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| |    The Developing Fetus, Weeks 1-8 by Robert Needlman, M.D., F.A.A.P. reviewed by Marjorie Greenfield, M.D. The first 8 weeks of gestation see the fetus develop from a single fertilized egg into a complete and very complex, organism. This articles traces some of the main steps of this remarkable transformation. (A note on dates: Obstetricians and pregnant women traditionally count the weeks of pregnancy from the last menstrual period. By this measure, the fetus is already two weeks old at the time of fertilization! In this article and the two that follow I'm counting gestational age starting with the moment of conception as day one.) Day 1: The sperm fertilizes the egg. The sperm and egg each carries half of the normal amount of genetic material (DNA.) After fertilization, the egg now has a full set of genes that is unique. Day 6: The embryo, which has been floating free, comes to rest on the lining of the uterus, and burrows in. The embryo is now a round mass of cells, with an empty space in the middle. Week 2: The embryo now receives nutrients from the rich blood supply to the uterus. The embryo now consists of two layers of cells, roughly an "inside" layer and an "outside" layer. Week 3: A third cell layer has appeared, roughly a "middle" layer. The inner, outer, and middle layers of cells are destined to become different tissues in the developing fetus, such as skin and nerves, intestines, and muscle. The heart has begun to pump; the brain and spinal cord begin as a tube-like structure. You miss your first menstrual period. Week 4: Through a complex series of folds, the disc-like embryo now has a head and a tail, with buds that will grow into limbs. The beginnings of a spinal column and muscles are apparent. The embryo is about 4 to 5 millimeters long -- a bit less than 1/4 inch. Week 5: Eyes are starting to form, a mouth-like opening appears near the "head"; finger and toes are beginning to form. The brain now has three recognizable divisions, as it does in the fully-developed baby and adult. Week 6: The beginnings of a nose and palate appear; eyelids cover the eyes. The brain is growing rapidly, making a prominent bulge in the head region. The length is not quite one inch. Week 7-8: More brain growth. Inside, intestines, liver, kidneys, lungs, heart are all taking shape. Ovaries and testes can be seen. The first muscle movements take place. By the end of the 8th week, all of the main organ systems are in place, although not in their final form. The embryo sits in the shape of a C, with its large head bowed, and legs flexed upward. It weighs about 1/3 of an ounce, and measures about 2 inches from top to bottom. Up until this point, scientists have called this wonder of development an embryo. After week 8, the convention is to call it a fetus. All of the most dramatic transformations are over. But there is still a lot of development that needs to take place before the baby is ready to emerge into the world.
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