients.
Two weeks after conception, a milestone in development takes place that coincides with the missed period. The cellular mass, which is about the size of a poppy seed, organizes itself into "top" and "bottom" and is now known as an embryo. A process called "gastrulation" begins, and layers begin to form. The center layer will become the embryo, and the cells that will form the liver, lungs and GI tract are already differentiating at the lower level. The middle layer of cells will become muscle, blood, heart and bone, while the upper layer is organizing into the nervous system, spinal column, brain, skin and hair.
Three weeks after fertilization, the neural tube forms and will eventually close to become the spinal cord, brain and bone surrounding these organs. When the neural tube does not close properly, a neural tube defect (NTD) occurs. Spina bifida and anencephaly are two common forms of NTD. Spina bifida is a defect of the spinal cord from which spinal nerves protrude. Depending upon location and severity, such defects may result in paralysis or death. Anencephaly is a birth defect that results in babies born with underdeveloped brains and incomplete skulls. Research has shown that if all women get 400 micrograms of folic acid every day before pregnancy and for the first three months of pregnancy, neural tube defects can be reduced by 50 to 70 percent. All multivitamins, even children's vitamins, contain 400 micrograms of this important nutrient.