Pregnancy and Lactation

Question 1

Maturation and Fertilization

Answer 1

 During ovulation, the secondary oocyte suspended in
Meiosis II is released
 Fertilization will occur if sperm cell interact with the
circulated oocyte
 Meiosis II will be completed, IF fertilization occurs.

Question 2

Entry of the secondary oocyte into

Fallopian tube

Answer 2

 During ovulation, the secondary oocyte (surrounded by
granulosa cells – Corona radiate – meaning “radiating
crown”) is released
 The oocyte is released and enter the fimbriae end of the
fallopian tube.
 Under estrogen stimulation, cilia in the fimbria are
activated and facilitate the entry of the oocyte into the
tube
 Cilia always beat towards the opening (ostium) of the
fallopian tube

Question 3

Fertilization of the secondary oocyte

Answer 3

 After male ejaculation into the vagina during intercourse,
the sperm cells travel from the vagina into the uterus and
enter the fallopian tube.
 Sperm cell movement is aided by uterine and fallopian
tube contraction under the influence of prostaglandin
secreted by the seminal fluid which forms part of the
ejaculate.
 Oxytocin released by the posterior pituitary gland during
female orgasm further aids in sperm transport.
 Only a few thousand out of the half billion sperm cells,
eventually reaches the ampullae – the site of fertlization

Before a sperm can enter the oocyte, it must first
penetrate the multiple layers of granulosa cells attached
to the outside of the ovum (the corona radiata) and
then bind to and penetrate the zona pellucida
surrounding the oocyte.

Once a sperm has entered the oocyte, Meiosis II is
complete (refer to previous section)
 The now mature ovum still carries in its 23 chromosomes.
One of which is the female chromosome, known as the X
chromosome.
 Upon entering the secondary oocyte, the sperm cell head
swells to form a male pronucleus. (Figure 83-1D)
 The 23 unpaired chromosomes of the male and the 23
unpaired chromosomes of the female align themselves to
re-form a complete complement of 46 chromosomes
(diploid) known as a zygote (fertilized ovum)

Question 4

Transport of the Fertilized Ovum in the

Fallopian Tube

Answer 4

 After fertilization has occurred, an additional 3 to 5 days is
required for transport of the fertilized ovum through the
remainder of the fallopian tube into the cavity of the uterus
 This transport is effected mainly by a fluid current in the tube
resulting from epithelial secretion plus action of the ciliated
epithelium that lines the tube; the cilia always beat toward the
uterus.
 Weak contractions of the fallopian tube may also aid passage
of the fertilized ovum.
 Progesterone secreted by the corpus luteum increase
receptors on the smooth muscles of the fallopian tube, relaxing
the tubules to facilitate ovum transport

Question 5

Implantation of the Blastocyst in the

Uterus

Answer 5

 During the delayed transport of the fertilized ovum, cellular division
(mitosis) takes place resulting in the formation of a Blastocyst
 A blastocyst consist of a inner cell mass and a fluid cavity that are
surrounded by single-layer of cells known as trophoblast.
 The inner cell mass will eventually become the fetus.
 The developing blastocyst usually remains in the uterine cavity an
additional 1 to 3 days before it implants in the endometrium;
 Implantation is driven by the action of trophoblast that secrete
proteolytic enzymes that digest pathways between adjacent cells of
the uterine endometrium, forming finger-like chords of the trophoblast
cells.
 Some of the fluid and nutrients released during this process are actively
transported by the same trophoblast cells into the blastocyst, adding
more sustenance for growth.

Question 6

Placenta development

Answer 6

 The endometrial tissue undergo dramatic changes to
support the conceptus – embryo and associated parts.
 The endometrial tissue is now known as decidua.
 While the trophoblastic cords from the blastocyst are
attaching to the uterus, blood capillaries grow into the
cords from the vascular system of the newly forming
embryo, that will form the placenta and the various
membranes of pregnancy.
 The major function of the placenta is to provide for diffusion
of foodstuffs and oxygen from the mother’s blood into the
fetus’ blood and diffusion of excretory products from the
fetus back into the mother.

Question 7

Hormonal Factors in Pregnancy

Answer 7

 In pregnancy, the placenta forms especially large
quantities of human chorionic gonadotropin, estrogens and
progesterone.
 Human chorionic gonadotropin (hCG) hormone – prevents
sloughing of the endometrial wall during (menstruation)
and secretion rate rises significantly during the 1st trimester
of pregnancy.
 The most important function of hCG is to prevent involution
of the corpus luteum at the end of the monthly female
sexual cycle.
 Instead, it causes the corpus luteum to secrete even larger
quantities of its sex hormones—progesterone and
estrogens—for the next few months.

Question 8

hCG function

Answer 8

 Under the influence of hCG, the corpus luteum in the
mother’s ovary grows to about twice its initial size by a
month or so after pregnancy begins. Its continued
secretion of estrogens and progesterone maintains the
decidual nature of the uterine endometrium, which is
necessary for early development of the fetus.
 hCG also stimulates the Leydig cells for testosterone
production in male foetuses.

Question 9

Estrogen during pregnancy

Answer 9

 During pregnancy, estrogen is secreted by the placenta.
 During pregnancy, the extreme quantities of estrogens
cause:

(1) enlargement of the mother’s uterus,
(2) enlargement of the mother’s breasts and growth of the
breast ductal structure, and
(3) enlargement of the mother’s female external genitalia.

 There is reason to believe that estrogens also affect many
general aspects of fetal development during pregnancy—
for example, by affecting the rate of cell reproduction in
the early embryo.

Question 10

Progesterone during pregnancy

Answer 10

 Progesterone is just as essential as estrogen for a successful pregnancy.
1. Progesterone causes decidual cells to develop in the uterine
endometrium. These cells play an important role in nutrition of the
early embryo.
2. Progesterone decreases contractility of the pregnant uterus, thus
preventing uterine contractions from causing spontaneous abortion.
3. Progesterone contributes to development of the conceptus even
before implantation because it specifically increases secretions of
the mother’s fallopian tubes and uterus to provide appropriate
nutrition for the developing morula and blastocyst. Progesterone
may also affect cell cleavage in the early developing embryo.
4. The progesterone secreted during pregnancy helps estrogen
prepare the mother’s breasts for lactation

Question 11

Parturition

Answer 11

 Toward the end of pregnancy, the uterus becomes
progressively more excitable, until finally it develops such
strong rhythmic contractions that the baby is expelled.
 The exact cause of the increased activity of the uterus is
not known, but at least two major categories of effects
lead up to the intense contractions responsible for
parturition:
(1) progressive hormonal changes that cause increased
excitability of the uterine musculature and
(2) progressive mechanical changes.

Question 12

Effects of female sex hormones on the

Uterus

Answer 12

 Estrogens tend to increase the degree of uterine
contractility, during pregnancy.
 Progesterone however inhibits uterine contractility during
pregnancy, thereby helping to prevent expulsion of the
fetus.
 Both progesterone and estrogen are secreted in
progressively greater quantities throughout most of
pregnancy, but from the seventh month onward,
estrogen secretion continues to increase while
progesterone secretion remains constant or perhaps
even decreases slightly (See Fig 83-7)

Question 13

Effects of Oxytocin on the Uterus

Answer 13

 Oxytocin, specifically causes uterine contraction.
 The reasons why oxytocin is important in increasing the contractility of
the uterus near term:
1. The uterine muscle increases its oxytocin receptors and therefore
increases its responsiveness to a given dose of oxytocin during the
latter few months of pregnancy.
2. Oxytocin secretion rate by the neurohypophysis is considerably
increased at the time of labor.
3. Although hypophysectomized animals can still deliver their young at
term, labor is prolonged.
4. Experiments in animals indicate that irritation or stretching of the
uterine cervix, as occurs during labor, can cause a neurogenic reflex
through the paraventricular and supraoptic nuclei of the
hypothalamus that causes the posterior pituitary gland to increase its
secretion of oxytocin

Question 14

Effect of fetal secretions on the Uterus

Answer 14

 The fetus’ pituitary gland secretes increasing quantities
of oxytocin, which might play a role in exciting the
uterus.
 Also, the fetus’ adrenal glands secrete large quantities
of cortisol, another possible uterine stimulant.
 Fetal membranes release prostaglandins in high
concentration at the time of labor. These prostaglandins,
too, can increase the intensity of uterine contractions.

Question 15

Mechanical Factors That Increase

Uterine Contractility

Answer 15

 Stretching smooth muscles of the uterus due to fetal
movements increases their contractility. Note especially
that twins are born, on average, 19 days earlier than a
single child, which emphasizes the importance of
mechanical stretch in eliciting uterine contractions.
 Stretch/Irritation of the Cervix is particularly important in
eliciting uterine contractions. For example, obstetricians
frequently induce labor by rupturing the membranes so
the head of the baby stretches the cervix more
forcefully than usual or irritates it in other ways.

Question 16

Onset of Labor

Answer 16

 Weak and slow rhythmic contractions called Braxton
Hicks contractions become progressively stronger
toward the end of pregnancy.
 These contractions suddenly change, to become
exceptionally strong contractions that start stretching
the cervix and later force the baby through the birth
canal, thereby causing parturition.
 This process is called labor, and the strong contractions
that result in final parturition are called labor
contractions.

Question 17

Labor

Answer 17

 Labor is driven by a positive feedback cycle.
 Stretching of the cervix by the fetus’ head finally
becomes great enough to elicit a strong reflex increase
in contractility of the uterine body.
 This pushes the baby forward, which stretches the cervix
more and initiates more positive feedback to the uterine
body. Thus, the process repeats until the baby is
expelled

Question 18

Theory for the onset of intensely strong contractions during labor

Answer 18

Babies head stretches the cervix

Cervical stretch excites fundic contraction

Fundic contraction pushes the baby and stretches the cervix more

Cycle repeats over and over again

Question 19

Mechanics of Parturition

Answer 19

 The uterine contractions during labor begin mainly at the
top of the uterine fundus and spread downward over the
body of the uterus.
 With greater intensity in contraction at the top then lower
segments.
 Contractions of labor occur intermittently, because strong
contractions impede or sometimes even stop blood flow
through the placenta.
 In the beginning of labor, the contractions might occur only
once every 30 minutes. As labor progresses, the
contractions appear as often as once every 1 to 3 minutes
and the intensity of contraction increases greatly.

 In majority of births, the head is the first part of the baby
to be expelled and, in most of the remaining cases, the
buttocks are presented first.
 Entering the birth canal with the buttocks or feet first is
called a breech presentation
 The first major obstruction to expulsion of the fetus is the
uterine cervix.
 Towards the end of pregnancy, the cervix becomes soft,
which allows it to stretch when labor contractions begin
in the uterus.

 Once the cervix has dilated fully, the fetal membranes
rupture and the amniotic fluid is lost suddenly through the
vagina.
 The head of the fetus moves rapidly into the birth canal,
and with additional force from above, it continues to
wedge its way through the canal until delivery occurs.
 10-45 minutes after birth of the baby, the uterus continues
to contract to a smaller and smaller size, which causes a
shearing effect between the walls of the uterus and the
placenta, thus separating the placenta from its
implantation site

Question 20

Involution of the Uterus After Parturition

Answer 20

 4 to 5 weeks after parturition, the uterus involutes.
 Its weight becomes less.
 During early involution of the uterus, the placental site on
the endometrial surface autolyzes.
 After this time, the endometrial surface becomes reepithelialized and ready for normal, nongravid sex life
again.

Question 21

Lactation

Answer 21

 The breasts, begin to develop at puberty.
 This development is stimulated by the estrogens of the
monthly female sexual cycle; estrogens stimulate growth
of the breasts’ mammary glands plus the deposition of
fat to give the breasts mass.
 In addition, far greater growth occurs during the highestrogen state of pregnancy, and only then does the
glandular tissue become completely developed for
production of milk.

Question 22

Estrogen in breast development

Answer 22

 The large quantities of estrogens secreted by the
placenta cause the ductal system of the breasts to grow
and branch.
 Simultaneously, the stroma of the breasts increases in
quantity, and large quantities of fat are laid down in the
stroma.
 Important for growth of the ductal system are at least four
other hormones: growth hormone, prolactin, adrenal
glucocorticoids, and insulin.
 Each of these hormones is known to play at least some
role in protein metabolism, which presumably explains
their function in the development of the breasts.

Question 23

Progesterone in breast development

Answer 23

 Final development of the breasts into milk-secreting
organs requires progesterone.
 Once the ductal system has developed, progesterone—
acting synergistically with estrogen, as well as with the
other hormones just mentioned—causes additional
growth of the breast lobules, with budding of alveoli and
development of secretory characteristics in the cells of
the alveoli.
 These changes are analogous to the secretory effects of
progesterone on the endometrium of the uterus during
the latter half of the female menstrual cycle.

Question 24

Prolactin promotes Lactation (1)

Answer 24

 Prolactin is secreted by the mother’s anterior pituitary
gland, and its concentration in her blood rises steadily from
the fifth week of pregnancy until birth of the baby, at
which time it has risen to 10 to 20 times the normal nonpregnant level.
 Because of the suppressive effects of estrogen and
progesterone, no more than a few milliliters of fluid are
secreted each day until after the baby is born.
 The fluid secreted during the last few days before and the
first few days after birth is called colostrum; it contains
essentially the same concentrations of proteins and lactose
as milk, but it has almost no fat.

Question 25

Prolactin promotes Lactation (2)

Answer 25

 After birth, when both estrogen and progesterone
secretion ceases from the placenta, the lactogenic
effect of prolactin from the mother’s pituitary gland
assume its natural milk-promoting role.
 The breasts begin to secrete copious quantities of milk
instead of colostrum.
 This secretion of milk requires an adequate background
secretion of most of the mother’s other hormones as well,
but most important are growth hormone, cortisol,
parathyroid hormone, and insulin. These hormones are
necessary to provide the amino acids, fatty acids,
glucose, and calcium required for the formation of milk.

Question 26

Prolactin after birth

Answer 26

 After the birth of the baby, the basal level of prolactin secretion returns to the
non-pregnant level during the next few weeks.
 However, each time the mother nurses her baby, nervous signals from the nipples
to the hypothalamus cause a 10- to 20-fold surge in prolactin secretion that lasts
for about 1 hour.
 This prolactin acts on the mother’s breasts to keep the mammary glands secreting
milk into the alveoli for the subsequent nursing periods.
 If this prolactin surge is absent or blocked as a result of hypothalamic or pituitary
damage or if nursing does not continue, the breasts lose their ability to produce
milk within 1 week or so.
 However, milk production can continue for several years if the child continues to
suckle, although the rate of milk formation normally decreases considerably after
7 to 9 months

Question 27

Suspension of Ovarian Cycle during

nursing

Answer 27

 In most nursing mothers, the ovarian cycle (and ovulation) does not
resume until a few weeks after cessation of nursing.
 The reason seems to be that the same nervous signals from the
breasts to the hypothalamus that cause prolactin secretion during
suckling, inhibit secretion of gonadotropin-releasing hormone by the
hypothalamus.
 This inhibition, suppresses formation of the pituitary gonadotropic
hormones—luteinizing hormone and follicle-stimulating hormone.
 However, after several months of lactation, in some mothers
(especially those who nurse their babies only some of the time), the
pituitary begins to secrete sufficient gonadotropic hormones to
reinstate the monthly sexual cycle, even though nursing continues.

Question 28

Ejection (or “Let-Down”) Process in Milk

Secretion—Function of Oxytocin (1)

Answer 28

 Milk is secreted continuously into the alveoli of the
breasts, but it does not flow easily from the alveoli into
the ductal system and, therefore, does not continually
leak from the nipples.
 Instead, the milk must be ejected from the alveoli into
the ducts before the baby can obtain it.
 This ejection is caused by a combined neurogenic and
hormonal reflex that involves the posterior pituitary
hormone Oxytocin.

Question 29

Ejection (or “Let-Down”) Process in Milk

Secretion—Function of Oxytocin (2)

Answer 29

 When the baby suckles, it receives virtually no milk for the first half
minute or so.
 Sensory impulses must first be transmitted through somatic nerves
from the nipples to the mother’s spinal cord and then to her
hypothalamus, where they cause nerve signals that promote
oxytocin secretion at the same time that they cause prolactin
secretion.
 The oxytocin is carried in the blood to the breasts, where it causes
myoepithelial cells (which surround the outer walls of the alveoli) to
contract, thereby expressing the milk from the alveoli into the ducts.
Then the baby’s suckling becomes effective in removing the milk.
 This process is called milk ejection or milk let-down.