function&abnormalities The Placenta function&abnormalities

Development of the placenta In human placenta the trophoblast erodes into the decidua,this invasion leads to transformation of small narrow spiral arteries to large sinusoidal vessels so that the endothelium is destroyed (haemochorialplacenta). The trophoblast soon arranged intrabeculae, which are covered by syncytiotrophoblast &blood vessels formed insid it,to form the chorionic villi.

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. After 20 w the cytotrphoblast disappear &finally only thin layer of syncytiun remain

At first the villi are formed over all the surface of the gestational sac (at 4th week) Between 12th-16th 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 12th week).

The placenta at term Is circular in shape, forming a spongy disc 20cmin 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 form2ry &3ry system from which terminal villi arise, 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.

Normal placentation The maternal flow to the placenta increases from 50mL/min in the 1st trimester to 600mL/min at term. In the first 12 week the decidual segment of thespiral a. is invaded by trophoblast & fibrinoid. At the end of this period,the trphoblast plugs, which occupy the spiral a ,released leading to incrsese blood flow to intervillous space

then, the invasion of the intramyometrial segment of the spiral arteries also invaded by trophoblastwhich further reduces resistance to blood flow to the placenta&associated with mid trimester reduction of blood pressure 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

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 a. these pathological condition associated with increased perinatal mortality.

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 haemorrahgein pregnancy or labour Bilobate & trilobate placenta Instead of a single disc , it may consist of2or 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,

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 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 distance

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. Placenta accreta- the placenta is partially or completely adherent to the uterus with penetration of villi into the superficial part of the myometrium. Placenta increta- the villi penetrate deeply through the decidua into the myometrium.

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 pregnancy.

There is delay in the 3rd 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 of 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 Hydropic placenta In sever cases of isoimmunization (hydrops fetalis) the placenta show the same changes of fetus, being enlarged, pale & odematous 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 , 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.

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