Describe the different stages of development that a fertilized egg undergoes to become a fully developed baby.

Question

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Answer 1

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When an egg becomes fertilized by a sperm, the resulting one-celled entity, called a zygote, immediately begins to develop. The zygote starts out as a microscopic speck. Three days after fertilization, though, the zygote increases to around 32 cells; within a week it has grown to 100-150 cells. These first two weeks are known as the germinal period.

Two weeks after conception, the developing individual enters the embryonic period, which lasts from week 2 through week 8, and he or she is now called an embryo. As an embryo develops through an intricate, preprogrammed process of cell division, it grows 10,000 times larger by 4 weeks of age and attains a length of about one-fifth of an inch. At this point it has developed a rudimentary beating heart, a brain, an intestinal tract, and a number of other organs. Although all these organs are at a primitive stage of development, they are clearly recognizable. Moreover, by week 8, the embryo is about an inch long and has discernible arms, legs, and a face.

From week 8 and continuing until birth, the developing individual enters the fetal period and is called a fetus. At the start of this period, it begins to respond to touch; it bends its fingers when touched on the hand. At 16 to 18 weeks, its movements become strong enough for the mother to sense them. At the same time, hair may begin to grow on its head, and the facial features become similar to those the child will display at birth. The major organs begin functioning, although the fetus could not be kept alive outside the mother. In addition, a lifetime's worth of brain neurons are produced-although it is unclear whether the brain is capable of thinking at this early stage. Within the womb the fetus continues to develop before birth. It begins to grow fatty deposits under the skin, and it gains weight. The fetus reaches the age of viability, the point at which it can survive if born prematurely, at about prenatal age 22 weeks. By week 24, a fetus has many of the characteristics it will display as a newborn. Before birth, a fetus passes through several sensitive periods. A sensitive period is the time when organisms are particularly susceptible to certain kinds of stimuli.

In the final weeks of pregnancy, the fetus continues to gain weight and grow. At the end of the normal 38 weeks of pregnancy, the fetus typically weighs 7 pounds and is about 20 inches in length.