Heredity and Prenatal Development

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CHAPTER 2

• Heredity and Prenatal Development

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The Influence of Heredity on Development
3
Chromosomes and Genes

• Genetics - the field of biology that studies
  heredity
• Fundamental in the transmission of physical
  traits
• Also plays a role in psychological traits
• Chromosomes Rod-shaped structures composed of
  genes that are found within the nuclei of cells
• Gene The basic unit of heredity. Composed of
  DNA
• Polygenic Resulting from many genes
• Deoxyribonucleic Acid (DNA) Genetic material
  that takes the form of a double helix composed of
  phosphates, sugars and bases.

4
Mitosis

• Life begins as a single cell or zygote that
  divides repeatedly.
• Mitosis
• Genetic code carried into new cells in our bodies
• DNA breaks apart double helix duplicates
• DNA forms two camps on either side of the cell,
  and then the cell divides

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Fig. 2-1, p. 24
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Fig. 2-2, p. 25
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Meiosis

• Sperm and ova are produced through meiosis or
  reduction division.
• 46 chromosomes within the cell nucleus first line
  up into 23 pairs.
• DNA ladders unzip, leaving unpaired halves of
  chromosome when cell divides, one member of each
  pair goes to each newly formed cell.
• Each new cell nucleus contains only 23
  chromosomes, not 46.

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Fig. 2-3, p. 25
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Identical and Fraternal Twins

• Monozygotic (MZ)
• - Zygote divides into two cells that separate
  so that each
• develops into an individual with the same
  genetic makeup.
• Dizygotic (DZ)
• - Two ova are produced in the same month, each
  fertilized by a different sperm cell.
• Ovulation
• - As women reach end of childbearing years,
  ovulation becomes less regular.
• -Results in months when more than one ovum is
  released

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Dominant and Recessive Traits

• Traits determined by pairs of genes
• each member of pairs of genes termed an allele
• Homozygous
• having two identical alleles
• Heterozygous
• having two different alleles
• Incomplete dominance/codominances
• when both alleles are shown

• Dominant trait
• trait that is expressed
• Recessive trait
• trait that is not expressed when genes have been
  paired with dominant genes
• Carriers
• people who bear one dominant gene and one
  recessive trait

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Fig. 2-4, p. 26
12
Table 2-1, p. 27
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Chromosomal and Genetic Abnormalities

• Down syndrome
• Caused by an extra chromosome of the 21st pair,
  resulting in 47 chromosomes
• Characteristic features
• -rounded face
• -protruding tongue
• -broad, flat nose
• -sloping fold of skin over the inner corners of
  the eyes
• Show deficits in cognitive development and motor
  development

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Fig. 2-5, p. 28
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Sex-Linked Chromosomal Abnormalities

• MALES
• 1 male in 700-1,000 has XYY
• leads to heightened male secondary sex
  characteristics
• most individuals with disorder are infertile
• 1 male in 500 has Klinefelter syndrome (XXY)
• leads to inadequate development of male primary
  and secondary sex characteristics

• FEMALES
• 1 female in 2,500 has a single X resulting in
  Turner Syndrome
• poorly developed ovaries, short, and infertile.
• 1 female in 1,000 has XXX sex chromosomal
  structure called Triple X syndrome
• normal in appearance but tend to show
  lower-than-average language skills and poorer
  memory for recent events

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Klinefelter Syndrome (XXY)

• 1 male in 500
• Caused by extra X on sex chromosome
• Produces less testosterone than normal males
• Characteristic features of testes, deepening of
  voice, musculature, and male pattern of body hair
  do not develop properly
• Typically treated with testosterone replacement
  therapy

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Turner Syndrome (X)

• 1 female in 2,500
• Single X chromosome
• External genitals are normal, ovaries poorly
  developed, producing little estrogen
• Shorter than average and infertile
• Cognitive deficits with low estrogen problems
  with visual-spatial skills, mathematics, and
  nonverbal memory

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Triple X Syndrome (XXX)

• 1 female in 1,000
• Appears normal, but demonstrates
  lower-than-average language skills
• Poorer memory for recent events
• Development of external sex organs appears normal
• Increased incidence of infertility

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Phenylketonuria (PKU)

• Enzyme disorder transmitted by a recessive gene
  affecting 1 child in 8,000
• Cannot metabolize an amino acid called
  phenylalanine builds up in body and impairs
  functioning of the central nervous system (CNS)
• Results are mental retardation, psychological
  disorders, physical problems
• No cure, but children can be placed on diets low
  in phenylalanine and develop normally

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Huntingtons Disease (HD)

• Affects 1 in 18,000 Americans
• Fatal, progressive degenerative disorder
• Dominant trait
• Physical symptoms include uncontrollable muscle
  movements
• Psychological symptoms include loss of
  intellectual functioning and personality change
• Onset of HD is delayed until middle adulthood
• Possibly half their offspring will develop it
• Medicines can deal with some symptoms

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Sickle-Cell Anemia

• Carriers
• -1 in 10 African Americans
• -1 in 20 Latino or Latina Americans
• Caused by a recessive gene
• Red blood cells take on the shape of a sickle and
  clump together, obstructing small blood vessels
  and decreasing the oxygen supply
• Lessened oxygen can impair cognitive skills
• Physical problems
• -painful and swollen joints
• -jaundice
• -potentially fatal conditions such as
  pneumonia, stroke, and heart and kidney failure

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Tay-Sachs Disease

• 1 in 30 Jewish Americans are carriers
• Caused by recessive gene
• Causes the central nervous system to degenerate,
  resulting in death
• Commonly found among children in Jewish families
  of Eastern European background

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Cystic Fibrosis

• Approx. 30,000 Americans have disorder, 10
  million more are carriers (1 in 31 people)
• Caused by recessive gene
• Most common fatal hereditary disease among
  Caucasians
• Children suffer from excessive production of
  thick mucus that clogs the pancreas and lungs
• Most victims die of respiratory infections in
  their 20s

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Sex-Linked Genetic Abnormalities

• Genetic defects only carried on the X sex
  chromosome
• Hemophilia genetic disorder in which blood does
  not clot properly
• Duchenne muscular dystrophy sex linked
• Involve recessive genes
• Females with two X sex chromosomes are less
  likely than males to show sex-linked disorder
• Sons of female carriers are more likely to be
  afflicted

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Genetic Counseling and Prenatal Testing

• Genetic counselors compile information about a
  couples genetic heritage to explore if their
  children will have a genetic abnormality.
• Couples with likelihood of passing on genetic
  abnormality tend to adopt or not have children of
  their own.
• Prenatal testing can indicate if the embryo or
  fetus is carrying genetic abnormalities.

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Amniocentesis

• Performed on mother 14-16 weeks after conception
• Syringe withdraws fluid from the amniotic sac
  contains cells sloughed off by fetus
• Cells separated, grown in culture, and examined
  for genetic and chromosomal abnormalities
• Routine for women over 35 to detect for Down
  syndrome other chromosomal abnormalities
  increase dramatically as women approach 40
• Amniocentesis carries some risk of miscarriage.

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Fig. 2-6, p. 30
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Chorionic Villus Sampling

• Carried out between 9th and 12th week of
  pregnancy
• Syringe inserted through vagina into uterus to
  suck out threadlike projections (villi) from the
  outer membrane that covers the amniotic sac and
  fetus
• Results available in days
• CVS slightly greater risk than amniocentesis of
  miscarriage both increase the risk of miscarriage

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Ultrasound

• Sound waves that are too high in frequency to be
  heard by human ear obtain information about the
  fetus
• Ultrasound waves are reflected by the fetus
  computer uses the information to generate a
  picture of the fetus
• Picture is termed a sonogram
• Used to guide the syringe in amniocentesis and
  CVS by determining the position of the fetus
• Used to track growth of fetus, detect multiple
  pregnancies, detect structural abnormalities

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Blood Tests

• Used to identify sickle-cell anemia, Tay-Sachs
  disease, and cystic fibrosis
• Alpha-fetoprotein (AFP) used to detect neural
  tube defects such as spina bifida and chromosomal
  abnormalities
• Neural tube defects cause elevation in the AFP
  level in the mothers blood
• Elevated AFP levels related to increased risk of
  fetal death

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Heredity and the Environment
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Heredity and Environment

• Inheritance, nutrition, learning, exercise,
  accident, and illness contribute to development
  of traits
• Genotypes
• -Set of traits we inherit from our parents
• Phenotypes
• -Actual set of traits
• -Both genetic and environmental influences

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Kinship Studies

• The more closely people are related, more genes
  they have in common
• Parents and children have 50 genetic overlap
• Siblings have 50 genetic overlap
• Aunts, uncles have 25 overlap with nieces and
  nephews grandparents, 25 overlap with
  grandchildren
• First cousins have 12.5 overlap

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Twin Studies

• Monozygotic (MZ) twins share 100 of genes
• -MZ twins resemble each other more closely than
  DZ twins on a number of physical and
  psychological traits.
• -MZ twins more likely to look alike and be
  similar in height
• -MZ twins more likely to share autism,
  depression, schizophrenia, and vulnerability to
  alcoholism
• Dizygotic (DZ) twins share 50 of genes same as
  other siblings

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Adoption Studies

• Children that are separated from their natural
  parents at an early age and reared by adoptive
  parents provide special opportunities for sorting
  out nature and nurture.
• When children who are reared by adoptive parents
  are nonetheless more similar to their natural
  parents in a trait, a powerful argument is made
  for a genetic role in the appearance of that
  trait.

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Conception Against All Odds
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Conception

• Conception refers to the union of an ovum and a
  sperm cell.
• Ova
• Women born with all the ova they will ever have,
  about 400,000
• Released from ovarian follicle and enter
  fallopian tube 3-4 days later, egg propelled by
  small, hairlike structures called cilia, and
  perhaps, by contractions in the wall of the tube
• If egg not fertilized, discharged through the
  uterus and vagina, along with endometrium that
  had formed to support an embryo, in the menstrual
  flow during reproductive years, about 400 ova
  will ripen and be released

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Conception (contd)

• Ova are larger than sperm, barely visible to the
  eye
• Sperm cells develop through stages sperm with Y
  sex chromosomes swim faster than sperm with X sex
  chromosomes.
• Male fetuses suffer a higher rate of miscarriage
  than females, often during the first month of
  pregnancy.
• 150 million sperm ejaculated, only 1 in 1,000 can
  approach an ovum

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Conception (contd)

• Sperm have to fight vaginal acidity, gravity, and
  swim against current fluid coming from the
  cervix.
• If sperm survive, reach fallopian tubes 60-90
  minutes after ejaculation approximately 2,000
  enter the correct tube
• Sperm are egged on by a change in calcium ions
  that occurs when an ovum is released.
• Only one enters have to thin the gelatinous
  layer of egg secrete an enzyme that briefly
  thins layer
• Once sperm enters, layer thickens and locks out
  other sperm

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Conception (contd)

• Chromosomes from the sperm cell line up across
  from corresponding chromosomes in the egg cell.
• Form 23 new pairs with unique set of genetic
  instructions
• The direction that sperm travel is guided by a
  change in calcium ions that occurs when an ovum
  is released.

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Fig. 2-8, p. 33
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Fig. 2-9, p. 34
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Infertility

• Infertility problems with men
• - Low sperm count (most common)
• - Lack of sperm
• - Genetic factors
• - Environmental poisons
• - Diabetes
• - Sexually transmitted infections
• - Overheating of the testes
• - Pressure (e.g., bike seats)
• - Aging
• - Certain prescription and illicit drugs

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Infertility (contd)

• Infertility problems with women
• - Irregular ovulation, lack of ovulation
• - Irregularities among the hormones that govern
  
• ovulation, stress, and malnutrition
• - Pelvic inflammatory disorder (PID)
• -infection that scars the fallopian tubes and
  other organs,
• impeding the passage of sperm
• -Endometriosis
• -can obstruct the fallopian tubes

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Infertility Options

• Artificial insemination
• -Sperm is collected and quick-frozen
• -Sperm then injected into womans uterus at
  time of ovulation
• addresses low sperm count as well as low
  sperm motility
• -Can be used for a woman to get pregnant who
  does not have a partner
• In vitro fertilization
• -Ripened ova are surgically removed from the
  mother and
• placed in laboratory dish fathers sperm also
  placed in dish
• -One or more ova fertilized and injected into
  mothers uterus to become implanted
• -Used when fallopian tubes are blocked or
  father has low sperm motility
• -A donor can be used

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Infertility Options (contd)

• Donor IVF
• -Mother does not produce ova
• -Ovum from another woman is fertilized and
  injected into the
• uterus of the mother-to-be
• Surrogate mothers
• -Mothers who bring baby to term for other women
  who are
• infertile
• -Can be artificially inseminated by partner of
  infertile woman
• Adoption
• -Another way for people to obtain children that
  results in the
• formation of loving new families

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Preimplantation Genetic Diagnosis (PGD)

• Reliable method for selecting the sex of the
  child prior to implantation
• Ova are fertilized in vitro
• After a few days of cell division, cell is
  extracted from each
• Sex chromosomal structure of the cell is examined
  microscopically to determine its sex
• Embryos of desired sex are implanted in the
  womans uterus

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Prenatal Development
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Prenatal Development

• Normal gestation period 280 days from date of
  fertilization 266 days
• Divided into three periods
• 1. Germinal stage (Weeks 0 - 2)
• 2. Embryonic stage (Weeks 3 - 8)
• 3. Fetal stage (Weeks 9 - Birth)

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The Germinal Stage

• The period from conception to implantation
• Blastocyst
• -Cells separate into groups that will become
  different structures
• -Inner part of blastocyst has two distinct
  layers that form a thickened mass of cells called
  embryonic disk
• -These cells eventually become the fetus
• Trophoblast
• Four membranes that will protect and nourish the
  embryo
• 1. One produces blood cells, then it
  disappears
• 2. One develops into umbilical cord and blood
  vessels of the
• placenta
• 3. One develops into the amniotic sac
• 4. One becomes the chorion, which will line
  the placenta

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The Embryonic Stage

• Development follows cephalocaudal (head to tail)
  and proximodistal (near to far).
• Growth of the head takes precedence over growth
  of the lower parts of the body.
• Relatively early maturation of the brain and
  organs that lie near the spine
• Ectoderm
• -Outer layer of cells
• -Develops into nervous system, sensory organs,
  nails, hair,
• teeth, and outer layer of skin
• Endoderm
• -Forms digestive and respiratory system, liver,
  and pancreas
• Mesoderm
• -Develops into excretory, reproductive, and
  circulatory systems, muscles, the skeleton,
  and the inner layer of skin

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The Embryonic Stage (contd)

• Head and blood vessels form third week after
  conception
• Heart starts beating
• Major organ systems develop within first 2 months
• -Arm buds and legs appear by end of 1st month
• -Eyes, ears, nose, and mouth take shape
• -Nervous system and brain begins to develop
• During 2nd month, nervous system begins to send
  messages
• Embryo is 1 inch long and weighs 1/30th of an
  ounce
• Teeth buds are formed by end of stage

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The Embryonic Stage (contd)

• At 5-6 weeks, internal and external genitals
  resemble primitive female structures.
• By 7th week, genetic code asserts itself, causing
  sex organs to differentiate
• Genetic activity on Y sex chromosome causes the
  testes to begin to differentiate.
• Ovaries begin to differentiate if the Y
  chromosome is absent.
• Androgens begin to produce.
• Embryo and fetus develop within protective
  amniotic sac sac is surrounded by clear membrane
  and contains amniotic fluid

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The Embryonic Stage (contd)

• Amniotic fluid acts as natural air bag allowing
  embryo and fetus to move without injury.
• Placenta is a mass of tissue that permits the
  embryo to exchange nutrients and waste with the
  mother.
• Mother and embryo have separate circulatory
  systems.
• Placenta permits oxygen and nutrients to reach
  the embryo from the mother, and permits carbon
  dioxide and waste products to pass to the mother
  from the embryo.

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The Embryonic Stage (contd)

• Mother eliminates waste through her lungs and
  kidneys.
• Drugs such as aspirin, narcotics, alcohol,
  tranquilizers, and others cross the placenta and
  affect the fetus.
• Placenta secretes hormones that preserve
  pregnancy, prepare breasts for nursing, and
  stimulate contractions.
• Placenta passes from the birth canal after the
  baby, hence the term afterbirth.

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The Fetal Stage

• Lasts from beginning of third month until birth
• Between 9th and 10th, week fetus responds to
  external stimulation
• Major organ systems formed at end of first
  trimester
• Second trimester consists of further maturation
  and gain in size
• Weight advances from 1 ounce to 2 pounds and
  grows from about 3 to 14 inches

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The Fetal Stage (contd)

• Fetus can open and shut eyes, suck thumb at end
  of second trimester
• Gains about 5 1/2 pounds and doubles in length
• By 7th month, fetus turns upside down toward
  delivery
• Doubles in weight by end of 7th month
• 90 survival if born at end of 7th month and
  given quality care

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The Fetal Stage (contd)

• By 13th week, fetus responds to sound waves
• Experiment by DeCasper and Fifer (1980)
  demonstrated that a fetus is learning while in
  uterus
• Fetal movements are noticeable during middle of
  4th month
• At 29-30 weeks, fetus moves limbs vigorously and
  turns summersaults
• Fetus gets cramped as it grows, becomes less
  active during 9th month

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Fig. 2-12, p. 41
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Environmental Influences on Prenatal Development

• Maternal malnutrition effects
• -low birth weight
• -prematurity
• -retardation of brain development
• -cognitive deficiencies
• -behavioral problems
• -cardiovascular disease
• Fetal malnutrition can sometimes be overcome by a
  supportive, care-giving environment.
• Supplementing diets of pregnant women shows
  positive effects on motor development of infants.
• Maternal obesity linked with higher risk of
  stillbirth

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Environmental Influences on Prenatal Development
(contd)

• Women should gain between 25-35 pounds
• -Overweight women may gain less
• -Slender women may gain more
• Teratogens
• -Environmental agents can harm the embryo or
  fetus
• -Includes drugs taken by mother, lead, mercury
• Pathogens
• -Disease-causing organisms
• -Bacteria and viruses

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Environmental Influences on Prenatal Development
(contd)

• Critical periods refer to the times when organs
  are developing.
• Particular teratogens at a particular time can be
  harmful to the fetus.
• Sexually transmitted infections such as syphilis
  and HIV/AIDS can affect the development of the
  fetus.
• -Routine blood tests are given early in
  pregnancy to diagnose syphilis

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Environmental Influences on Prenatal Development
(contd)

• Rubella
• -Women affected by rubella (German measles)
  during first 20 weeks of pregnancy stand 20
  chance of bearing children with birth defects
  such as deafness, mental retardation, heart
  disease, eye problems (including blindness)
• Preeclampsia (or Toxemia)
• -Life-threatening characterized by high blood
  pressure that may afflict women late in 2nd
  trimester or early in 3rd trimester
• Rh incompatibility
• -Consists of antibodies produced by the mother
  and transmitted to a fetus or newborn infant that
  cause brain damage or death

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Drugs Taken by the Parents

• Thalidomide
• -A sedative used for treatment of insomnia and
  nausea
• -Causes missing or stunted limbs during second
  month of pregnancy

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Drugs Taken by the Parents (contd)

• Hormones
• -Women at risk for miscarriages have been
  prescribed hormones to help maintain their
  pregnancies.
• -Progestin
• -Synthetic version of naturally occurring
  progesterone
• -Can masculinize the external sex organs
  of female embryos
• -Diethylstilbestrol (DES)
• -Used to prevent miscarriage during 1940s
  and 1950s
• -Caused cervical and testicular cancer in
  some offspring
• -1 in 1,000 daughters of DES users will
  develop cancer in the reproductive
• tract

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Drugs Taken by the Parents (contd)

• High doses of vitamins A and D
• -Associated with central nervous system damage,
  small head size, and heart defects
• Narcotics (heroine, methadone) easily pass
  placental membrane and cause fetuses to become
  addicted.
• -After birth, drug is substituted so serious
  withdrawal symptoms are minimized
• -Addicted newborns may have behavioral effects,
  
• motoric delays, language delays

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Drugs Taken by the Parents (contd)

• Marijuana
• Contributes to slower fetal growth, low birth
  weight
• Babies of women who regularly used marijuana show
  increased tremors and startling, suggesting
  immature development of the nervous system.
• Study by Goldschmidt et al. (2000) indicates
  prenatal exposure to marijuana can result in
• -increased hyperactivity
• -impulsivity
• -problems paying attention
• -increased delinquency and aggressive behavior

68
Drugs Taken by the Parents (contd)

• Cocaine
• Cocaine use during pregnancy increases risk of
  stillbirth, low birth weight, and birth defects.
• Infants are excitable, irritable, or lethargic
  sleep is disturbed
• Suggestions of delays in cognitive development
  even at 12 months of age
• Prenatal exposure linked to lower receptive and
  expressive language abilities at older ages

69
Drugs Taken by the Parents (contd)

• Alcohol
• Heavy alcohol consumption during pregnancy can
  result in fetal alcohol syndrome (FAS).
• FAS babies
• -often smaller, with smaller brains
• -facial features include widely spaced eyes,
  underdeveloped upper jaw, flattened nose
• Psychological characteristics appear to reflect
  dysfunction of the brain.
• Maladaptive behaviors such as poor judgment,
  distractibility, and difficulty perceiving social
  cues are common.

70
Fig. 2-13, p. 44
71
Drugs Taken by the Parents (contd)

• Caffeine
• Research regarding caffeine consumption is
  inconsistent
• -Several studies have found that pregnant women
  who take in a good deal of caffeine are more
  likely than nonusers to have a miscarriage or a
  low-birth-weight baby.

72
Drugs Taken by the Parents (contd)

• Cigarettes
• Consist of nicotine, carbon monoxide, and
  hydrocarbons (tars)
• -Nicotine and carbon monoxide pass through
  placenta and reach the fetus
• -Nicotine stimulates the fetus long-term
  effects unknown
• -Carbon monoxide decreases amount of oxygen
  available to the fetus
• -Connected with impaired motor development
  academic delays, learning disabilities, mental
  retardation, and hyperactivity
• Smokers babies likely to be smaller than those
  of nonsmokers
• -Babies of smokers more likely to be stillborn
  or to
• die soon after birth
• Men who smoke are more likely to produce abnormal
  sperm.
• -Babies of fathers who smoke have higher rates
  of birth defects, infant mortality, lower
  birth weights, and cardiovascular problems.

73
Environmental Hazards

• Environmental hazards consist of heavy metals
  such as lead, mercury, and zinc.
• Exposure to lead related to delayed mental
  development at 1 and 2 years of age
• Consumption of PCB-contaminated fish from Lake
  Michigan resulted in babies that were smaller and
  showed poorer motor functioning and memory
  defects.
• Exposure to radiation has been linked to mental
  retardation and physical deformity suggestion to
  avoid unnecessary x-rays

74
Parents Age

• Older fathers more likely to produce abnormal
  sperm
• 20s ideal age for women to bear children
• Teenage pregnancy can result in higher incidence
  of infant mortality and low birth weight.
• Stillborn or preterm babies increase as age of
  mother increases adequate prenatal care
  decreases this likelihood even for first-time
  older mothers.