Prenatal Development (page 2)
Prenatal refers to the time from conception to birth. The prenatal period may be divided into three stages. The first stage is called the germinal stage and lasts for about 2 weeks. The embryonic stage is next and lasts from about 2 to 8 weeks. The third stage, the fetal stage, follows and continues until birth. Although the variation of ovulation among women, as well as within the same woman, creates minor difficulties in accurately dating the pregnancy, this sequence of prenatal development holds true for all children. According to Deiner (1997), a general guideline to estimate the due date is to count back 3 months from the first day of the last menstrual cycle and then add 7 days, resulting in approximately 266 days postconception.
The cell created by the union of the sperm and the egg is the zygote. Within 36 hours of fertilization, mitosis begins; this single cell rapidly divides. During the beginnings of mitosis, the zygote slowly moves down the fallopian tube toward the uterus. This process takes about 3 to 4 days. Once its destination is reached, the zygote has transformed into a liquid-filled structure called a blastocyst, which floats in the uterus for about 24 to 48 hours. Mitosis continues and some of the cells of the blastocyst begin to clump on one side of the uterus to form the embryonic disk. This is the group of cells from which the baby will develop.
As the embryonic disk thickens, it begins to divide into three layers: the ectoderm, the endoderm, and the mesoderm. The ectoderm is the upper layer of cells in the embryonic disk and ultimately will become the epidermis, nails, hair, teeth, sensory organs, and central nervous system. The lower layer of cells, the endoderm, will eventually become the child’s digestive system, respiratory system, and various other internal organs. The mesoderm is the last of the three layers to develop. It will evolve into the dermis, muscles and connective tissue, the skull, and parts of the circulatory and reproductive systems. The remaining parts of the blastocyst will create the prenatal structures required for intrauterine life. These include the placenta, which will nourish and protect the developing infant; the umbilical cord, which will connect the placenta to the developing child; and the amniotic sac, which will house the baby for the entire prenatal period. The outer layer of the blastocyst, called the trophoblast, eventually will produce microscopic, hair-like structures called villi. These villi adhere to the uterine lining until the blastocyst is totally implanted. Once uterine implantation is complete, the germinal stage is over, and the embryonic period begins.
This stage of development has been divided into 23 separate stages termed Carnegie Stages. It is beyond the scope of this chapter to review each of these stages in detail (for a review, see O’Rahilly & Muller, 1996); the main point to note is that during the embryonic stage, the embryo, as it is now called, experiences rapid growth. The amniotic sac, placenta, and umbilical cord are fully developed, and mitosis has progressed to the point that the embryo resembles a miniature human being. During this period, the developing embryo is extremely sensitive to toxic and infectious agents. Nearly all major birth defects, such as malformed limbs, cleft palate, blindness, and deafness, occur during this period or slightly after, but typically within the first 3 months of pregnancy (O’Rahilly & Muller, 1996). In fact, as noted earlier, many embryos spontaneously abort when their defects are markedly severe.
By the end of this period, at about 8 weeks, the embryo has a beating heart, the beginnings of a skeleton, and a rapidly growing brain. This tiny developing human is now only about 2.5 to 3.8 cm (1–11/2 in) long and weighs about 0.9 gm (1/30 oz), but it clearly has begun to show distinct human characteristics (Moore, Persaud, & Torchia, 2007). For example, the head and brain are now visible, with the head accounting for approximately one half the length of the fetus, and a thin pink skin covers the body.
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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