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Third Month to Birth:
The Fetus and Placenta
The length o the fetus is usually indicated as the crown-rump length (CRL – sitting height)
or as the crown-heel length (CHL – standing) measuring from the vertex of the skull to the
heel.
The length of pregnancy is considered to be 280 days, or 40 weeks after the onset of the
last normal menstrual period (LNMP). More accurately, it is 266 days or 38 weeks after
fertilization.
Monthly changes
At the beginning of the third month the head constitutes approximately half of the CRL. At the
beginning of the fifth month the head is one-third the CRL, and at birth the head is one-fourth the
CRL. During the third month the face becomes human looking. In 12th week primary
ossification centers appear in long bone and skull and ultrasound can define the sex of external
genitalia. During the 6th week intestinal loops are herniated in the umbilical cord, but by 12
weeks the loops withdraw into the abdominal cavity.
During the fifth month movements of the fetus can be felt by the mother. The fetus is
covered with fine hair, lanugo. During the last two months the skin is covered with a white,
fatty substance called vernix caseosa.
Time of Birth
Time of birth is calculated at 280 days or 40 weeks from the first day of the LNMP. For women
with regular menstrual cycles this method is fairly accurate. An error can be made if there was
bleeding two weeks after fertilization when the blastocyst implanted into the uterus. Most
fetuses are born within 10 to 14 days of the calculated delivery date. If they are born much
earlier they are premature; if born later, they are postmature.
Fetal Membranes and Placenta
Changes in the Trophoblast – By the end of the second month, the trophoblast is characterized
by many secondary and tertiary villi. The villi are anchored in the mesoderm of the chorionic
plate and are attached peripherally to the maternal decidua by way of the outer cytotrophoblast
shell.
During the following months, small extensions sprout from the existing villi and dangle in the
surrounding lacunar or intervillous spaces. Initially, from the inside to outside, these villi have
(1) mesoderm (from the chorionic plate), (2) cytotrophoblastic cells, and (3) syncytium. The
chorionic plate mesoderm will form blood vessels (endothelium) and blood cells; much of the
cytotrophoblast will disappear. The syncytium remains, thus forming the functional tertiary
villus. The only two cell types that form the barrier between the maternal and fetal circulation is
capillary endothelium and syncytial tissue.
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Chorion Frondosum and Decidua Basalis
In human embryology, the chorion is defined as the layer of trophoblast plus the inner layer of
extraembryonic mesoderm (chorionic plate). Within the chorion is the chorionic cavity.
The entire surface of the chorion will have villi early during development. Later, the
embryonic pole will have an expansion and proliferation of villi called the chorion frondosum
(bushy chorion). At the abembryonic pole, villi will degenerate and this part of the chorionic
plate is called the chorion laeve (smooth chorion).
The endometrium has two parts. One is the functionalis, which is sloughed off during
menstruation and after delivery of the placenta. The other part is the basalis, which always
remains to provide new epithelial cells for the uterine lining. During pregnancy, connective
tissue stroma of the basalis become loaded with glycogen and lipid (decidual reaction), thus
forming the decidua basalis. The transformed connective tissue cells are called decidual cells.
The decidua basalis overlies the chorion frondosum. The decidual basalis, also called the
decidual plate, is tightly connected to the chorion via the villi, mainly.
The decidual layer over the abembryonic pole (chorion laeve) is called the decidua
capsularis. With growth the decidual capsularis gets stretched, degenerates, and subsequently
comes in contact with the rest of the uterine wall, the decidua parietalis. The decidua capsularis
and parietalis fuse, obliterating the uterine cavity. The chorion frondosum and decidua basalis
form the placenta. The amnion and chorion form the amniochorionic membrane. The
amniochorionic membrane is what ruptures when braking water.
Structure of the Placenta
By the beginning of the fourth month, the placenta has two compartments: (1) a fetal portion,
formed by the chorion frondosum, and (2) a maternal portion formed by the decidual basalis.
On the fetal side, the placenta is bordered by the chorionic plate. On the maternal side, the
placenta is bordered by the decidua basalis.
During the fourth and fifth months, the decidua basalis will form septa in the placenta that
extend toward, but do not reach, the chorionic plate. The septa will delineate as many as 35
lobes on the maternal surface of the placenta. These lobes are slightly bulging areas called
cotyledons. Cotyledons are covered by a thin layer of decidua basalis. Most of the basalis
remains to line the uterus.
The mature placenta is disklike in shape, with a thickness of 3 cm and a diameter of 20 cm. A
typical placenta weighs about 500 gm. The fetal side is shiny because of the apposed amniotic
membrane. The maternal side is dull and has the lobes of the cotyledons.
Because the villi and outer surface of the chorionic plate are continually bathed in maternal
blood, the human placenta is designated as a hemochorial type placenta.
Circulation of the Placenta
Cotyledons receive blood through 80 – 100 spiral arteries that pierce the decidual plate and enter
the intervillous spaces. Fetal blood reaches the placenta through the two umbilical arteries,
which ramify throughout the chorionic plate. In contrast to the fetal circulation, which is totally
contained within blood vessels, the maternal blood supply to the placenta is a free-flowing lake
that is not bound by blood vessels. Blood flow back into the maternal circulation through
endometrial veins.
The placental membrane (barrier) separates the maternal from the fetal circulation. Initially
it is composed of four layers: (1) fetal endothelial cells, (2) connective tissue of villous core,
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(3) cytotrophoblastic cells, and (4) syncytium. During and after the fourth month the placental
membrane consists of the fetal endothelial cells and syncytium for gas and metabolic exchange,
although many substances can still pass through the "placental barrier."
Function of the Placenta
In addition to gas and metabolic exchange, the placenta produces both protein and steroid
hormones. The first protein hormone it produces is human chorionic gonadotropin (HCG).
the hormone is responsible for maintaining the corpus luteum and its production of progesterone
and estrogens. HCG is the basis for the pregnancy test. Its production peaks about the eighth
week of gestation then gradually declines. By the end of the first trimester, the placenta
produces enough progesterone and estrogens so that pregnancy can be maintained even if the
corpus luteum is removed.
Another placental protein hormone is chorionic somatomammotropin, (human placental
lactogen or growth hormone). It is similar to human growth hormone and influences growth,
lactation, plus, lipid and carbohydrate metabolism. The placenta also produces small amounts of
chorionic thyrotropin and corticotropin.
Human placental growth hormone is produced by the syncytiotrophoblast. During the first
15 – 20 weeks of pregnancy, the main form of growth hormone is maternal growth hormone
(produced by the pituitary adenohypophysis). But, from 15 weeks to term the placental hormone
replaced the maternal growth hormone. A major function of this hormone appears to be the
regulation of maternal blood glucose so that the fetus is ensured of an adequate supply of
nutrients. This hormone can make the mother somewhat diabetogenic.
Immunoglobins for the fetus consist almost entirely of maternal immunoglobin G (IgG) that
begins to be transported from mother to fetus at approximately 14 weeks. The fetus gains
passive immunity against various infections. Newborns produce their own IgG at about 3 years
of age.
Amnion and Umbilical Cord
The umbilical cord is a conduit for the umbilical vessels, which are embedded in a mucoid
connective tissue that is often called Wharton's jelly. The umbilical cord is commonly about 50
to 60 cm long and twisted many times. In about 1% of full-term pregnancies, true knots occur in
the umbilical cord. If they tighten as the result of fetal movements they can cause anoxia and
even death of the fetus.
Occasionally, an umbilical cord contains two umbilical veins if the right umbilical vein does
not undergo degeneration.
The line of reflection between the amnion and embryonic ectoderm is the amnio-ectodermal
ring or primitive umbilical ring. Passing through the primitive ring at the fifth week of
development are: (1) the connecting stalk (containing the allantois, two umbilical arteries, and
one vein), (2) the yolk stalk or vitelline duct, and (3) the canal connecting the intraembryonic
and extraembryonic cavities. The yolk sac is in the chorionic cavity.
Amniotic Fluid
The amniotic fluid is produced in part by amniotic cells and partly by maternal blood. At ten
weeks there is about 30 ml and at thirty-seven weeks about 800 – 1000 ml. The volume is
replaced about every three hours. From the fifth week the fetus swallows about half or up to 400
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ml of it day. Fetal urine is added to it, being mostly water since the placenta is responsible for
removing metabolic wastes.
Dizygotic Twins
About two-thirds of twins are dizygotic (non-identical or fraternal twins) and their incidence is
9/1000 births. They are the result of simultaneous ovulation of two oocytes and their subsequent
fertilization by two different sperm.
Monozygotic Twins
These twins develop from a single fertilized egg and are described as monozygotic (identical or
maternal) twins. The rate of occurrence is about 4/1000 births. They result from splitting of
the zygote at various stages of development. Splitting usually occurs at the early blastocyst stage
within the same blastocyst cavity. Rare cases will split at the bilaminar disc stage, just before the
appearance of the primitive streak.
Parturition
During the last 2 to 4 weeks of the thirty-eight week pregnancy the myometrium gets prepared
for parturition (birth, labor).
Labor consists of three stages: (1) effacement (thinning and shortening) and dilation of the
cervix, (2) delivery of the fetus, and (3) delivery of the placenta and fetal membranes.
Problems in pregnancy
Erythroblastosis fetalis and fetal hydrops – Over 400 red blood cell antigens have been
identified. These include the ABO blood groups and the D or Rh antigen. The Rh antigen is
the most dangerous of all the antigens the mother can respond to. If the mother is Rh negative
and the father is Rh positive, the fetus inherits the Rh + blood. The mother will make antibodies
against the fetus's blood cells. The placental barrier is not complete and some fetal blood will
mix with maternal blood in the placenta, hence, the antibody response by the mother. For the
first pregnancy this response may not cause much of a problem for the fetus when the mother's
anti-RH+ antibodies get into the fetal circulation. Usually, the mother's first antibody response is
using IgM, which does not cross the placental barrier. By time a second pregnancy occurs, class
switching of the plasma cells will produce IgG that will cross the placenta resulting in hemolytic
disease of the newborn (erythroblastosis fetalis). The anemia may become so severe that fetal
hydrops (edema and effusions into body cavities) occurs.
Amniotic Fluid
Hydramnios or polyhydramnios is the term used to describe an excess of amniotic fluid (1500
– 2000 ml). Primary causes include idiopathic, maternal diabetes, and congenital malformations
of the central nervous system and gastrointestinal tract that prevent the fetus from swallowing the
fluid.
Oligohydramnios refers to a decreased amount of fluid (less that 400 ml). Oligohydramnios is
rare and may result from renal agenesis. Clubfoot, lung hypoplasia, and Potter's facies can be
caused by oligohydramnios.
Twin Defects
Vanishing twin refers to the death of one fetus. This occurs during the firsts or second trimester
and may result in a mummified fetus papyraceus.
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When separation of the embryo is incomplete conjoined twins or Siamese twins are made.
They are classified according to the nature and degree of union. Thoracopagus is joining at the
chest. Pygopagus is rump-to-rump fusion. Craniopagus or cephalopagus is joining of the head.
Cephalothoracopagus is joining of both head and thorax.
Questions
1. After fertilization the normal length of pregnancy is about __________weeks.
a. 30
b. 34
c. 38
d. 42
2. One of the two cell types that forms a functional tertiary villus are _________ cells.
a. syncytial
b. cytotrophoblast
c. amniotic
d. endometrial
3. The chorion over the abembryonic pole is called the _____________.
a. chorion frondosum
b. chorionic plate
c. chorion laeve
d. chorionic cavity
4. There are three functional parts of the endometrium that lines the uterine wall. The part
that is not attached to the chorion in any way is called the ___________.
a. decidua parietalis
b. decidua capsularis
c. decidua basalis
d. amniochorionic membrane
5. Cotyledons are formed from septal growth of the ________________.
a. amnion
b. extraembryonic mesoderm
c. chorionic plate
d. decidua basalis
6. Which of the following terms describes a human placenta?
a. epitheliochorial
b. hemochorial
c. synepitheliochorial
d. endotheliochorial
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7. Which of the following placentally derived molecules influences growth and is
diabetogenic?
a. human chorionic gonadotropin
b. chorionic thyrotropin
c. chorionic somatommamotropin
d. maternal immunoglobins G
8. Which of the following does not pass through the primitive umbilical ring?
a. connecting stalk
b. yolk stalk
c. canal connecting the intra and extraembryonic cavities
d. outer cytotrophoblastic shell
9. Fraternal twins are characterized as the result of which of the following?
a. Two eggs fertilized by two different fathers.
b. One fertilized egg splitting in the early blastocyst stage.
c. Two ovulated eggs, each fertilized by a sperm
d. Conjoined twins.
10. If there is poor development of the gastrointestinal tract of the embryo, which of the
following would occur?
a. Hydramnios
b. Erythroblastosis fetalis
c. Oligohydramnios
d. Effacement