Download Period of the Fetus - People Server at UNCW

Lecture Outline
• Conception
• Periods of Prenatal Development
– Period of the zygote
– Period of the embryo
– Period of the fetus
• Conception
– Ovulation: Release of ovum (egg) from one of
a woman’s two ovaries
• Occurs approximately every 28 days
• Egg moves through one of the fallopian tubes
toward the uterus
• If sexual intercourse occurs near ovulation,
conception can occur
– A sperm must penetrate the outer membrane of the egg
» Each sperm and egg cell has only 23 chromosomes
» When merged, the resulting cell has 46 chromosomes
• Conception usually occurs in the fallopian tube
• Fertilized egg is called a zygote
Period of the Zygote (or Germinal Period)
• Lasts about 2 weeks, from conception to
implantation
• Zygote undergoes mitosis (cell duplication) as it
travels down the fallopian tube to the uterus
• By approximately the 4th day after conception, the
zygote has become a blastocyst
– Fluid-filled sphere of about 60-70 cells
Two parts of the blastocyst:
• Inner cell mass: Cells on the inside of the
blastocyst
– Will become the embryo
• Trophoblasts: Cells on the outside of the
blastocyst
– Will develop into tissues that protect and nourish the
embryo
• Implantation of the blastocyst into the
uterine wall occurs approximately between
7 and 9 days post-conception
• Support structures begin to develop from
the trophoblasts after implantation
Conception and Implantation
Support Structures:
• Amnion: Membrane that encloses the
embryo in amniotic fluid
– Cushions organism from injury
– Helps to keep temperature constant
• Chorion: Membrane surrounding the
amnion
– Develops by the end of the second week postconception
• Tiny blood vessels (villi) emerge from the
chorion and burrow into the uterine wall
– Placenta begins to develop from these blood
vessels
• Placenta is fed by blood vessels from the mother
and from the embryo
• Placenta is connected to the embryo by the
umbilical cord
• Placenta is semi-permeable
– Some substances can pass through, but not all
• Prevents blood of mother and embryo from mixing directly
• Placenta allows nutrients and oxygen to reach the
organism and waste products and carbon dioxide
to be carried away
The Placenta
and
Umbilical
Cord
Period of the Embryo
• Lasts from the 3rd through the 8th week of
pregnancy (about 6 weeks)
• Inner cell mass differentiates into three
layers (about 3rd week post-conception):
– Ectoderm --> nervous system, outer layer of
skin, nails, teeth, ears, eyes
– Mesoderm-->muscles, skeleton, circulatory
system, inner layers of skin
– Endoderm-->digestive system, lungs, urinary
tract, glands
• Ectoderm folds over to form a neural tube
(primitive spinal cord)
– Top of the neural tube swells to form a brain (about 3.5
weeks post-conception)
• External body structures (e.g., arms, legs) and
internal organs (e.g., heart) begin to develop
• Rapid brain development occurs
Period of the Fetus
• Lasts from the ninth week post-conception
until the end of pregnancy (approximately
38 weeks)
Between approximately 9-12 weeks:
– Organs, muscles, and nervous system start to
become organized and connected
• By about 12 weeks, fetus engages in most
movements that are present at birth
– Exs: kicking, thumb-sucking, grasping, swallowing
– Expansion and contraction of lungs (“fetal
breathing”)
– Sexual differentiation has begun
• Sex of the fetus can be detected with ultrasound by
approximately 12 weeks
Period of the Fetus (2nd trimester):
• Between approximately 16-20 weeks,
mother starts to feel movement of fetus
Period of the Fetus (3rd trimester):
• Fetus triples its weight during the last trimester
– Brain growth also continues
• Cerebral cortex enlarges
• Fetus reacts to a variety of sounds
• Age of viability: The point at which a fetus can
first survive outside the womb
– Approximately 22-26 weeks (with medical
intervention)
Period of the Fetus (3rd trimester) con’t:
• Near the end of pregnancy, fetus is awake
more often than earlier in pregnancy
– But still spends most of its time sleeping (like
newborns)
• REM sleep is present
• Fetal activity level is correlated with infant
activity level
• Teratogen: Any environmental agent that
can cause damage during the prenatal
period
General Principles of Teratogenic Effects:
• Dose: Larger doses over longer time periods
usually have more negative effects
• Heredity: The genetic makeup of the mother
and embryo/fetus influence the effect of a
teratogen
• Timing: Effects of a teratogen vary with the
age of the organism at the time of exposure
– Sensitive Period: Time during which basic
structures are being formed
• Each major organ system has its own sensitive
period
– An organ system is most vulnerable to teratogens during
its sensitive period
• Cumulative Risk: Effect of a teratogen may
be worse if there are other risk factors
present (e.g., poor nutrition, lack of medical
care, other teratogens)
Article: Brown et al. (2004)
1. Users would differ from non-users
•
Users: Lower socioeconomic status; greater obstetric
risk
2. Children prenatally exposed to cocaine would
differ from children without exposure in social
development, but not in physical or cognitive
development
Users only:
3. Mothers who did not keep custody of their
children after birth would differ from
mothers who did keep custody
•
Heavier prenatal drug use, lower SES, greater
obstetric risk in mothers who did not have
custody
•
Their newborns would also show more birth
complications (e.g., prematurity)
4. At 2 years of age, children in non-parental
care would differ from children in parental
care:
•
Non-parental care would be higher-quality
than parental care
•
Children in non-parental care would show
better cognitive and social development than
children in parental care
Findings:
1. Users differed from non-users
2. Children with prenatal cocaine exposure
(PCE) did not differ from children without
exposure in terms of physical, social, or
cognitive development
3. Mothers who retained custody did differ
from mothers who did not retain custody
4. Children in non-parental care:
•
Received higher-quality care than children in
parental care
•
Showed better social and cognitive
development than children in parental care