Prenatal Development (page 2)

By — Pearson Allyn Bacon Prentice Hall
Updated on Mar 1, 2011

Fetal Stage

The fetal stage begins with the development of the first bone cell, which is produced from the cartilage of the developing skeleton at about 8 to 9 weeks. The embryo now becomes a fetus. This final stage of the prenatal period lasts until birth. It is during this time that the organism experiences the most extensive rate of growth in its lifetime. The length of the fetus increases until normal body proportions are achieved, and all internal systems and organs continue to increase their efficiency. At the end of the third month, the fetus is approximately 8 cm (3 in) long and begins to show human characteristics. Its musculature is developing, and some spontaneous movements may be noted by the mother. Eyelids, teeth, fingernails, toenails, and external genitalia begin to form, and the gender is easily distinguished. During the fourth month the fetus grows another 2.5 to 5 cm (1 to 2 in) in length and begins to develop hair, called lanugo, on its body. The eyelids begin to blink, the mouth begins to open, and the hands are capable of grasping. The fifth month is characterized by the mother typically feeling the fetus move for the first time. Eyebrows and hair appear, and the skin begins to take on the human shape. The length of the fetus is now roughly 25 cm (10 in). At the end of the second trimester, the fetus has eyes that can open, taste buds on the tongue, and a functional respiratory system; it also is capable of making crying sounds. The fetus now weighs approximately 680 gm (1 lb 8 oz), and life outside of the uterus is possible.

The final trimester is a period of further growth and development of the fetus. This is manifested in greater structural differentiation and definition. At this point the fetus is viable; that is, its respiratory system and central nervous system are developed to a point at which it could survive independently outside of the uterine environment without undue difficulty. This growth continues until the point of delivery, or birth, typically somewhere between weeks 37 to 41 of the pregnancy.

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