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Forth stage
Obstetric
Lec-5
.د
ولدان
1/1/2016
Development of the placenta
In human placenta the trophoblast erodes into the decidua, so that the endothelium of the
maternal blood vessels is destroyed & maternal blood is in direct with the chorion, without
the intervention of any decidual tissue(haemochorial placenta).The trophoblast soon
arranged in trabeculae, which are covered by syncytiotrophoblast.
When the embryonic mesoderm appears it extends into each of these trabecula & finally the
vascularization of the mesoderm complete the formation of chorionic villi by about the 16
th
day after fertilization.
The trophoblast at some point comes into direct contact with the decidua, thus anchoring
villi are formed, then by budding from both them & the chorion, true chorionic villi are
formed.
The trophoblast extend for a variable distance into maternal spiral arterioles where they
enter the intervillous space. After the 20
th
week the cytotrophoblast disappear & finally only
thin layer of syncytium remains.
At first the villi are formed over all the surface of the gestational sac (at 4
th
week). Between
12
th
-16
th
week the villi on the capsular surface degenerate & become smooth called the
chorion leave. In compensation the villi on the decidua basalis undergo great hypertrophy
called chorion frondosum & its matted into solid disc which is the fully developed placenta
(formed by the 12
th
week).
The placenta at term
Is circular in shape, forming a spongy disc20cm in diameter, about 3cm in thickness 500gm
in wt (directly related to the fetal wt).
The functional unit of the placenta is fetal cotyledon & the mature placenta has about 120
cotyledons, which are grouped into visible lobes. Each cotyledon contains a primary villus
stem arising from the chorionic plate, which is divided to form secondary & tertiary stems
from which arise the terminal villi, where the fetal-maternal exchange takes place.
The placenta as a functioning organ is a space containing maternal blood, bounded on the
maternal side by the decidual space & on the fetal side by the chorionic plate.
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Function of placenta:
1. Enables the fetus to take oxygen & nutrients from the maternal blood.
2. Excretory function when CO2 & other waste products pass from the fetus to the maternal
circulation.
3. Barrier against the transfer of infection to the fetus.
4. Secrete hormones like hCG, oestrogen and progesterone.
Normal placentation
The maternal flow to the placenta increases throughout pregnancy from 50mL/min in the 1
st
trimester to 600mL/min at term. This increase in perfusion can only be accomplished by the
anatomical conversion of the maternal spiral arteries by trophoblast, from narrow tortuous
muscular vessels to wide-bored flaccid vessels.
In the first 12 weeks the decidual segment of the spiral a. is invaded by trophoblast &
fibrinoid. Following this, the trophoblast invasion of the intramyometrial segment of the
spiral arteries which further reduces resistance to blood flow to the placenta.
This process should be complete by 20 week, also because they lack smooth muscle, they are
less likely to respond to vaso-active compound.
Abnormal placentation
Pre-eclampsia.
Intrauterine growth restriction(IUGR).
Abruptio placentae.
Collagen vascular disease.
Antiphospholipid syndrome.
Sever D.M.
Chronic hypertension.
All these are clinical manifestations of total or patchy failure of trophoblast invasion of the
myo-metrial segments of the spiral arteries. All these result in a small placenta with gross
morphological changes which are :
Infracts represents an area of ischemic necrosis of cotyledon resulting from spiral a.
occlusion, usually by thrombosis.
Basal haematomas consist of a mass of blood in the centre of the cotyledon due to the
rupture of the damaged spiral artery these pathological condition associated with
increased perinatal mortality.
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Other abnormalities of the placenta
Anomalies in weight: In cases of diabetes & haemolytic disease of the newborn the
placental wt may increase to up to half the wt of the fetus.
Site of implantation of the placenta: The placenta usually attached to the uterine wall near
the fundus, to either the anterior or posterior surface. In about 1 in 250 pregnancies the
placenta is implanted wholly or partially on the lower segment of the uterus (placenta
previa). This is a serious abnormality which may cause severe haemorrahge in pregnancy
or labour.
Bilobate & trilobate placenta: Instead of a single disc , it may consist of 2 or 3 lobes partly
fused( of no clinical importance).
Placenta succenturiata: This is not uncommon. One or more accessory lobes of placenta
are found on the chorion at a distance from the edge of the main placenta, this lobe is of
clinical importance because its liable to be retained in the uterus after the placenta proper
has been expelled, causing post partum hemorrhage(PPH) this abnormality should be
discovered after inspection of placenta & membrane, when a round defect seen in the
membrane.
Placenta circumvallata: Where the original area of attachment of the chorionic plate to
the uterine wall is small & placental growth has continued beyond its margin, a fibrous
ring is seen on the fetal surface of the placenta , this variation is not important as the
placenta continue to function normally.
Morbid adherence of the placenta: In 3rd stage of labour the placenta normally
separates through the stratum spongiosum of the maternal decidua( the superficial part
of the decidua comes away with the placenta & the deeper part remains on the uterine
wall), normally the chorionic villi only penetrate as far as this Morbid adherence of the
placenta results from increased penetration of the decidua & myometrium by the villi.
The degree of morbidity is determined by the depth of invasion.
1. Placenta accrete the placenta is partially or completely adherent to the uterus with
penetration of villi into the superficial part of the myometrium.
2. Placenta increta the villi penetrate deeply through the decidua into the myometrium.
3. Placenta percreta penetration can even be seen on the serosal surface.
Placenta previa, c.s, curettage are the most pre-disposing causes. It occur 1 in 500
pregnancies.
There is delay in the 3
rd
stage of labour with PPH & the abnormalities is only discovered when
an attempt to remove the placenta manually & no plane of cleavage is found.
Morbid adherence is of great importance clinically because it makes it impossible to remove
the placenta completely thus exposing the mother to risk of sever PPH & it may end with
hysterectomy.
Tumours of the placenta: Apart from choriocarcinoma, tumours of the placenta is rare like
vascular tumours known as haemangiomas or chorangiomas
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Hydropic placenta: In sever cases of isoimmunization (hydrops fetalis) the placenta show
the same changes of fetus, being enlarged, pale & oedematous with a marked increase in
wt.
The umbilical cord
Abnormal length: The usual length, same as fetus at term 50cm. Excessive length
predispose to prolapse of the cord, formation of loops round some part of the fetus may
cause IUD in very rare cases. Short cord predispose to delay in 2nd stage of labour,
premature separation of the placenta, inversion of uterus are theoretical accidents.
Knots in the cord: These may be formed by fetal movement, knots are rarely tight enough
to obstruct the circulation, but they do occasionally cause IUD.
Abnormal insertion of the cord: The cord usually attached to the centre of the placenta,
but sometimes attached to the edge of placenta ( squash racket placenta) of no clinical
importance. In very rare cases the cord is attached to the membrane at some distance
from the edge of the placenta, at this point the vessle may divide into branches which
run on the membrane before reaching the placenta (velamentous insertion of the cord
).This can be dangerous to the fetus if the vessels happen to pass across part of the
chorion that lies below the presenting part( vasa previa), as a branch may be torn when
the membrane rupture, leading to fetal blood loss.
Single umbilical artery: This is uncommon, but can be associated with other abnormalities
of the fetus, notably those of the kidneys, ureters or bladder.