Section 4, Article 1 - Prenatal development usually takes between 266 and 280 days, or 38 to 40 weeks, which can be divided into three stages. The germinal period is the first 2 weeks of prenatal development after conception, which involves rapid cell division and the start of cell differentiation.
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First Stages of the Germinal Period.
The original zygote as it divides into two cells, four cells and eight cells. Occasionally at this early stage, the cells separate completely, forming the beginning of monozygotic twins, quadruplets, or octuplets.
Approximately 10 days after conception, the process of implantation starts, involving the developing organism attaching to the placenta lining the uterus, in which nourishment and protection is provided as development continues. Another key element of this stage is the placenta, which is the conduit between the expectant mother and the developing organism that provides nourishment and oxygen via the umbilical cord, which also removes fetal waste and carbon dioxide.
The period of prenatal development from about the third week through the eighth is the embryonic period, characterized by the development of the basic forms of all structures of the body, including internal organs. The embryonic stage focuses on the embryo, which is a developing human organism between about the third and the eighth week after conception. This period involves rapid growth of nerve cells called neurons. In fact, during the second month of prenatal development, as many as 100,000 neurons are produced every minute (Balter, 2000Source: Balter, L. (2000). Parenthood in America: An encyclopedia (Vol. 1). Santa Barbara, CA: ABC-CLIO. ).
Last is the fetal period, which is the stage of prenatal development that occurs from the ninth week after conception until the fetus is born. This period consists of the fetus gaining approximately 7 pounds (more than 3,000 grams) and the organs continuing to develop, slowly gaining the ability to function independently. In the fetal stage, the developing human organism is called a fetus. This is the longest stage, starting in the ninth week after conception and continuing until birth. During this stage, pregnant individuals often have an ultrasound done, which is an image made using high frequency sound waves that can show a fetus or an internal organ (also called sonogram) (Goncalves, 2016Source: Goncalves, L.F. (2016). Three-dimensional ultrasound of the fetus: How does it help? Pediatric Radiology, 46, 177–189.; Li, 2015Source: Li, X. (2015). Sonographic markers of fetal a-thalassemia major. Journal of Ultrasound Medicine, 34,197–206.).
The Embryonic Period. At 4 weeks conception, the embryo is only about 1/8 inch long, but already the head has taken shape. By 7 weeks, the organism is somewhat less than an inch long. Eyes, nose, digestive system and even the first stage of toe formation can be seen.
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Approximately 22 weeks after conception, the fetus reaches the age of viability, or the age that a fetus could possibly survive outside of the mother’s uterus if specialized medical care were received. During this stage, many changes are seen in the developing brain. For instance, the two halves, or hemispheres, of the brain experience rapid growth. Likewise, in a process called myelination, an insulating material called myelin surrounds neurons, which improves the speed at which messages can be transmitted from the brain to the rest of the body (Gash & Dean, 2015Source: Gash, D. M. & Deane, A. S. (2015). Neuron-based heredity and human evolution. Frontiers in Neuroscience, 9, 209. doi: 10.3389/fnins.2015.00209).