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.
First Signs You're Pregnant
At about this time (three weeks from conception, five weeks from the start of the last period), the mother's breasts begin to feel very sensitive and tender. This is often the first symptom of pregnancy. The queasy stomach, nausea and vomiting typically begin the following week. When a woman is a few days (or hours in some cases!) late, the pregnancy test may be positive. Some test kits are sensitive enough to pick up small amounts of hCG (human chorionic gonadotropin), a hormone secreted by the cells that are burrowing into the endometrial lining of the uterus. By six weeks from the last menstrual period, four weeks from conception, all pregnancy tests should be positive if the pregnancy is developing appropriately.
Among the first organs to develop, the nervous system is represented by the "neural groove," which makes its appearance now. One month after conception, the embryo is about four millimeters long and takes on a "C" shape. Early arm and leg buds appear, and there is rapid cellular development of every organ system. The placenta is functional now, and there is a beginning exchange of blood from mother to baby. A baby at this stage becomes more and more vulnerable to disease and infection as well as alcohol, substance abuse and some medications. The eyes and ears are undergoing rapid and precise development now. If the mother should contract a rubella infection at this time, the risk for eye and ear damage would be approximately 81 percent, whereas it would decrease to 25 percent if she were infected at the end of the second trimester.