Phys Chapter 82: Pregnancy and Lactation
In the ovary, the ovum is a primary oocyte
-
-
Just before it’s released by the follicle, its nucleus divides by meiosis to form a 1st polar body and
a secondary oocyte
o This causes each of the 23 pairs of chromosomes to lose one of its partners, which goes
into the polar body
o This leaves 23 unpaired chromosomes in the secondary oocyte
The secondary oocyte is then ovulated with tons of granulosa cells attached to it, called the
corona radiata
The ovum is expelled into the peritoneal cavity, and must then enter one of the fallopian tubes and
travel to the uterus
-
The inner surface of the fimbria of the fallopian tubes are lined with ciliated epithelium
The cilia are activated by estrogen from the ovaries, causing them to beat toward the opening
of the fallopian tube, called the ostium
Fertilization of the ovum:
-
-
-
-
-
After the male ejaculates semen into the vagina, a few sperm are sent within 5-10 minutes from
the vagina up through the uterus and fallopian tubes to the ampulla of the fallopian tubes
o This is helped by contractions of the uterus and fallopian tubes stimulated by
prostaglandins in the male seminal fluid, and also by oxytocin released from the
posterior pituitary gland of the female during her orgasm
o Of the many sperm that get deposited into the vagina, only a small % reach the ampulla
Fertilization of the ovum usually happens in ampulla
Before a sperm can enter the ovum, it has to first penetrate the many layers of granulosa cells
attached to the outside of the ovum, called the corona radiata, and then penetrate the zona
pellucida surrounding the ovum
Once a sperm enters the ovum (currently a secondary oocyte), the oocyte divides again to from
the mature ovum and a second polar body, which gets expelled
o After this happens, the mature ovum still has 23 chromosomes in it’s female pronucleus
 One of these will be the X chromosome
At the same time, the male sperm head swells into a male pronucleus
The 23 unpaired chromosomes of the male pronucleus and the 23 unpaired chromosomes of
the female pronucleus then align themselves to form a new set of 23 pairs, giving you 46
chromosomes total, in the fertilized ovum – page 1004 is a pic
When a sperm forms, half of them will have an X chromosome, and half will have a Y
o So if an x carrying sperm fertilizes the ovum, you get an XX girl
o If a Y carrying sperm fertilizes the ovum, you get an XY boy
After fertilization, the fertilized ovum spends 3-5 days going through the rest of the fallopian tube into
the uterine cavity
-
-
This is helped by epithelial secretions and cilia on the epithelium, which always beats toward the
uterus, and weak contractions of the fallopian tube
The isthmus of the fallopian tube is the last part of it before you get to the uterus
o The isthmus remains contracted for the first 3 days after ovulation
o Once the progesterone made by the ovum increases enough, it increases the # of
progesterone receptors on the fallopian tube smooth muscle, and then activates
receptors to relax the muscle and allow entry of the ovum into the uterus
The time it takes to get to the uterus allows for the ovum to divide enough times to become a
blastocyst, which is the stage it’s in when it enters the uterus
The fallopian tube secretes lots of secretions to be nutrition for the developing blastocyst
After reaching the uterus, the developing blastocyst usually remains in the uterine cavity for another 13 days before it implants in the endometrium
-
-
So implantation is at about the 5th-7th day after ovulation – pic page 1005
Before implantation, the blastocyst gets its nutrition from uterine endometrial secretions, called
“uterine milk”
Implantation happens from trophoblast cells that develop over the surface of the blastocyst
o Trophoblast secrete proteolytic enzymes to digest and liquefy the cells of the uterine
endometrium
o Some of the fluid and nutrients released are transported by the trophoblast into the
blastocyst
Once implantation happens, trophoblast and nearby cells of both the fetus and mom proliferate
rapidly to form the placenta and other membranes of pregnancy
Early nutrition of the embryo:
-
-
-
Progesterone secreted from the ovarian corpus luteum causes the uterine endometrium to
convert it’s stromal cells into large swollen cells filled with glycogen, proteins, and lipids, for
the development of the fetus
When the embryo implants in the endometrium, the continued secretion of progesterone
causes the endometrial cells to swell more and store even more nutrients
o At this point, these cells are called decidual cells, and the total mass of cells is called
the decidua
As the trohpoblast cells invade the decidua, they digest it and release stored nutrients that the
embryo uses
o During the 1st week of development, this this the only way the embryo gets nutrients
o The embryo will continue to get nutrition this way for up to 8 weeks, but by the 16th
day, the placenta starts to provide nutrition too
-
-
-
-
While the trophoblast cords from the blastocyst are attaching to the uterus, blood capillaries
grow into the cords from the embryo’s forming vascular system
o At about 21 days, the embryo’s heart can start pumping blood
At the same time, blood sinuses from mom develop around the outsides of the trophoblastic
cords
The trophoblast then keeps sending out more and more projections, which become placental
villi that the fetal capillaries grow into
So the villi carry fetal blood, and are surrounded by sinuses that have mom’s blood
The final placenta will have fetal blood flow through 2 umbilical arteries, then into the capillaries
of the villi, and then back through an umbilical vein into the fetus – page 1006
o At the same time, mom’s blood flows from her uterine arteries into large maternal
sinuses that surround the villi, and then back into uterine veins of the mother
o Nutrients pass through the placental membrane by diffusion, just like in alveoli
The major job of the placenta is to provide for diffusion of nutrition and oxygen from mom into
the fetus’s blood, and diffusion of excretory stuff from the fetus back into mom
In the early months of pregnancy, the placental membrane is still thick because it hasn’t fully
developed, so it’s permeability is low
o It also has a low surface area because it hasn’t grown enough yet
o Because of all this, there isn’t much diffusion at first
Later in pregnancy, the permeability increases because the placental membrane thins and the
surface area increases, so there’s more diffusion
Rarely, a break can happen in the placental membrane, which allows fetal blood cells to pass
into mom
Diffusion through the placenta:
-
-
Oxygen in the blood of the maternal sinuses passes into fetal blood through simple diffusion,
driven by an oxygen pressure gradient from moms blood to the fetus’s
The fetal Po2 is only about 30 mm Hg, compared to mom’s 50 Po2 in the maternal sinuses
o 3 reasons why this low Po2 is still enough to supply the tissues:
 The fetus Hgb is mainly fetal hemoglobin made by the fetus
 Fetal Hgb can carry up to 50% more oxygen than mom Hgb can
 The Hgb concentration of the fetal blood is about 50% higher than in the mom
 Bohr effect – Hgb can carry more oxygen at a low Pco2 than it can at a high Pco2
 Fetal blood going to the placenta carries CO2 that is then sent to mom
o All these changes cause the ability of the fetal blood to combine with oxygen to
increase, and mom’s ability to bind the oxygen to decrease
 This will force even more oxygen from mom blood, and promote more oxygen
uptake in the fetus
 This is called a double Bohr effect
The fetus can only get rid of its carbon dioxide by sending it through the placenta to mom
o
-
-
The Pco2 of the fetal blood is slightly higher than mom’s blood, which causes diffusion
of carbon dioxide into mom
In late pregnancy, the fetus can use as much glucose as mom’s whole body does
o To provide enough glucose, the trophoblast cells lining the placental villi can do
facilitated diffusion of glucose through the placental membrane with carrier molecules
Fetal wastes are also sent through the placenta to mom, and excreted with her wastes
Hormones of pregnancy:
-
-
In pregnancy, the placenta forms lots of human chorionic gonadotropin (hCG), estrogens,
progesterone, and human chorionic somatommatropin
hCG causes persistence of the corpus luteum, and prevents menstruation
o Syncytial trophoblast cells secrete hCG into mom’s fluids
o hCG can first be measured in the blood 8-9 days after ovulation, shortly before
implantation
 The rate of hCG secretion then increases rapidly to max out at 10-12 weeks of
pregnancy, and then decreases to a lower amount by 16-20 weeks, and stays at
that level for the rest of the pregnancy
o hCG is a glycoprotein that is very similar to LH
o The most important job of hCG is to prevent the corpus luteum from involuting at the
end of the female sexual cycle
 The corpus luteum then instead secretes even more sex hormones for the next
few months, which prevent menstruation and cause the endometrium to
continue to grow and store lots of nutrients
 This forms decidual cells, which are swollen with nutrition and ready for
implantation
 The corpus luteum keeps secreting the sex hormones to keep the endometrium
deciduous, which is needed for the early development of the fetus
 If the corpus luteum is removed before the 7th week of pregnancy, you almost
always have an abortion
 After 7 weeks, the placenta secretes enough progesterone and estrogens to
maintain pregnancy
 The corpus luteum involutes slowly after the 13th-17th week
o hCG also stimulates the interstitial cells of the fetal testes to make testosterone,
allowing for male sex organs to grow for a boy instead of female
 At the end of pregnancy, the test also causes the testes to descend into the
scrotum
The syncytial trophoblast cells of the placenta also secrete estrogen and progesterone
o The estrogens in the placenta are not made at the placenta as cholesterol to estrogen
o Instead, the placenta uses DHEA and 16-hydroxy-DHEA from mom and the fetus’s
adrenals to make estrogen

-
-
The baby’s adrenals are big and mostly made of a fetal zone that secretes tons
of DHEA
o In pregnancy, estrogens enlarge mom’s uterus, breasts, breast ducts, and external
genitalia, and also relax mom’s pelvic ligaments
o Effects of progesterone in pregnancy:
 Progesterone causes the decidual cells to develop
 Progesterone decreases the contractility of the pregnant uterus – prevents
abortion
 Progesterone triggers mom’s fallopian tubes before implantation to secrete
things and tells the uterus get nutrition ready
 Progesterone helps estrogen prepare mom’s breasts for lactation
Human chorionic somatomammotropin – similar to GH, it decreases insulin sensitivity and
makes glucose less available to mom, allowing more to be there for the baby
o Glucose is the major substrate the baby uses to grow
o It also promotes fatty acid movement into mom’s blood, so she can use that for energy
The corpus luteum and the placenta also release relaxin, which relaxes pubic ligaments
o This is increased by hCG
Hormone changes in mom from pregnancy:
o The anterior pituitary enlarges a lot during pregnancy and increases release of ACTH,
TSH, and prolactin, while very little FSH and LH are released thanks to all the sex steroids
o Glucorticoid release is increased in pregnancy
o Aldosterone is released in pregnancy, causing water retention and possible pregnancyinduced hypertension
o The thyroid enlarges during pregnancy and releases more thyroxine, thanks to hCG
o The parathyroids enlarge in pregnancy, causing calcium reabsorption from mom’s bones
to maintain blood calcium and provide it for the baby to ossify it’s own bones
 This is even more pronounced during lactation when the baby needs even more
calcium than the fetus did
How mom’s body responds in pregnancy:
-
-
Mom’s sex organs get a lot bigger in pregnancy
o Uterus gets bigger, breasts get bigger, vagina enlarges, the introitus widens
All the sex steroids in pregnancy can also give the woman edema, acne, or masculine features
The average weight gain during pregnancy is about 25-35 lbs, with most of this happening in the
last 2 trimesters
o This includes the baby, enlarged sex organs (so far about half), extra fluid in blood, & fat
accumulation
o After birth, the extra fluid is excreted, since the fluid retaining hormones are gone with
the placenta
During pregnancy, mom gets very hungry, due to the baby stealing her food & all the hormones
-
-
-
-
-
-
Because of the all the increased metabolic hormones, mom’s metabolism increases in
pregnancy
o This makes her feel overheated
o Mom also expends more energy than normal because of the extra weight she’s carrying
By far the greatest amount of fetal growth happens in the last trimester of pregnancy
o Normally, the mom doesn’t absorb enough protein, calcium, phosphates, and iron from
her diet at this time to supply for these extra needs of the fetus
o In preparation for this though, mom has been storing nutrients up to this point that the
fetus can use
To form the baby’s blood, it needs a lot of iron
o So if mom doesn’t take in enough iron, it can cause a hypochromic anemia
Mom also needs enough vitamin D to absorb enough calcium from the GI
Often leading up to birth, you give a mom vitamin K, so that the baby can clot enough to handle
birth
Blood flow through the placenta, and the increase in mom’s metabolism, will increase mom’s
cardiac output during pregnancy up to the 27th week
o In the last 8 weeks, the cardiac output falls to a little above normal, for reasons we don’t
know
Mom’s blood volume increases during pregnancy, mainly during the second half of pregnancy
o This is due to increased aldosterone and estrogen causing fluid retention, and the bone
marrow making more RBCs
o This provides a safety factor for bleeding during birth
Because of mom’s increased metabolic rate and her larger size, mom uses more oxygen during
pregnancy and forms more carbon dioxide, causing increased respiration and respiratory rate
to keep up
o This is helped by progesterone, and the baby pushing up on the diaphragm, which
decreases ability to breathe deep
Mom’s increased fluid intake and more accumulation of wastes makes her form more urine
Also, the renal tubules ability to reabsorb sodium, chloride, and water is increased
o Due to all the hormones made to increase reabsorption of salt and water
In pregnancy, the renal vessels vasodilate for reasons we don’t know, causing an increased
glomerular filtration rate
Most of the fluid in the amniotic fluid is formed by the fetus’s urine
About 5% of pregnant women have a rapid rise in blood pressure in the last few months, along
with protein leaking into the urine, which is called preeclampsia
o Symptoms of preeclampsia are excess salt and water retention, weight gain, edema, and
hypertension
o Renal blood flow and glomerular filtration rate will be decreased in preeclampsia
o In preeclampsia, there’s failure of the placenta trophoblasts to invade the arterioles of
the uterine endometrium and turn them into large blood vessels with low resistance
 This causes insufficient blood to the placenta, causing it to release things that
cause a lot of the problems seen in preeclampsia,
o
Ecclampsia happens when there’s seizures, possible coma, greatly decreased urine
output, liver malfunction, and extreme hypertension
 It usually happens shortly before birth, and results in a lot of deaths if they’re
not treated
 If you treat quickly with vasodilators and C-section, few die from ecclampsia
Parturition – birth of the baby
-
-
-
Towards the end of pregnancy, the uterus gets progressively more excitable, until finally the
rhythmic contractions are strong enough to expel the baby
o Progesterone inhibits uterine contractility during pregnancy, while estrogens increase
uterine contractility
 More estrogen is secreted after the 7th month
o Oxytocin causes uterine contractility
 Later in pregnancy, uterine muscle makes more receptors for oxytocin
 More oxytocin is released by the posterior pituitary at labor
o Stretching a smooth muscle organ, like the uterus, will increase its contractility
 Stretch of the cervix is especially important
During most of the months of pregnancy, the uterus has periodic episodes of weak and slow
rhythmic contractions called Braxton Hicks contractions
o These get progressively stronger toward the end of pregnancy, but then get suddenly
exceptionally strong
o This surge in contractions starts stretching the cervix and later forces the baby through
the birth canal
o These stronger contractions are called labor contractions
o The positive feedback theory says these contractions get this strong when stretching of
the cervix by the fetuses head finally becomes 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 ot the uterine body
 This process repeats until the baby is expelled
 So the more the cervix is stretched, the more contractile the uterus gets
 Also, stretching the cervix causes release of oxytocin
Once uterine contractions get strong enough in labor, pain signals cause reflexes in the spinal
cord to the abdominal muscles to intensely contract, helping to birth the baby
The uterine contractions during labor start mainly at the top of the uterine fundus, and spread
down over the body of the uterus
o The intensity of contraction is great at the top and body of the uterus, but weak in the
lower uterus and cervix
o So each uterine contraction forces the baby downward towards the cervix
o In the early part of labor, contractions may happen once every 30 minutes
o
-
-
-
As labor progresses, the contractions show up as often as every few minutes, and the
intensity increases a lot, with only a short period of relaxation in between
 If uterine contraction was more constant instead of rhythmic, like what
happens from excess oxytocin, it blocks blood flow and kills the fetus
In more than 95% of births, the head is the first part of the baby expelled
o If not the head, it’s probably going to be the butt or feet, called breech presentation
o The head acts as a wedge to open the birth canal as the fetus is forced downward
o The first hurdle in the birth path is the cervix
 In the first stage of labor, the cervix progressively stretches and dilates, until the
cervical opening is as wide as the head
 The first stage of labor can last from 8-24 hours in the 1st pregnancy, but can
take just minutes in later pregnancies
o Once the cervix has fully dilated, the fetal membranes rupture and the amniotic fluid is
lost suddenly through the vagina
o This starts the 2nd stage, where the baby’s head moves quickly into the birth canal
leading to delivery
 This doesn’t take anywhere near as long as the first stage
For up to 45 minutes after the baby is born, the uterus continues to contract to a smaller and
smaller size, which shears off the placenta from the uterine wall, causing bleeding
o Bleeding is limited cause the uterine smooth muscle arranged itself in a way during
pregnancy where contractions aroundthese bleeding vessels will constrict them
Cramping pains up to the first stage of labor are from hypoxia of the uterine muscles from blood
vessel compression
Labor pains in the second stage of labor are way more severe from all the stretching and tearing
For a month after birth, the uterus involutes and gets smaller
-
Gets to half its pregnancy weight within a week
Lactation suppresses pituitary gonadotropes, which will make the uterus get smaller quicker
For 10 days after pregnancy, the uterine changes also lead to vaginal discharge called “lochia,”
which starts bloody, and then gets clear
After 10 days, the uterus has re-epithelialized, and is ready for normal non pregnant function
again
The breasts begin to develop at puberty – page 1014 pic of what’s in a breast
-
-
This is stimulated by estrogens from the menstrual cycle, which stimulate growth of the breast
mammary glands and deposition of fat to give the breasts mass
The large amounts of estrogen in pregnancy from the placenta cause the breast duct system to
grow and branch
o At the same time, the breast stroma increases, and more fat is laid down in the stroma
Final development of the breasts into a milk-secreting organ needs progesterone
o
-
-
-
-
Once the ducts develop, progesterone works with other hormones to cause the breast
lobules to grow, along with budding of the alveoli and development of secretory
changes in the alveoli
Although estrogen and progesterone are needed for the physical development of the breast
during pregnancy, they also inhibit actual secretion of milk
The job of prolactin is to cause milk secretion
o Prolactin from the anterior pituitary starts to increase at the 5th week of pregnancy up
until birth
o Despite this, in the first few days after birth, mom can’t lactate much because estrogen
and progesterone still inhibit it
o The fluid secreted in the last few days before, and the first few days after birth, is called
colostrum
 It’s the same as milk, but without fat
o After birth, the sudden loss of estrogen and progesterone from the placenta allows
lactation by prolactin, and over the next week the breasts start releasing more milk and
less colostrum
o Secretion of milk needs GH, cortisol, parathyroid hormone, and insulin, to provide the
amino acids, fatty acids, glucose, and calcium to make milk
o Over the next few weeks after birth, prolactin levels decrease back to normal levels
o Each time mom nurses the baby though, nervous signals from the nipples to the
hypothalamus cause a surge in prolactin that lasts for an hour
 This keeps moms mammary glands secreting milk into the alveoli during that
period of nursing
While nursing, the ovarian cycle for ovulation won’t start again until a few weeks after you stop
nursing
o The same signals that suckling causes to go to the hypothalamus and release prolactin,
will also inhibit release of FSH and LH
o After a few months though, the pituitary progressively secretes more gonadotropes
Milk is secreted continuously into the alveoli of the breasts, but doesn’t flow easily from the
alveoli into the duct system, so it doesn’t leak, and instead needs ejected from the alveoli
o When the baby suckles, it gets no milk at first
o Suckling sends impulses to the spinal cord and then hypothalamus to cause release of
oxytocin with prolactin
o Oxytocin is then carried through blood to the breast, where it causes myoepithelial cells
around the alveoli to contract, which ejects the milk into the ducts
o This is called milk ejection aka milk-letdown
o Many neural things can interfere with oxytocin release
Nursing drains mom of energy