Dr. FARHAT AAMIR
Lecturer of Anatomy and Embryology
Objectives
-Enlist the monthly changes in fetus from third month
to birth.
- Describe changes in trophoblast during formation
of placenta.
- Discuss formation of chorionic frondosum and
decidua basilis.
- Describe the structure of placenta.
- Discuss circulation of placenta.
- Identify clinical application’s i.e., placental barrier,
hydrops fetalis, preeclampsia and low birth weight.
Changes in fetus from 3rd month to birth
- At the beginning of the 3rd month the head
constitutes approximately ½ of the
crown-heel length (CHL).
- By the beginning of the 5th month the size
of the head is about 1/3 of the CHL
- At birth it is approximately 1/4 of the
CHL.
During the 3rd month:
- The face becomes more human looking.
- The eyes initially directed laterally then
moved to the ventral aspect of the face.
- The ears come to lie close to their position
at the side of the head.
- The limbs reach their relative length in
comparison with the rest of the body.
- Also by the 12th week the sex of the fetus
can be determined by (ultrasound).
An 11-week fetus. The umbilical cord still shows a swelling at its base,
caused by herniated intestinal loops. Toes are developed, and the sex of the
fetus can be seen. The skull of this fetus lacks the normal smooth contours.
A 12-week fetus in utero. Note the extremely thin skin and underlying blood
vessels. The face has all of the human characteristics, but the ears are still
primitive. Movements begin at this time but are usually not felt by the
mother.
An 18-week-old fetus connected to the placenta by its umbilical cord.
The skin of the fetus is thin because of lack of subcutaneous fat. Note
the placenta with its
cotyledons and the amnion.
During the 4th and 5th months:
- The fetus length increase rapidly and at the end of the first half of
intrauterine life its crown rump length CRL is about 15 cm
(about half the total length of the newborn).
- The weight of the fetus increases little during this period and by the
end of the 5th month is still <500 g.
- The fetus is covered with fine hair called lanugo hair.
- Eyebrows and head hair are also visible.
- Movements of the fetus can be felt by the mother.
During the 6th month: The skin of the fetus
is reddish and has a wrinkled appearance.
- A fetus born early in the 6th month has great
difficult to survive. The respiratory system and
the central nervous system have not
differentiated sufficiently and coordination
between the two systems is not well established.
By 6.5 to 7th months:
- The fetus has a CRL of about 25 cm and weighs
about 1,100 g. If born at this time the infant has
90% chance of surviving.
During the last 2 months: The fetus obtains
rounded contours as the result of deposition of
subcutaneous fat. By the end of intrauterine life
the skin is covered by a whitish fatty substance
(vernix caseosa).
A 7-month-old fetus. This fetus would be able to survive. It has
well-rounded contours as a result of deposition of subcutaneous
fat. Note the twisting of the umbilical cord
cord
.
At the end of the ninth month:
- The skull has the largest circumference of all
parts of the body.
- The weight of the fetus is 3,000 - 3,400 g.
- Its CRL is about 36 cm and its CHL is about
50 cm.
- Sexual characteristics are pronounced and the
testes should be in the scrotum
Changes in the Trophoblast
- The fetal part of the placenta is derived from the
trophoblast and extraembryonic mesoderm.
- At the 2nd month the trophoblast is characterized
by a great number villi.
1. Primary villi: It is formed by trophoblast.
2. Secondary villi: It is formed by trophoblast
and mesoderm.
3. Tertiary villi: It is formed by trophoblast,
mesoderm and blood vessels.
- The stem of the villi extend from the mesoderm
of the chorionic plate to the cytotrophoblast cell.
- The cytotrophoblast cell is covered by
syncytiotrophoblast layer (syncytium).
- During the following months small extensions
grow out from the stem villi and form free villi
into the lacunar or intervillous space.
Chorion frondosum and decidua basalis
Chorion frondosum:
- In early pregnancy the villi cover the whole
surface of the chorion.
- As the pregnancy advances the villi on the
embryonic pole continue to grow and form
chorion frondosum. The villi away from the
embryonic pole degenerate and form
chorion laeve at the 3rd month.
Decidua:
- The decidua is endometrium of the uterus
It includes 3parts:
1. Decidua basalis: It is the endometrium in
contact with chorion frondosum.
2. Decidua capsularis: It is the part related
to chorion laeve.
3. Decidua parietalis: It is the rest of the
uterine wall.
Structure of the placenta
- At the beginning of the 4th month the
placenta has two components:
1. Fetal portion: Formed by chorion
frondosum
2. Maternal portion: Formed by the
decidua basalis.
- During the 4th and 5th month the decidua
form a number of decidual septa which
project in the intervillous space but don’t
reach to the chorionic plate.
- These septa have a core of maternal tissue
and their surface is covered by layer of
syncytial cells, so that the syncytial cells
separates the maternal blood in the
intervillous space from fetal blood in the
villi.
Full term placenta
Shape: It is discoid shape
Diameter: 15 – 20 cm Thickens: 3 cm
Weight: 500 – 600 g
Separation of the placenta after birth:
Within 30 minutes.
Maternal side: Formed of 15 – 20 cotyledons
separated by decidual septa and covered by
thin layer of decidua basalis.
Fetal surface: It is covered by chorionic plate.
- A number of large arteries and veins
converge toward the umbilical cord.
- The chorion is covered by the amnion.
- Attachment of the umbilical cord is usually
eccentric, sometimes it is marginal and rarely
to the chorionic membranes.
Circulation of the placenta
- The cotyledons receive their blood through
100 spiral arteries that pierce the decidual
plate and enter the intervillous space.
- The blood return from the intervillous space
to the decidua in the endometrial veins.
- The fetal blood moves in the blood vessels
along the chorionic villi.
Placental membrane: It is the separation
between the fetal and maternal blood.
It is formed of:
1. Endothelial lining of the blood vessels.
2. Connective tissue in the villus core.
3. The cytotrophoblastic layer.
4. The syncytium.
- syncytium.
Clinical application
1. Hydrops fetalis: It is edema and effusion of
the body cavities of the fetus.
Causes: 1. Rh incompatibility: The mother Rh
–ve while the baby Rh is +ve. is
2. ABO incompatibility: The mother blood
group is O while the baby has different group.
- Few drops from fetal blood passes to maternal
blood leading to antibody formation against
fetal blood
2. Preeclampsia: It is characterized by
maternal hypertension, edema and proteinuria.
- It leads to growth retardation or death of fetus
3. Low birth weight: The wight of the newborn
is less than 2.5 kg (Normal 2.5 – 4 kg)