The Placenta
Introduction
o At full term the placenta is a discoid tissue
o Diameter of 15 to 25 cm
o 3.3 cm thick
o ˝ kg
o At birth it is separated from the uterine wall and approximately 30 minutes after delivery it is expelled from the uterine cavity.
o Maternal side - cotyledons - covered by a thin layer of decidua basalis.
o Grooves between the cotyledons are formed by decidual septa.
The foetal surface
o Covered entirely by the chorionic plate.
o Large arteries and veins - chorionic vessels, converge toward the umbilical cord.
o The chorion, in turn, is covered by the amnion.
o Attachement of the umbilical cord is usually eccentric, may be central and occasionally even marginal
o Rarely it is inserted into the chorionic membranes outside the placenta (velamentous) insertion.
Circulation of the placenta
o Cotyledons receive blood through vessels that pierce the decidual plate and enter the intervillous spaces. The placental membrane which separates maternal and foetal blood, is initially composed of four layers (a) the endothelial lining of fetal vessels, (b) the connective tissue in the villous core, (c)the cytotrophoblastic layer and (d) the syncytium. The placental membrane is called the placental barrier.
Function of the placenta
o Exchange of metabolic and gaseous products between maternal and fetal bloodstreams and production of hormones.
o Transmissionof Maternal Antibodies.
o Hormone production 4th month end - progesterone - estrogenic hormones.
o During the first two months of pregnancy the syncytiotrophoblast also produces human chorionic gonadotropin which maintains the corpus luteum. Excreted in the urine - pregnancy test
Abnomalities of the placenta
o Placenta Accreta occurs when the placenta attaches too deep in the uterine wall but it does not penetrate the uterine muscle.
o Placenta Increta occurs when the placenta attaches even deeper into the uterine wall and does penetrate into the uterine muscle. Placenta increta accounts for approximately 15% of all cases.
o Placenta Percreta occurs when the placenta penetrates through the entire uterine wall and attaches to another organ such as the bladder. Placenta percreta is the least common.
o The fertilized ovum normally implants in the uterine fundus.
o Placenta implanted over the cervical os - placenta previa can lead to extensive hemorrhage as the cervix dilates during labour
o Twin placenta
o Placental Infarct
o Placental abruption.
Molar pregnancy
Due to abnormal fertilization of the ovum.
No fetus develops
an abnormal placenta consisting of a mass of tissue with grape-like swollen villi is formed
the uterus is large for
HCG levels are markedly elevated.
Hyperemesis gravidarum +
Patients may present with bleeding, and may pass some of the grape-like villi.
Large retroplacental blood clot - known as abruptio placenta.
