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Prenatal Development.docx

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PSYC 302

Prenatal Development and the Newborn Period A. Conception • Results from the union of two gametes, the egg and the sperm • Gametes are produced through a specialized cell division, which results in each gamete’s having only half the genetic material of all other normal cells in the body. Female Reproductive System • The process of reproduction starts with the launching of an egg from one of the woman’s ovaries into the fallopian tube. • If sperm are available near the time the egg is released, then conception will be possible. Sex Differences Begin at Conception • Approximately 120 to 150 males are conceived for every 100 females because the sperm bearing the Y chromosome are lighter and swim faster to the egg than those bearing an X chromosome. • However, male embryos are miscarried at higher rates than female embryos, and boys are more vulnerable to developmental disorders • Males are also more vulnerable to illness throughout the life span The Zygote • The fertilized egg, or zygote, has a full complement of human genetic material, half from each parent. • Marks the beginning of the three periods of prenatal development • Germinal (conception to two weeks) • Embryonic (3 to 8 week) • Fetal (9 week to birth) B. Developmental Processes • Four major developmental processes transform a zygote into an embryo and then into a fetus. 1. Cell division results in the proliferation of cells 2. Cell migration is the movement of cells from their point of origin to somewhere else in the embryo 3. Cell differentiation transforms the embryo’s unspecialized stem cells into different types of cells 4. Apoptosis, genetically programed cell death, also enables prenatal development Phylogenetic Continuity • The idea that because of our common evolutionary history, humans share some characteristics and developmental processes with other animals, especially mammals. The Role of Hormones • Hormones play a crucial role in sexual differentiation. o All humans’fetuses can develop either male or female genitalia, depending on the presence or absence of testosterone. • One of the many ways in which the fetus acts as an instigator of its own development C. Early Development • By the 4th day after conception, the zygote arranges itself into a hollow sphere of cells with a bulge of cells, the inner cell mass, on one side. o The inner cell mass eventually forms into the embryo Twins • Identical twins originate from the splitting in half of the inner cell mass, resulting in the development of genetically identical individuals • Fraternal twins result when two eggs are released into the fallopian tube at the same time and are fertilized by different sperm The Embryo • After implantation, the inner cell mass becomes the embryo and the rest of the cells develop into its support system. • The neural tube is a U-shaped groove formed from the top layer of differentiated cells in the embryo o It eventually becomes the brain and the spinal cord. • The support system includes: • Placenta: permits the exchange of materials between the bloodstream of the fetus and that of the mother • Umbilical cord: the tube that contains the blood vessels that travel from the placenta to the developing organism and back again Protecting the Fetus • The placental membrane is a barrier against some, but not all toxins and infectious agents. • The amniotic sac, a membrane filled with fluid in which the fetus floats, provides a protective buffer for the fetus D. Fetal Behavior • By 12 weeks after gestation, most of the movements that will be present at birth have appeared. • Prenatal to postnatal continuity • Swallowing amniotic fluid promotes the normal development of the palate and aids in the maturation of the digestive system • Movement of the chest wall and pulling in and expelling small amounts of amniotic fluid help the respiratory system become functional Behavioral Cycles • Become stable during the second half of pregnancy • Circadian rhythms are also apparent • Near the end of pregnancy, the fetus’sleep and wake states are similar to those of the newborn E. Fetal Experience • The sensory structures are present relatively early in prenatal development and play a vital role in fetal development and learning. o Prenatal visual experience is negligible o Fetus experiences tactile stimulation as a result of its own activity, and tastes and smells the amniotic fluid th o It responds to sounds from at least the 6 month of gestation G. Fetal Learning • At 32 weeks gestation, the fetus decreases responses to repeated or continued stimulation, a simple form of learning called habituation. • Newborn infants have been shown to recognize rhymes and stories presented before birth. • Newborns also prefer smells, tastes, and sound patterns that are familiar because of prenatal exposure H. Hazards to Prenatal Development • So far the focus has been on the normal course of prenatal development, but sometimes there are hazards that occur during prenatal development. Miscarriage • By far the most common misfortune in prenatal development is spontaneous abortion (miscarriage). • Around 45% or more of conceptions result in very early miscarriages, and about 15-20% of pregnancies of which women are aware are miscarried • The majority of embryos that miscarry very early have severe defects Environmental Influences • Teratogens are environmental agents that have the potential to cause harm during prenatal development. • Timing is a crucial factor in the severity of the effects of potentially harmful agents o Many agents cause damage only if exposure occurs during a sensitive period in development • Most teratogens show a dose-response relation. o Increases in exposure to potential teratogens are associated with greater probabilities of fetal defects and with more severe problems. • Individual differences also influence the effects of teratogens. • Identifying teratogens is made difficult by the existence of sleeper effects in which the impact of a given agent may not be apparent for many years • Teratogens include legal as well as illegal substances... Legal Drugs • Cigarettes o Cigarette smoking during pregnancy is linked to retarded growth and low birth weight. o Cigarette smoking has also been linked to SIDS (sudden infant death syndrome), although the ultimate causes of SIDS are still unknown o Parents can reduce the risk of SIDS by not smoking, putting babies to sleep on their backs rather than on their stomachs, using firm mattresses and no pillows as bedding for infants, and avoiding wrapping infants in lot
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