Understanding Prenatal Development: From Zygote to Newborn
This chapter explores the stages of prenatal development: the germinal period (first 14 days), embryonic period (3rd to 8th weeks), and fetal period (9th week to birth). It examines key processes like cell division, implantation, organ formation, and fetal growth. The discussion includes risks during prenatal development, such as teratogens, and emphasizes the importance of the critical periods for susceptibility to harm. It highlights the role of genetic factors and environmental influences, impacting the health of the newborn.
Understanding Prenatal Development: From Zygote to Newborn
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Presentation Transcript
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 • 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
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
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
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
Fetus: From the Ninth Week Until Birth • Called a fetus from 9th week on
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
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)
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%
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
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
Determining Risk • Risk analysis—weighing of factors that affect likelihood of teratogen causing harm
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
Amount of Exposure • Dose and/or frequency • Threshold effect—teratogen relatively harmless until exposure reaches a certain level
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
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
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
