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Chapter 3

Chapter 3

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University of Toronto Scarborough
Konstantine Zakzanis

Prenatal Development and Birth – Chapter 3 Stages of Prenatal Development Conception takes place during a woman's ovulation or within a few days of it; the ovum, or egg, once released from the mother's ovaries, lives only about three to five days. Prenatal development then encompasses the 38 weeks or approximately 9 months, between conception and birth. Traditionally, pregnancy has been described as occurring in three trimesters, three periods of three months each. Increasingly however, we talk about the three periods of: 1) The zygote: The developing organism from the time of the union of the sperm and the egg to about the second week of gestation; the period of the zygote is comprised of the implantation of the fertilized egg in the wall of the uterus. When this occurs, about one week after conception, the zygote is very small. At this point the zygote forms the physiological dependence with the mother that will continue throughout the course of prenatal development. 2) The Embryo: This is a period of rapid growth that lasts from the beginning of the third week of gestation (the carrying of an embryo or fetus during pregnancy, usually for nine months in humans) until the end of the eighth week. During this period the embryo becomes recognizable as a tiny human. During this period the infant increases 2 million percent in size. During this period three crucial structures develop to support the embryo: 1. The amniotic sac: a membrane containing a watery fluid that encloses the developing organism, protecting it from physical shocks and temperature changes. 2. The placenta: A fleshy, disc-like structure formed by cells from the lining of the uterus and from the zygote, and that, together with the umbilical cord, serves to protect and sustain the life of the growing organism. 3. Umbilical cord: A tube that contains blood vessels that carry blood back and forth between the growing organism and its mother by way of the placenta; it carries oxygen and nutrients to the growing infant and removes carbon dioxide and waste products. During the embryonic period, the inner mass of the developing organism differentiates into three layers: 1. The ectoderm: The hair, nails, parts of the teeth, outer layer of the skin, skin glands, the sensory cells and the nervous system develop from this. 2. The mesoderm: Forms into the muscles, skeleton, circulatory and excretory systems, and the inner skin layer. 3. The endoderm: Forms into the gastrointestinal tract, trachea, bronchia, Eustachian tubes, glands, and vital organs, such as the lungs, pancreas, and liver. Prenatal development is guided by two principles: 1. Cephalocaudal: The pattern of human physical growth in which development begins in the area of the brain and proceeds downward, to the trunk and legs. 2. Proximal-distal: The pattern of human physical growth wherein development starts in central areas, such as the internal organs, and proceeds to more distant areas, such as arms and legs. 3) The Fetus: The term for the developing organism from the third month of gestation to delivery. During this fetal period the development of bodily structures and systems becomes complete. At the end of the third month the fetus has all of its body parts, by the end of the fourth month mothers usually report movement of the fetus. At around five months, reflexes such as sucking, swallowing and hiccupping, usually appear. After the fifth month the fetus develops nails, and sweat glands, coarser, more adult-like skin and soft hair called lanugo (soft hair that covers the fetus's body from about the fifth month of gestation on; may be shed before birth or after). By six month, the eyes can open and close. - Respiratory distress syndrome: A condition of the newborn marked by laboured breathing and a bluish discolouration of the skin or mucous membranes, and which often leads to death. Usually occurs in children that are born prematurely at six months because their regulatory processes are not mature enough, and cannot produce or maintain an adequate amount of surfactant (a liquid that allows the lungs to transmit oxygen from air to blood). - The age of viability: The age of 22 to 26 weeks, by which point the fetus's physical systems are well enough advanced that it has a chance at survival if born prematurely. However, babies born before 28 weeks can still have many difficulties, since many systems are still developing. Risks in Prenatal Environment Teratogen: An environmental agent, such as a drug, medication, dietary imbalance, or polluting substance that may cause developmental deviations in a growing human organism; most threatening in the embryonic stage but capable of causing abnormalities in the fetal stage as well. Teratogens exert their affects on prenatal development in specific ways: 1. A teratogen exerts its effects largely during critical periods. 2. Each teratogen exerts certain specific effects. 3. Either maternal or fetal genotypes may counteract a teratogen's effects. 4. The effects of one teratogen may intensify the effects of another. 5. Different teratogens may produce the same defect. 6. The longer a fetus is exposed to a particular teratogen and the greater the intensity of the teratogen's effects, the more likely it is that the fetus will be seriously harmed. Environmental Dangers Nicotine and alcohol Sudden infant death syndrome (SIDS): Infants under the age of six months stop breathing and die without apparent cause. This syndrome is more common in the offspring of mothers who smoke, drink, or take narcotic drugs. Fetal alcohol syndrome (FAS): A disorder exhibited by infants of alcoholic mothers and characterized by stunted growth, a number of physical and physiological abnormalities, and often, mental retardation. The fetal damage from alcohol appears to be the greatest in the last trimester. Fetal alcohol spectrum disorder (FASD): An umbrella term used to describe the range of effected associated with prenatal exposure to alcohol. Unfortunately it is not yet known how much or how little alcohol consumption leads to problems in infants. Heroin, Cocaine, and Other Drugs Mother who are addicted to heroin or use cocaine have offspring who are also addicted or sustain toxic effects from these drugs. Babies addicted to one of these drugs go through withdrawal symptoms. In some, cases the symptoms can be severe enough to result in infant's death. Although not all mothers that take cocaine have babies with developmental anomalies, cocaine use is related to a series of physical defects in infants, including bone, genital, urinary tract, kidney, eye and heart deformities, and brain hemorrhages. Environmental Toxins − Lead: Exposure during pregnancy has been associated with a variety of problems in newborns, including prematurity and low birthweight, brain damage, and physical defects, as well as with long-term problems in cognitive and intellectual functioning. − Polychlorinated biphenyls (PCBs): Were routinely used in electrical transformer and capacitators until they were banned in the mid 70s. Women who PCB contaminated fish gave birth to infants that were smaller, less responsive, and less neurologically advanced. − Father's exposure to environmental toxins can affect their sperms, and this ultimately have harmful effects on the developing fetus. The exposure may lead to chromosomal abnormalities that may affect their fertility or may increase the risk of their pregnant wives miscarrying or bearing infants with birth defects. Medical Interventions in Pregnancy and Children Physicians may prescribe drugs or diagnostic procedures to alleviate discomfort during pregnancy, sometimes these drugs too have proven dangerous. − Diethylstilbestrol (DES): A synthetic hormone once prescribed to pregnant women to prevent, miscarriages but discontinued when cancer and precancerous conditions were detected in the children of such women. − Thalidomide: A drug once prescribed to relieve morning sickness in pregnant women but discontinued when found to cause serious malformations of the fetus. Current controversy surrounds possible use in treating symptoms of such diseases as AIDS, cancer, and leprosy. Medications used in labour and delivery Babies of mothers who received large amounts of obstetrical medication during labour showed less responsiveness, less smiling, and more irritability for several days after birth, as ell as depression, motoric disorganization, and disruptions in feeding responses. They also have impaired attention and motor abilities at 1 month of age but not longer. Maternal Factors Some factors affecting the fetus are directly related to characteristics of the mother herself. − Age and parity (the number of children she has already borne): A woman's age and parity may interact in influencing the development of her fetus. Women who have their first child when they are under 15 or over 35 years are likely to experience more problems during pregnancy and complications during delivery than other women. The chance of becoming pregnant declines s
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