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Chapter Four

Chapter Four . Prenatal Development and Birth. From Zygote to Newborn. Germinal period —first 14 days Embryonic period —3rd through 8th weeks Fetal period —9th week through birth. Germinal: The First 14 Days.

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Chapter Four

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  1. Chapter Four Prenatal Development and Birth

  2. From Zygote to Newborn • Germinal period—first 14 days • Embryonic period—3rd through 8th weeks • Fetal period—9th week through birth

  3. Germinal: The First 14 Days • Zygote divides and keep dividing (at least though 3rd doubling they are the same) • At this stage (8 cells) differentiation begins • early “stem” cells take on distinct characteristics • they gravitate to locations, foreshadowing the type of cells they will become

  4. Germinal: The First 14 Days, cont. • At about a week after conception the multiplying cells separate into two masses • outer layer forms a shell (later the placenta) and the inner cells from a nucleus (later the embryo) • first task of out cells to achieve implantation— embed themselves into the nuturant environment of the uterus • 60% of all natural conceptions fail to implant; 70% of in vitro procedures fail to implant

  5. Embryo: From the Third to the Eighth Week • First sign of human structure: thin line down the middle (22 days) that becomes the neural tube, which eventually forms the central nervous system, including brain and spinal column • fourth week • head begins to take shape • heart begins with a miniscule blood vessel that begins to pulsate

  6. Embryo: From the Third to the Eighth Week, cont. • fifth week • arm and leg buds appear • tail-like appendage extends from the spine • eighth week • embryo weighs 1 gram and is 1 inch long • head more rounded; face formed • all basic organs and body parts (but for sex) present • 20% of all embryos spontaneously abort now

  7. Fetus: From the Ninth Week Until Birth • Called a fetus from 9th week on

  8. Third Month • Sex organs take shape (Y cell sends signal to male sex organs; for females, no signal occurs) • genital organs fully shaped by 12th week • All body parts present • Fetus can move every part of body • Fetus weighs 3 ounces and is 3 inches long

  9. Middle Three Months: Preparing to Survive • Heartbeat stronger • Digestive and excretory systems develop more fully • Impressive brain growth (6X in size and responsive) • new neurons develop (neurogenesis) • synapses—connections between neurons (synaptogenesis)

  10. Middle Three Months: Preparing to Survive, cont. • Age of viability—age at which preterm baby can possibly survive (22 weeks) • 26 weeks survival rate about 50% • brain maturation critical to viability • weight critical to viability • 28 weeks survival rate about 95%

  11. Final Three Months— Viability to Full Term • Maturation of the respiratory and cardiovascular systems • critical difference • Gains weight—4.5 lbs. in last 10 weeks

  12. Risk Reduction • Despite complexity, most babies are born healthy • Most hazards are avoidable • Teratology—study of birth defects • teratogens—broad range of substances that can cause environmental insults that may cause prenatal abnormalities or later learning abilities

  13. Determining Risk • Risk analysis—weighing of factors that affect likelihood of teratogen causing harm

  14. Timing of Exposure • Critical period—in prenatal development, the time when a particular organ or other body part is most susceptible to teratogenic damage • entire embryonic period is critical

  15. Amount of Exposure • Dose and/or frequency • Threshold effect—teratogen relatively harmless until exposure reaches a certain level

  16. Amount of Exposure, cont. • Interaction effect—risk of harm increases if exposure to teratogen occurs at the same time as exposure to another teratogen or risk

  17. Genetic Vulnerability • Genetic susceptibilities: product of genes combined with stress • Folic-acid deficiency may cause neural- tube defects • occurs most commonly in certain ethnic groups and less often in others • Males are more genetically vulnerable

  18. Specific Teratogens • No way to predict risk on an individual basis • Research has shown possible effects of most common and damaging teratogens • AIDS and alcohol extremely damaging • pregnant women with AIDS transmit it to their newborns; high doses of alcohol cause FAS; alcohol + drug use increase risk to developing organism

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