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M08_MURR8663_08_SE_CH08.QXD 214 5/27/08 10:12 AM Page 214 Unit II Influences on the Developing Person and Family Unit organs. The fetal stage, from 8 weeks until birth, is characterized by rapid growth and changes in body form caused by different rates of growth of different parts of the body (34). The fetus is a very small but rapidly developing human being who is influenced by the maternal and external environment. The mother responds to the fetus, especially when fetal movement begins. Table 8-1 summarizes some major physiologic milestones in the sequential development of the conceptus, the new life that has been conceived (34). Table 8-2 summarizes major physiologic changes in the pregnant woman (34). TABLE 8-1 Summary of the Sequence of Prenatal Development Time Period After Fertilization Developmental Event Germinal Stage 30 hours 40 hours 60 hours 3 days 4 days 4 1/2–6 days 6–7 days 7–14 days 2–8 weeks First division or cleavage occurs. Four-cell stage occurs. Morula, a solid mass of 12 to 16 cells; total size of mass not changed because cells decrease in size with each cleavage to allow morula to pass through lumen of fallopian tube. Ectopic pregnancy within fallopian tube occurs if morula is wedged in lumen. Zygote has divided into 32 cells; travels through fallopian tube to uterus. Zygote contains 70 cells. Morula reaches uterus; forms a blastocyst, a fluid-filled sphere. Blastocyst floats in utero. Embryonic disc, thickened cell mass from which baby develops, clusters on one edge of blastocyst. Mass of cells differentiates into two layers: (1) ectoderm, outer layer of cells that become the epidermis, nails, hair, tooth enamel, sensory organs, brain and spinal cord, cranial nerves, peripheral nervous system, upper pharynx, nasal passages, urethra, and mammary glands; (2) endoderm, lower layer of cells that develops into gastrointestinal system, liver, pancreas, salivary glands, respiratory system, urinary bladder, pharynx, thyroid, tonsils, lining of urethra, and ear. Nidation, implantation of zygote into upper portion of uterine wall, occurs. Remainder of blastocyst develops into the following: (1) Placenta, a multipurpose organ connected to the embryo by the umbilical cord that delivers oxygen and nourishment from the mother’s body, absorbs the embryo’s body wastes, combats internal infection, confers immunity to the unborn child, and produces the hormones that (a) support pregnancy, (b) prepare breasts for lactating, and (c) stimulate uterine contractions for delivery of the baby. Placenta circulation evidenced by 11 to 12 days. (2) Umbilical cord, a structure that contains two umbilical arteries and an umbilical vein and connects embryo to placenta. Approximately 20 inches long and 1⁄2 inch in diameter. Rapid cell differentiation occurs. (3) Amniotic sac, a fluid-filled membrane that encases the developing baby, protecting it and giving it room to move. Period during which embryo firmly establishes uterus as home and undergoes rapid cellular differentiation, growth, and development of body systems. This is a critical period when embryo is most vulnerable to deleterious prenatal influences. All development birth defects occur during first trimester (3 months) of pregnancy. If embryo is unable to survive, a miscarriage or spontaneous abortion, expulsion of conceptus from the uterus, occurs. Embryonic Stage 15 days 16 days 19–20 days 21 days 22 days 26–27 days 28 days Cranial end of elongated disk has begun to thicken. Mesoderm, the middle layer, appears and develops into dermis, tooth dentin, connective tissue, cartilage, bones, muscles, spleen, blood, gonads, uterus, and excretory and circulatory systems. Yolk sac, which arises from ectoderm, assists transfer of nutrients from mother to embryo. Neural fold and neural groove develop. Thyroid begins to develop. Neural tube forms, becomes spinal cord and brain. Heart, the first organ to function, initiates action. Eyes, ears, nose, cheeks, and upper jaw begin to form. Cleft palate may occur if development is defective. Cephalic portion (brain) of nervous system formed. Leg and arm buds appear. Stubby tail of spinal cord appears. Crown to rump length, 4–5 mm. M08_MURR8663_08_SE_CH08.QXD 5/27/08 10:12 AM Page 215 Chapter 8 Prenatal and Other Developmental Influences 215 TABLE 8-1 Summary of the Sequence of Prenatal Development —continued Time Period After Fertilization 30 days 31 days 32 days 34 days 35–38 days 40 days 42 days 50 days 55–56 days 8 weeks Developmental Event Rudimentary body parts formed. Limb buds appear. Cardiovascular system functioning. Heart beats 65 times per minute; blood flows through tiny arteries and veins. Lens vesicles, optic cups, and nasal pits forming. By end of first month, new life has grown more quickly than it will at any other time in life. Swelling in head where eyes, ears, mouth, and nose will be. Crown to rump length, 7–14 mm (1⁄4 to 1⁄2 inch). Eye and nasal pit developing. Primitive mouth present. Paddle-shaped hands. Lens vesicles and optic cups formed. Head is much larger relative to trunk. Digital rays present in hands. Feet are paddle-shaped. Crown to rump length, 11–14 mm. Olfactory pit, eye, maxillary process, oral cavity, and mandibular process developing. Brain has divided into three parts. Limbs growing. Beginning of all major external and internal structures. Crown to rump length, 15–16 mm. Elbows and knees apparent. Fingers and toes distinct but webbed. Yolk sac continues to (1) provide embryologic blood cells during third through sixth weeks until liver, spleen, and bone marrow assume function; (2) provide lining cells for respiratory and digestive tracts; (3) provide cells that migrate to gonads to become primordial germ cells. Crown to rump length, 21–23 mm. All internal and external structures present. External genitalia present but sex not discernible; yolk sac disappears, incorporated into embryo; limbs, hands, feet formed. Nerve cells in brain connected. Eye, nostril, globular process, maxilla, and mandible almost completely formed. Ear beginning to develop. Stubby end of spinal cord disappears. Distinct human characteristics. Head accounts for half of total embryo length. Brain impulses coordinate function of organ systems. Facial parts formed, with tongue and teeth buds. Stomach produces digestive juices. Liver produces blood cells. Kidney removes uric acid from blood. Some movement by limbs. Weight, 1 g. Length, 1–1 1⁄2 inches (2.5–3.75 cm). Fetal Stage 9–40 weeks 9–12 weeks 13–16 weeks 17–20 weeks 21–24 weeks Remainder of intrauterine period spent in growth and refinement of body tissues and organs. Eyelids fused. Nail beds formed. Teeth and bones begin to appear. Ribs and vertebrae are cartilage. Kidneys function. Urinates occasionally. Some respiratory-like movements exhibited. Begins to swallow amniotic fluid. Grasp, sucking, and withdrawal reflexes present. Sucks fingers and toes in utero. Makes specialized responses to touch. Moves easily but movement not felt by mother. Reproduction organs have primitive egg or sperm cells. Sex distinguishable. Head one third of body length. Weight, 30 g (1 oz). Length, 3–3 1⁄2 inches at 12 weeks. Ova formed in female. Much spontaneous movement. Sex determination possible. Quickening, fetal kicking or movement, may be felt by mother. Moro reflex present. Rapid skeletal development. Meconium present. Uterine development in female fetus. Lanugo, downy hair, appears on body. Head one fourth of total length. Weight, 120–150 g (4–6 oz). Length, 8–10 inches. Fetus frowns, moves lips, turns head; hands grasp, feet kick. First hair appears. Umbilical cord as long as fetus. Placenta fully developed 16 weeks. New cells exchanged for old, especially in skin. Quickening occurs by 17 weeks. Vernix caseosa appears. Eyebrows, eyelashes, and head hair appear. Sweat and sebaceous glands begin to function. Skeleton begins to harden. Grasp reflex present and strong. Permanent teeth buds appear. Fetal heart sounds can be heard with stethoscope. Weight, 360–450 g (12 oz–1 lb). Length, 12 inches. Extrauterine life, life outside uterus, is possible but difficult because of immature respiratory system. Fetus looks like miniature baby. Mother may note jarring but rhythmic movements of infant, indicative of hiccups. Body becomes straight at times. Fingernails present. Skin has wrinkled, red appearance. Alternate periods of sleep and activity. May respond to external sounds. Weight, 720 g (1 1⁄2 lb). Length, 14 inches. continued M08_MURR8663_08_SE_CH08.QXD 216 5/27/08 10:12 AM Page 216 Unit II Influences on the Developing Person and Family Unit TABLE 8-1 Summary of the Sequence of Prenatal Development—continued Time Period After Fertilization 25–28 weeks 29–32 weeks 33–36 weeks 37–40 weeks Gestational age— 280 days in utero—266 days Developmental Event Jumps in utero in synchrony with loud noise. Eyes open and close with waking and sleeping cycles. Able to hear. Respiratory-like movements. Respiratory and central nervous systems sufficiently developed; some babies survive with excellent and intensive care. Assumes head-down position in uterus. Weight, 1,200 g (2 1/2 lb). Begins to store fat and minerals. Testes descend into scrotal sac in male. Reflexes fully developed. Thumb-sucking present. Mother may note irregular, jerky, crying-like movements. Lanugo begins to disappear from face. Head hair continues to grow. Skin begins to lose reddish color. Can be conditioned to environmental sounds. Weight, 1,362–2,270 g (3–5 lb). Length, 16 inches. Adipose tissue continues to be deposited over entire body. Body begins to round out. May become more or less active because of space constriction. Increased iron storage by liver. Increased lung development. Lanugo begins to disappear from body. Head lengthens. Brain cells number same as at birth. Weight, 2,800 g (6 lb). Length, 18–20 inches. Organ systems operating more efficiently. Heart rate increases. More wastes expelled. Lanugo and vernix caseosa disappear. Skin smooth and plump. High absorption of maternal hormones. Cerebral cortex well defined; brain wave patterns developed. Skull and other bones becoming more firm and mineralized. Continued storage of fat and minerals. Glands produce hormones that trigger labor. Ready for birth. Weight, 3,200–3,400 g (7–7 1⁄2 lb). Length, 20–21 inches. Baby stops growing approximately 1 week before birth. Prenatal Influences on Development Maternal and prenatal health is a major goal of the Healthy People 2010 Initiative in the United States. Refer to the Box Healthy People 2010: Examples of Objectives for Maternal and Prenatal Health Promotion (139). This section summarizes major health risks to the embryo and fetus. More information is available in maternity nursing texts. Preconception risk factors include the following diseases (23): (a) diabetes onset before conception, (b) folic acid deficiency, (c) hepatitis B, (d) HIV/AIDS, (e) hypothyroidism, (f ) maternal phenylketonuria (PKU), (g) obesity, and (h) sexually transmitted diseases such as Chlamydia trachomatis and gonorrhea. Other preconception risk factors include (23) (a) use of isotretinoins such as Accutane, (b) alcohol intake, (c) use of antineoplastic drugs, (d) folic acid deficiency, (e) oral anticoagulant use, and (f ) smoking. HEREDITY Genetic information is transmitted from parents to the offspring through a complex series of processes. The gene is the basic unit of heredity. Relatively few people are born without abnormal chromosomes. Everyone carries abnormal genes that could produce serious diseases or handicaps in the next generation. These genes are usually recessive or additive. Most of the 7,000 known genetic disorders are dominant, since the effects of dominant genes are apparent in the person’s phenotype (34). Refer to Chapter 1 for additional explanation. Congenital malformations are physical or mental disabilities that occur before birth for a variety of reasons. However, sometimes diseases or defects are genetic, the result of dominant or recessive transmission of abnormalities in the genes or chromosomes. An example is Fragile X syndrome, which involves an abnormal fragile section of DNA at a specific location on the X chromosome. This is a sex-linked inherited disorder. The female is usually a carrier, and the male is more likely to demonstrate the effects. It is estimated that 5% to 7% of all mental retardation is caused by this syndrome. Recessive genes may cause inherited diseases, such as cystic fibrosis, sickle-cell syndrome, muscular dystrophy, PKU disease, and Tay-Sachs disease (57). An example of a congenital but not inherited disorder is Down syndrome (trisomy 21), in which the child has three of the chromosome 21 instead of the normal two. Various factors contribute to the chromosomal break. The level of mental retardation varies. An inherited predisposition to a disorder may also interact with an environmental factor before or after birth and lead to expression of a disorder. Some abnormalities or diseases that are inherited appear months or years later (34, 57). Hereditary factors do not by themselves fully determine what the person will become. Reaction range, a range of potential expression of a trait (for example, weight), depends on environmental conditions. Then, there are some traits, such as eye color, that are so strongly programmed by genes that they are canalized, there is little variance in their expression (57). For example, diabetes mellitus has a familial (hereditary) or genetic etiology although other factors contribute to the cause. Incidence in African Americans, Hispanic Americans, and many