Placenta (plah-sen’tah) is a
It means “a flat cake”.
• Placenta is an organ characteristic of
mammals, developing during pregnancy,
joining mother and offspring, providing
necessary provisions for the sustenance
of developing human in intrauterine life.
• The placenta is an organ that connects
the developing fetus to the uterine wall to
allow nutrient uptake, waste elimination,
and gas exchange via the mother's blood
• "True" placentas are a defining
characteristic of eutherian or "placental"
• Eutheria (/ju θ əriəːˈ ɪ /; from Ancient Greek
ευθήριον, euthērion, meaning "true/good
beasts") is the clade consisting of
primates and all other mammals—in many
orders—that are more closely related to
them than they are to marsupials.
• Life restoration of Juramaia sinensis
• Kingdom Animalia
• Phylum Chorda
• Class Mammalia
• Branch Eutheria
• Marsupials are
an infraclass of mammals living primarily
in the Southern Hemisphere; a distinctive
characteristic, common to most species, is
that the young are carried in a pouch.
• Female Eastern Grey
a joey in her pouch
• Kingdom Animalia
• Phylum Chorda
• Class Mammalia
• Infraclass Marsupials
• A clade (from Ancient
Greek κλάδος, klados, "branch")
or monophylum (see monophyletic) is a
group consisting of an ancestor and all its
descendants, a single "branch" on the
"tree of life"
• The fully developed placenta is discoid-
shape with a diameter of 15 to 25 cm and
is approximately 3 cm thick.
• It weighs about 500 to 600 gm (about one-
sixth that of the fetus).
• In humans, the placenta averages 22 cm
(9 inch) in length and 2–2.5 cm (0.8–
1 inch) in thickness, with the center being
the thickest, and the edges being the
• It typically weighs approximately
500 grams (1 lb).
• It has a dark reddish-blue or crimson
• It connects to the fetus by an umbilical
cord of approximately 55–60 cm (22–
24 inch) in length, which contains
two umbilical arteries and one umbilical
• The umbilical cord inserts into the
chorionic plate (has an eccentric
• Vessels branch out over the surface of the
placenta and further divide to form a
network covered by a thin layer of cells.
This results in the formation of villous tree
• On the maternal side, these villous tree
structures are grouped into lobules
• In humans, the placenta usually has a disc
shape, but size varies vastly between
different mammalian species
• Placenta covers 15 to 30 % of the decidua
(endometrium during pregnancy)
Placenta is a fetomaternal organ that has
• A fetal portion that develops from a
portion of chorionic sac, called chorionic
• A maternal portion that is derived from a
portion of endometrium, called decidua
• The placenta begins to develop upon
implantation of the blastocyst into the
• Placenta grows throughout pregnancy.
• Development of the maternal blood supply
to the placenta is complete by the end of
the first trimester of pregnancy
(approximately 12–13 weeks).
Fertilization - Zygote
The cleavage starts in the zygote immediately after
fertilization and on 4th day morula has formed.
The morula consists of two groups of cells:
• Inner Cell Mass (Central Cells)
• Outer Cell Mass (Peripheral Cells)
Within one day morula is converted into blastocyst
consisting of same two groups of cells, now with
– Embryoblast derived from Inner Cell Mass
– Trophoblast derived from Outer Cell Mass
• Embryoblast forms the embryo proper
• Trophoblast forms the placenta and
Development of placenta starts as soon as blastocyst is
attached to the endometrium.
Trophoblasts start proliferating rapidly and
differentiate into two layers:
• Cytotrophoblast or cellular trophoblast
• Syncytial trophoblast (syncytiotrophoblast)
• Meanwhile, the cytotrophoblastic cells in the
villi penetrate progressively into the overlying
syncytial trophoblast until they reach maternal
endometrium. Here they establish contact with
similar extensions of neighboring villous stems,
thus forming a thin outer cytotrophoblast shell.
• This shell gradually surrounds the trophoblast
• Cytotrophoblastic shell attaches chorionic sac
to endometrial tissue.
• Stem Villi
Villi that are attached to the maternal tissues
via cytotrophoblastic shell are called stem villi
or anchoring villi.
• Floating Villi
The villi that branch from stem villi and float
free in intervillous space are called branching
villi or floating villi. These villi are not attached
to maternal tissue.
A section of chorionic sac showing two stem villi.
The developing fetus during
week of intrauterine life, showing
different parts of Decidua.
The developing fetus during 10th
of intrauterine life.
The developing fetus during 11th
of intrauterine life.
Fetus of 14 week showing large amniotic cavity. Decidua
capsularis and decidua parietalis have fused and uterine
cavity is obliterated
A. Note Chorion frondosum and Chorion laeve.
B. Note Placenta consisting of two parts:
1. Chorion Frondosum (Fetal portion)
2. Decidua Basalis (Maternal portion)
STRUCTURE OF PLACENTA
By the beginning of the 4th month,
placenta has two components:
• A fetal component formed by the chorionic
frondosum, which has developed in the
compact layer of endometrium.
• A maternal portion formed by decidua
basalis or decidual plate, which has
development from spongy layer of
Consists of chorionic plate, anchoring and floating
villi, and outer cytotrophoblastic shell.
The chorionic plate and outer cytotrophoblastic
shell is separated by lake of maternal blood.
The anchoring villi connect chorionic plate and
outer cytotrophoblastic shell and pass through
the blood lake, thus converting this maternal
blood lake into so-called intervillous spaces
seen in two dimension pictures.
The floating villi branch from anchoring villi and
float freely in the maternal blood lake.
Decidual plate is intimately incorporated into the
outer cytotrophoblastic shell. Here the
trophoblast cells and decidua cells intermingle.
During the 4th and 5th months, a number of septa
project into the maternal blood lake, from the
They never reach the chorionic plate. As a result of
this septum formation, the maternal blood lake
is divided into incomplete compartments.
• Growth in the size and thickness of the
placenta continues rapidly until the fetus is
about 18 weeks old (20 weeks’ gestation).
• The placenta at this stage is fully
• Later on it enlarges only due to
arborization of existing villus tree.
• After the delivery of fetus, rapid reduction
in the size of the uterus leads to
detachment of placenta from the uterine
• The uterus than undergoes a second
series of contraction due to which
placenta and its associated membranes
(amnion and chorion leave) are forced
The fetal surface is smooth and shiny. The umbilical cord attaches
near the center of this surface. Amnion, which is a translucent
membrane, covers this surface. Under the amnion the chorionic
plate can be seen with chorionic blood vessels radiating from the
attachment of the umbilical cord.
The maternal surface of the placenta is dark red in color.
It is covered by a thin, grayish, somewhat shaggy layer of
decidua basalis. This surface presents a number of convex
somewhat polygonal areas, which are separated from each
other by shallow grooves. These bulging areas are 15-30 in
number and represent the cotyledons of placenta.
A.Type I Lateral
B.Type II Marginal
C.Type III Paracentral
D.Type IV Central
Classification of degrees of Placenta Previa
A B C D
Placenta may be larger than the
normal size. It is sometimes very large
in size, almost lining the whole uterine
cavity but it is reciprocally very thin.
It is called Placenta Membranacea.
The normal weight of placenta is about 500 to 600 gm (about
one-sixth that of the fetus).
Very small placentas are found in women suffering from
Excessively large placentae occur with fetal hydrops.
Abnormalities of Shape
Normally the placenta has a circular shape. But sometimes it may
have oval, fusiform, crescentic or other shapes.
The placenta may be notched or lobed.
Notched Placentae: When there is
one notch in placenta dividing it into
two incomplete lobes the placenta
is called Placenta Bipartita and
when there are two notches dividing
the placenta into three incomplete
lobes the placenta is given the name
Lobed Placentae: here the placenta
may have two, three or more
separate lobes and the umbilical
vessels also divide accordingly. These
placentae are called Placenta Duplex,
Placenta Triplex etc. and Placenta
Multiplex, as the number of lobes
Sometimes placenta fails to develop
over a small area. There are only
fetal membranes in the deficient place.
The deficient area appears like a window.
This is called Placenta Fenestrata.
Sometimes the placenta has a
central depression on the
fetal surface. The fetal membranes
are attached to the margin of this
depression. This is called Placenta
The side view of the Placenta Circumvallata shown above.
The depression is visible in the center where umbilical cord is attached
Occasionally there are one or more accessory placentas,
which are of smaller size than the main placenta. This
condition is called placenta succenturiata.
The accessory placenta is connected to the main placenta only by
membranes and blood vessels.
The accessory placenta may be adjacent to main
placenta as shown below.
Main Placenta Accessory Placenta
Abnormal Attachment of the Umbilical Cord
Normally the umbilical cord is
attached near the center of the
placenta. Sometimes, the cord is
attached to the edge of the placenta.
Such a placenta is called battledore
Rarely the umbilical cord is attached
to the membranes at some distance
from the margin of placenta. The
umbilical blood vessels run on the
membranes for some distance before
reaching the placenta. This condition
is called velamentous insertion of
the cord and such placenta is
called placenta velamentosa
Abnormal attachment to the uterus.
When the placenta is abnormally adherent to the endometrium, it
is called placenta accrete. Here the placenta has gone deep into
endometrium and it is difficult to remove.
Sometimes the chorionic
and enters myometrium.
There is partial or
of decidua basalis.
The placenta is
called placenta increta.
When chorionic frondosum penetrates the myometrium all
the way to the perimetrium, the abnormality is called