Pregnancy, Growth and Development

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Pregnancy, Growth and Development

• Chapter 23

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Conception

• A secondary oocyte can be fertilized for about 24
  hours after ovulation
• Sperm remain viable for up to 48 hours within the
  female reproductive tract
• This gives a three day window for intercourse
  to result in fertilization two days before to
  one day after ovulation

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• Fertilization usually takes place in the outer
  one-third of the uterine tube, but can take place
  in the abdominal cavity
• Sperm swim up the female reproductive tract,
  aided by muscular contractions of the uterus
  stimulated by prostaglandins in the semen.
• The oocyte may also secrete a chemical that
  attracts sperm

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• Sperm undergo a functional change in the female
  tract called capacitation
• During this process the membrane around the
  acrosome becomes fragile, and its enzymes are
  released.
• It requires the combined action of many sperm to
  allow one sperm to penetrate the oocyte.

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• When the first sperm enters the egg, the cell
  depolarizes causing the release of calcium ions
  inside the cell.
• This stimulates the release of granules that
  cause changes in the zona pellucida to prevent
  entry of other sperm.
• Secondary oocyte completes division, and nuclei
  of ovum and sperm unite to form a zygote.

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Twins

• Dizygotic or fraternal twins occur when two
  separate eggs are ovulated. May be of different
  sexes.
• Monozygotic or identical twins occur when a
  single egg is fertilized but dividing cells break
  into two groups and develop into two individuals.
  Genetically identical (clones)

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• Zygote undergoes rapid mitotic cell division, but
  these do not increase the size of the zygote
  called cleavage divisions
• Cleavage produces a solid sphere of cells, still
  surrounded by zona pellucida now called a
  morula.
• At 4.5 to 5 days, cells have developed into a
  hollow ball of cells blastocyst.
• It is at this stage that it enters the uterus.

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• Blastocyst has an outer layer of cells called the
  trophoblast, an inner cell mass, and a fluid
  filled cavity called the blastocele.
• The trophoblast and part of the inner cell mass
  will form the membranes of the fetal portion of
  the placenta, the rest of the inner mass forms
  the embryo.

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Implantation

• The blastocyst remains free in the uterus a short
  time, during which the zona pellucida
  disintegrates.
• Blastocyst nourished by glycogen from glands of
  the endometrium.
• At about 6 days after ovulation blastocyst
  implants orients cell mass toward endometrium,
  and secretes enzymes which allow it to penetrate
  (digest) the endometrial wall. This nourishes
  the blastocyst for about a week after
  implantation.

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• Implantation can also occur in uterine tube,
  cervix, or the abdominal cavity.
• Implantation anywhere outside the uterus is
  called an ectopic pregnancy.
• It is possible for fetus to grow in the abdominal
  cavity, but growth inside the uterine tube causes
  the tube to rupture, resulting in severe bleeding.

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• As early as 8 -12 days after fertilization, the
  blastocyst begins to secrete human chorionic
  gonadotropin or hCG.
• hCG keeps the corpus luteum active until the
  placenta can produce estrogens and progesterone.
• The presence of hCG is the basis for pregnancy
  tests.

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• Inner cell mass forms two cavities
• The yolk sac
• Amniotic cavity
• In humans the yolk sac produces blood cells and
  future sex cells
• The amniotic cavity becomes the cavity in which
  the embryo floats. Fluid is produced from fetal
  urine, and secretions from the skin, respiratory
  tract, and amniotic membranes.

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Primary germ layers

• In between the yolk sac and the amniotic cavity
  is the embryonic disc, which gives rise to the
  primary germ layers
• Endoderm
• Mesoderm
• Ectoderm

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Gestation period

• Divided into three trimesters.
• During first trimester individual starts out as a
  zygote, then morula, blastocyst, and after
  implantation, is called an embryo.
• Embryonic phase of development lasts from
  fertilization until the 8th week of gestation,
  when it becomes a fetus.
• By day 35 the heart is beating, and eye and limb
  buds are present.

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• By month four, the rudiments of all organ systems
  are formed and functioning, and from then on,
  fetal development is primarily a matter of
  growth.
• By the end of the third month the placenta is
  functioning.

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The placenta

• The chorion develops into the fetal part of the
  placenta.
• The chorionic villi connect the fetal circulation
  to the placenta
• Composed of both fetal and maternal tissues

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Functions of the placenta

• 1 Transfer gasses
• 2 Transport nutrients
• 3 Excretion of wastes
• 4 Hormone production temporary endocrine organ
  estrogen and progesterone
• 5 Formation of a barrier incomplete,
  nonselective alcohol, steroids, narcotics,
  anesthetics, some antibiotics and some organisms
  can cross

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Quickening

• The first movement of the fetus felt by the
  mother, usually occurring during the fourth or
  fifth month of pregnancy
• By month seven the fetus is quite active
• During the last month the fetus becomes less
  active (usually due to space considerations.)

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• At the end of pregnancy both the mother and the
  uterus become irritable
• The uterus undergoes Braxton-Hicks contractions
  intermittent, painless contractions which can
  come 10 to 20 minutes apart.
• Become more frequent as gestation progresses, and
  can be mistaken for onset of labor
• Cervix begins to thin and dilate

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Labor (parturition)

• Stage one the period from the onset of true
  labor contractions until the cervix is completely
  dilated at 10 cm.
• The uterine contractions cause the cervix to
  dilate, and the amniotic sac may rupture.
• Usually lasts 6 24 hours depending on the
  number of previous deliveries.

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Stage 2

• Period from maximal cervical dilation until the
  birth of the baby
• Lasts minutes to an hour
• Contractions become more intense and frequent.

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Stage 3

• The expulsion of the placenta
• Usually occurs within 15 minutes after the birth
  of the baby, but can range from 5 to 60 minutes.

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The End !!

• Thats it !
• Youve made it !
• Study well !
• Good luck !