Prenatal Development Truth or Fiction?

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Chapter 3Prenatal Development
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The Germinal Stage

• Wanderings

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What Happens During the Germinal Stage?

• Period from conception to implantation
  (approximately 2 weeks)
• Zygote repeatedly divides but does not gain in
  mass
• Travels down Fallopian tube to uterus
• After 3 or 4 days, implants in uterine wall
• Takes form of blastocyst
• Trophoblast, outer membrane of blastocyst,
  differentiates into membranes that will protect
  and nourish the embryo

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Figure 3.1 The Ovarian Cycle, Conception and the
Early Days of the Germinal Stage
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How Does the Dividing Mass of Cells Obtain
Nourishment During the Germinal Stage?

• Prior to implantation, the dividing cells are
  nourished by the yolk of the original egg
• Once implanted, nourishment is obtained from the
  mother

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The Embryonic Stage
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What Happens During the Embryonic Stage of
Prenatal Development?

• Begins with implantation (3rd week) through 8th
  week
• Major organs systems differentiate
• Developmental trends
• Cephalocaudal head takes precedence over lower
  parts of the body
• Proximodistal central axis of body outward

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Figure 3.2 Human Embryos and Fetuses at Various
Stages of Development
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What Happens During the Embryonic Stage of
Prenatal Development?

• Embryonic disk
• Ectoderm (outer layer) develops into nervous
  system, sensory organs, nails, hair, teeth and
  skin
• Endoderm (inner layer) forms the digestive and
  respiratory systems, liver, and pancreas
• Mesoderm (middle layer) develops into the
  excretory, reproductive and circulatory system,
  muscles, skeleton and inner layer of skin

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When Does the Heart Begin to Beat?

• Heart begins to beat (4th week)
• Arm buds and leg buds appear (4th week)
• Eyes, ears, nose and mouth begin to take shape
• Limbs are elongating, facial features become
  distinct (8th week)
• Teeth buds, working kidneys and liver (8th week)

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Sexual Differentiation

• Nondescript sex organs formed (5 to 6 weeks)
• Internal and external genitals at this stage
  resemble female structures
• Sex organs begin to differentiate, based on
  genetic code (7th week)
• Y chromosome causes testes to differentiate
• No Y chromosome causes ovaries to differentiate

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Figure 3.3 Development of the Internal Genital
Organs from an Age of 5-6 Weeks Following
Conception
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Figure 3.4 Development of the External Genital
Organs from an Undifferentiated Stage at 5-6
Weeks Following Conception
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Sex Hormones and Sexual Differentiation

• Male hormonal influences
• Testes produce androgens
• Testosterone differentiates male duct system
  (Wolffian)
• DHT (dihydrotestosterone) triggers development of
  male external genital organs

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Sex Hormones and Sexual Differentiation

• Female hormonal influences
• Small amounts of androgens are produced
• play a role in secondary sexual characteristics
  in adolescence
• important in sex drive of females
• Wolffian ducts degenerate and Mullerian ducts
  develop into Fallopian tubes, uterus and inner
  part of the vagina

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A Closer Look

• Genetic Factors in Sexual Differentiation

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How Does the Embryo Get Nourishment from the
Mother?

• Amniotic Sac
• Protects the embryo/fetus within the uterus
• Sac is surrounded by amniotic fluid
• Placenta
• Mass of tissue exchanges nutrients and wastes
  between embryo/fetus and mother
• Umbilical Cord
• Connects the fetus to the placenta

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Do Germs or Drugs in the Mother Pass Through the
Placenta and Affect the Baby?

• Placenta acts as a filter between the bloodstream
  of the mother and the bloodstream of the
  embryo/fetus
• Oxygen and nutrients reach the embryo
• Carbon dioxide and waste pass to the mother
• Many germs and drugs may also reach the embryo

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The Fetal Stage
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What Happens during the Fetal Stage of Prenatal
Development?

• Third month through birth
• End of first trimester
• Major organ systems formed
• Fingers and toes formed
• Eyes can be distinguished
• Sex of fetus can be determined visually

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What Happens during the Fetal Stage of Prenatal
Development?

• End of second trimester
• Opens and shuts eyes
• Sucks thumb
• Alternates between sleep and wakefulness
• Responds to light and sound

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What Happens during the Fetal Stage of Prenatal
Development?

• End of third trimester
• Heart and lungs increasingly able to sustain life
• Gains in weight and length
• During 7th month, fetus turns upside down in
  uterus
• Born at end of 7th month 90 survival rate

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Fetal Perception

• Sound
• 13th week fetus responds to sound waves
• During third trimester can discriminate pitch
• The Cat in the Hat study (DeCasper and Fifer,
  1980)
• During final six weeks of pregnancy, Mothers read
  The Cat in the Hat twice daily
• Using a special pacifier, newborns chose The Cat
  in the Hat read by their mother
• Newborns prefer Mothers voice

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When Does the Mother Begin to Detect Fetal
Movements?

• Middle of 4th month detect first fetal
  movements
• End of second trimester
• Fetus moves vigorously, turns somersaults
• Begins slow squirming movements
• Begins sharp jabbing and kicking movements
• As fetus grows, movements becomes restricted
• Prenatal activity predicts activity levels after
  birth

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Environmental Influences on Prenatal Development
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How Does the Nutrition of the Mother Affect
Prenatal Development?

• Malnutrition in mother (esp. during last
  trimester)
• Low birthweight, prematurity, stunted growth,
  retardation of brain development, cognitive
  deficiencies, behavioral problems
• Overly slender mother
• Risk preterm deliveries and low birthweight
• Maternal obesity
• Linked to higher risk of stillbirth

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How Does the Nutrition of the Mother Affect
Prenatal Development?

• Folic acid supplements
• Reduce risk of neural tube defects
• Obesity during pregnancy
• Increases risk of neural tube defects
• Mother can expect to gain 25 to 35 pounds during
  pregnancy
• Inadequate weight gain increase chance of
  premature or low-birth-weight baby

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How do Teratogens Affect Prenatal Development?

• Teratogen environmental agents that can harm
  embryo or fetus
• Drug ingested by mother
• Substance produced by mother
• Heavy metals such as lead and mercury
• Excessive hormones
• Radiation
• Pathogens bacteria and viruses

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Does it Matter When, during Pregnancy, a Women
is Exposed to a Teratogen?

• Critical periods of exposure to teratogens
• Based on development of organs
• Embryo is more vulnerable than fetus due to major
  organ systems differentiating

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Figure 3.5 Critical Periods in Prenatal
Development
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A Closer Look

• Spacing Children the Goldilocks Way What is
  Just Right?

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What are the Effects of Maternal Health Problems?

• Sexually Transmitted Infections (STIs)
• Syphilis can cause miscarriage, stillbirth, or
  congenital syphilis
• About one-quarter babies born to HIV/AIDS
  infected mothers are infected also
• Many are infected during childbirth
• HIV is also found in breast milk

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A Closer Look

• Preventing Ones Baby from Being Infected with HIV

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What are the Effects of Maternal Health Problems?

• Rubella (German measles)
• Viral infection may cause birth defects
• Toxemia
• Characterized by high blood pressure, may be
  linked to malnutrition
• May cause premature or undersized babies
• Cause of maternal death
• Rh Incompatibility
• Transfer of maternal antibodies that may cause
  brain damage or death

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What are the Effects of Drugs Taken by the Mother?

• Thalidomide
• missing or stunted limbs
• Antibiotics
• Tetracycline may lead to yellowed teeth and bone
  abnormalities
• Hormones
• Progestin can masculinize external sex organs of
  female embryo
• DES can cause cervical and testicular cancer

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What are the Effects of Drugs Taken by the Mother?

• Vitamins
• High doses of vitamins A and D are associated
  with central nervous system damage, small head
  size and heart defects
• Heroin and Methadone
• Maternal addiction linked to low birthweight,
  prematurity and toxemia
• Baby may be born addicted
• Marijuana
• Risk of low birthweight, immature development of
  nervous system

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What are the Effects of Drugs Taken by the Mother?

• Cocaine
• Maternal use of cocaine increases risk of
  stillbirth, low birthweight, and birth defects
• In utero exposure results in problems throughout
  childhood
• Alcohol
• Alcohol passes through placenta and poses risks
  for death of fetus and neonate, malformations,
  growth deficiencies
• Fetal Alcohol Syndrome (FAS)
• physical and psychological defects

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What are the Effects of Drugs Taken by the Mother?

• Caffeine
• Connected with miscarriage and low-birthweight
• Cigarettes
• Nicotine and carbon monoxide pass through the
  placenta
• nicotine stimulates fetus
• carbon monoxide is toxic and decreases oxygen to
  fetus

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What are the Effects of Environmental Hazards?

• Heavy metals (lead, zinc, mercury)
• Threatens cognitive development
• Prenatal exposure to PCBs
• Connected with smaller, less responsive babies
• More likely to develop cognitive deficits
• Fetal exposure to radiation
• Risks for neural and skeletal problems
• Fathers exposure to heavy metals and radiation
  may also cause abnormalities in baby

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What are the Effects of Maternal Stress?

• Maternal stress linked to secretion of hormones
• Adrenaline passes through placenta
• Connected to complications during pregnancy and
  labor
• Connected to low preterm, low-birthweight, and
  irritable babies

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Is the Parents Age Connected with the Outcome
of Pregnancy?

• Teenage mothers
• Higher incidence of infant mortality and
  low-birthweight children
• Women over age 30
• Increasing risk of chromosomal abnormalities,
  stillborn or preterm babies