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THE COURSE OF PRENATAL DEVELOPMENT Typical prenatal development begins with fertilization and ends with birth, lasting between 266 and 280 days (from 38 to 40 weeks). It can be divided into three periods: germinal, embryonic, and fetal. The Germinal Period The germinal period is the period of prenatal development that takes place in the first two weeks after conception. It includes the creation of the fertilized egg, called a zygote, cell division, and the attachment of the zygote to the uterine wall. Rapid cell division by the zygote continues throughout the germinal period (recall from Chapter 2 that this cell division occurs through a process called mito- sis). By approximately one week after conception, the differentiation of these cells—their specialization for different tasks—has already begun. At this stage, the group of cells, now called the blastocyst, consists of an inner mass of cells that will eventually develop into the embryo, and the trophoblast, an outer layer of cells that later provides nutrition and support for the embryo. Implantation, the attachment of the zygote to the uterine wall, takes place about 11 to 15 days after conception. Figure 3.1 illustrates some of the most significant developments during the germinal period. The Embryonic Period The embryonic period is the period of prenatal development that occurs from two to eight weeks after conception. During the embryonic period, the rate of cell differentiation intensifies, support systems for cells form, and organs appear. This period begins as the blastocyst attaches to the uterine wall. The mass of cells is now called an embryo, and three layers of cells form. The embryo’s endoderm is the inner layer of cells, which will develop into the digestive and respiratory systems. The mesoderm is the middle layer, which will become the circulatory system, bones, muscles, excretory system, and reproductive system. The ectoderm is the out- ermost layer, which will become the nervous system and brain, sensory receptors (ears, nose, and eyes, for example), and skin parts (hair and nails, for example). Every body part eventually develops from these three layers. The endoderm primar- ily produces internal body parts, the mesoderm primarily produces parts that sur- round the internal areas, and the ectoderm primarily produces surface parts. As the embryo’s three layers form, life-support systems for the embryo develop rapidly. These life-support systems include the amnion, the umbilical cord (both of which develop from the fertilized egg, not the mother’s body), and the placenta. The amnion is like a bag or an envelope and contains a clear fluid in which the developing embryo floats. The amniotic fluid provides an environment that is temperature and humidity controlled, as well as shockproof. The umbilical cord contains two arteries and one vein, and connects the baby to the placenta. The placenta consists of a disk-shaped group of tissues in which small blood vessels from the mother and the offspring intertwine but do not join. Figure 3.2 illustrates the placenta, the umbilical cord, and the blood flow in the expectant mother and developing organism. Very small molecules—oxygen, water, salt, food from the mother’s blood, as well as carbon dioxide and digestive wastes from the offspring’s blood—pass back and forth between the mother and embryo or fetus (Wick & others, 2010). Large molecules cannot pass through the placental wall; these include red blood cells and harmful substances, such as most bacteria and maternal wastes. The mechanisms that govern the transfer of substances across the placental barrier are complex and are still not entirely understood (Barta & Drugan, 2010). By the time most women know they are pregnant, the major organs have begun to form. Organogenesis is the name given to the process of organ formation during the first two months of prenatal development. While they are being formed, the organs are especially vulnerable to environmental changes (Rojas & others, 2010; Torchinsky & Toder, 2010). In the third week after conception, the neural tube that eventually becomes the spinal cord forms. At about 21 days, eyes begin to appear, and at 24 days the cells for the heart begin to differentiate. During the fourth week, the urogenital system becomes apparent, and arm and leg buds emerge. Four chambers of the heart take shape, and blood vessels appear. From the fifth to the eighth week, arms and legs differentiate further; at this time, the face starts to form but still is not very recognizable. The intestinal tract develops and the facial structures fuse. At eight weeks, the developing organism weighs about 1/30 ounce and is just over 1 inch long. The Fetal Period The fetal period, lasting about seven months, is the prenatal period between two months after conception and birth in typical pregnancies. Growth and development continue their dramatic course during this time. Three months after conception, the fetus is about 3 inches long and weighs about 3 ounces. It has become active, moving its arms and legs, opening and closing its mouth, and moving its head. The face, forehead, eyelids, nose, and chin are distinguishable, as are the upper arms, lower arms, hands, and lower limbs. In most cases, the genitals can be identified as male or female. By the end of the fourth month of pregnancy, the fetus has grown to 6 inches in length and weighs 4 to 7 ounces. At this time, a growth spurt occurs in the body’s lower parts. For the first time, the mother can feel arm and leg movements. By the end of the fifth month, the fetus is about 12 inches long and weighs close to a pound. Structures of the skin have formed—toenails and fingernails, for example. The fetus is more active, showing a preference for a particular position in the womb. By the end of the sixth month, the fetus is about 14 inches long and has gained another half pound to a pound. The eyes and eyelids are completely formed, and a fine layer of hair covers the head. A grasping reflex is present and irregular breathing movements occur. As early as six months of pregnancy (about 24 to 25 weeks after conception), the fetus for the first time has a chance of surviving outside of the womb—that is, it is viable. Infants are born early, or between 24 and 37 weeks of pregnancy, usually need help breathing because their lungs are not yet fully mature. By the end of the seventh month, the fetus is about 16 inches long and now weighs about 3 pounds. During the last two months of prenatal development, fatty tissues develop, and the functioning of various organ systems—heart and kidneys, for example—steps up. During the eighth and ninth months, the fetus grows longer and gains substantial weight—about another 4 pounds. At birth, the average American baby weighs 71⁄2 pounds and is about 20 inches long. Figure 3.3 gives an overview of the main events during prenatal development. Notice that instead of describing development in terms of germinal, embryonic, and fetal periods, Figure 3.3 divides prenatal development into equal periods of three months, called trimesters. Remember that the three trimesters are not the same as the three prenatal periods we have discussed. The germinal and embryonic periods occur in the first trimester. The fetal period begins toward the end of the first trimester and continues through the second and third trimesters. Viability (the chances of surviving outside the womb) occurs at the very end of the second trimester. The Brain One of the most remarkable aspects of the prenatal period is the development of the brain (Nelson, 2011). By the time babies are born, they have approximately 100 billion neurons, or nerve cells, which handle information processing at the cellular level in the brain. During prenatal development, neurons spend time moving to the right locations and are starting to become connected. The basic architecture of the human brain is assembled during the first two trimesters of prenatal development. In typical development, the third trimester of prenatal development and the first two years of postnatal life are characterized by connectivity and functioning of neurons (Moulson & Nelson, 2008). As the human embryo develops inside its mother’s womb, the nervous system begins forming as a long, hollow tube located on the embryo’s back. This pear-shaped neural tube, which forms at about 18 to 24 days after conception, develops out of the ectoderm. The tube closes at the top and bottom ends at about 24 days after conception. Figure 3.4 shows that the nervous system still has a tubular appear- ance six weeks after conception. Two birth defects related to a failure of the neural tube to close are anencephaly and spina bifida. The highest regions of the brain fail to develop when fetuses have anencephaly or when the head end of the neural tube fails to close, and they die in the womb, during childbirth, or shortly after birth (Levene & Chervenak, 2009). Spina bifida results in varying degrees of paralysis of the lower limbs. Individuals with spina bifida usually need assistive devices such as crutches, braces, or wheelchairs. Both maternal diabe- tes and obesity place the fetus at risk for developing neural tube defects (McQuire, Dyson, & Renfrew, 2010; Yazdy & others, 2010). A strategy that can help to prevent neural tube defects is for women to take adequate amounts of the B vitamin folic acid, a topic we will further dis- cuss later in the chapter (Rasmussen & Clemmense n, 2010; Shookhoff & Ian Gallicano, 2010).