The document summarizes the development and anatomy of the human placenta. It begins by defining the placenta and outlining its objectives. It then describes the development of the placenta from the chorion and decidua, the structures of the mature placenta including the chorionic plate, basal plate, intervillous space and villi, and the maternal and fetal circulations that occur through it. Finally, it discusses placental aging and the degenerative changes that occur over the course of a pregnancy.

1. Development of Placenta

2. Objectives
• Define placenta
• Explain the development of placenta
• State the gross anatomy of placenta at term
• Describe the structures of placenta
• Explain the placental circulation
• State the placental ageing
• List out the functions of placenta
• Explain the umbilical cord
• Describe the amniotic cavity, amnion and amniotic
fluid
• References
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3. The placenta
• Only the eutherian mammals possess placenta.
The human placenta is discoid, haemochorial
and deciduate.
• The placenta is attached to the uterine wall
and establishes connection between the
mother and fetus through the umbilical cord.
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4. Development of placenta
• Developed from two sources.
• The principal component is fetal which
develops from the chorion frondosum and the
maternal component consists of decidua
basalis.
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5. Development of placenta contd…
• When the interstitial implantation is completed on 11th
day, the blastocyst is surrounded on all sides by lacunar
spaces around cords of syncytial cells, called trabaculae.
• From the trabaculae develops the stem villi on 13th day
which connect the chorionic plate with the basal plate.
• Primary, secondary and tertiary villi are successively
developed from the stem villi.
• Arterio-capillary-venous system in the mesenchymal core
of each villus is completed on 21st day.
• This ultimately makes connection with the intraembryonic
vascular systems through the body walls.
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6. Development of placenta contd…
• Simultaneously, lacunar spaces become confluent with one
another and by 3rd-4th week, form a multilocular
receptacle lined by syncytium and filled with maternal
blood.
• This space becomes the future intervillous space.
• As the growth of the embryo proceeds, decidua capsularis
becomes thinner beginning at 6th week and both the villi
and the lacunar spaces in the abembryonic area get
obliterated. This is compensated by
– Exuberant growth and proliferation of the decidua basalis and
– Enormous and exuberant division and subdivision of the
chorionic villi in the embryonic pole (chorion frondosum).
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7. Development of placenta contd…
• These two i.e., chorion frondosum and the
decidua basalis form the discrete placenta. It
begins at 6th week and is completed by 12th
week.
• Until the end of 16th week, the placenta
grows both in thickness and circumference
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8. The placenta at term
Gross anatomy
• Circular disc with a diameter of 15-20 cm and thickness
of about 2.5 cm at its center.
• It thins off towards the edge.
• It feels spongy and weight about 500 gm
• Proportion to the weight of the baby being roughly 1:6 at
term and occupies about 30% of the uterine wall.
• It presents two surfaces, fetal and maternal, and a
peripheral margin.
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10. The placenta at term contd…
Fetal surface
• Covered by the smooth and glistening amnion with the
umbilical cord attached at or near its center.
• Branches of the umbilical vessels are visible beneath the
amnion.
• The amnion can be peeled off from the underlying
chorion except at the insertion of the cord.
• At term, about four-fifths of the placenta is of fetal
origin.
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11. The placenta at term contd…
Maternal surface
• Rough and spongy
• Dull red colour
• A thin greyish, somewhat shaggy layer which is the
remnant of decidua basalis and has come away with
the placenta, may be visible.
• 15-20 convex polygonal areas known as lobes or
cotyledons which are limited by fissures.
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12. Maternal surface of placenta contd…
• Each fissure is occupied by the decidual septum.
• Numerous small greyish spots are visible.
• Amounts to less than one fifth of the total placenta.
• Only the decidua basalis and the blood in the
intervillous space are of maternal origin.
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13. The placenta at term contd…
• Margin: peripheral margin of the placenta is limited
by the fused basal and chorionic plates and is
continuous with the chorion leave and amnion.
• Attachment:
upper part of the body of the uterus encroaching to
the fundus adjacent to the anterior or posterior wall
with equal frequency.
Separation: Placenta separates after the birth
through the decidua spongiosum.
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14. Structures of placenta
• The placenta is limited internally by the amniotic
membrane and chorionic plate; externally by the
basal plate and in between these two lies the
intervillous space containing the stem villi with their
branches, the space being filled with maternal blood.
• Amniotic membrane: It consists of single layer of
cubical epithelium loosely attached to the adjacent
chorionic plate. It takes no part in formation of
placenta.
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15. Structures of placenta contd…
Chorionic plate: From within outwards, it consists of
– Primitive mesenchymal tissue containing branches of umbilical
vessels
– A layer of cytotrophoblast and
– Syncytiotrophoblast
Basal plate: It consists of the following structures from
outside inwards.
– Part of the compact and spongy layer of the decidua basalis
– Nitabuch's layer of fibrinoid degeneration of the outer
syncytiotrophoblast at the junction of the cytotrophoblastic
shell and decidua.
– Cytotrophoblastic shell
– Syncytiotrophoblast
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18. Structures of placenta contd…
• The basal plate is perforated by the spiral
branches of the uterine vessels through which
the maternal blood flows into the intervillous
space but fail to reach the chorionic plate.
• The septum consists of decidual elements
convered by trophoblastic cells.
• The areas between the septa are known as
cotyledons (lobes), which are observed from the
maternal surface, numbering 15-20.
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19. Structures of placenta contd…
• Intervillous space: It is bounded on the inner side by
the chorionic plate and the outer side by the basal
plate, limited on the periphery by the fusion of the
two plates.
• It is lined internally on all sides by the
syncytiotrophoblast and is filled with slow flowing
maternal blood.
• Numerous branching villi which arise from the stem
villi project into the space and constitute chief
content of the intervillous space.
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20. Structures of placenta contd…
Stem villi
• Arise from the chorionic plate and extend to the
basal plate.
• With the progressive development-
primary, secondary and tertiary villi are formed.
• Functional unit of the placenta is called the fetal
cotyledon or placentome.
• Major villi pass down through the intervillous
space to anchor onto the basal plate.
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21. Stem villi contd…
• Functional subunit is called lobule which is derived
from the tertiary stem villi.
• About 60 stem villi persist in human placenta.
• Thus, each cotyledon (totaling 15-29) contains 3-4 major
stem villi.
• The total surface, for exchange, varies between 4-14
square meters.
• The fetal capillary system within the villi is almost 50 km
long.
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22. Structures of placenta contd…
Structure of terminal villus: In the early placenta, each
terminal villus has got the following structures from
outside inward:
– Outer syncytiotrophoblast
– Cytotrophoblast
– Basement membrane
– Central stroma containing fetal capillaries, primitive
mesenchymal cells, connective tissue and a few
phagocytic (Hofbauer) cells.
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23. Structure of terminal villus contd…
• In the term placenta the syncytiotrophoblast becomes
relatively thin at places overlying the fetal capillaries and
thicker at other areas containing extensive endoplasmic
reticulum.
• The cytotrophoblast is relatively sparse.
• Basement membrane becomes thicker.
• Stroma contains dilated vessels along with all the
constituents and few Hofbauer cells.
• These cells have IgG surface receptors and can express
class II MHC molecules.
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24. Placental Circulation
Placental
circulation
Utero-
Feto-Placental
Placental
Circulation
circulation
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25. Uteroplacental circulation
(Maternal circulation)
• A mature placenta has a volume of about about 500 ml of
blood; 350ml being occupied in the villi system and 150 ml
lying in the intervillous space.
• Intervillous blood flow at the term is estimated to be 500-
600 ml/minute, blood in the intervillous space is
completely replaced about 3-4 times/minute.
• Intervillous space pressure is about 10 to 15 mm Hg during
uterine relaxation and 30-50 mm Hg during uterine
contraction.
• In contrast, the fetal capillary pressure in the villi is 20-40
mm Hg.
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26. Utero-placental circulation contd…
Arterial circulation:
• About 120-200 spiral arteries open into the intervillous
space.
• Normally, there is cytotrophoblastic invasion into the
spiral arteries initially upto the intra decidual portion
within 12 weeks of pregnancy.
• There is a secondary invasion of trophoblast between
12-16 weeks extending upto radial arteries within the
myometrium, thus spiral arteries are converted to large
bore uteroplacental arteries.
• The net effect is funneling of arteries which reduce the
pressure of the blood to 70-80 mm Hg before it
reaches the intervillous space.
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27. Utero-placental circulation contd…
• Venous drainage: Through the uterine veins
which pierce the basal plate randomly like the
arteries.
• Circulation of the intervillous space: The arterial
blood enters the space under pressure. Lateral
dispersion occurs, after it reaches the chorionic
plate. Villi help in mixing and slowing of the blood
flow. Mild stirring by the villi pulsation aided by
uterine contraction help migration of the blood
towards the basal plate and thence to uterine
veins.
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28. Fetoplacental circulation
• The two umbilical arteries enter the chorionic plate
underneath the amnion, each supplying one half of the
placenta.
• The arteries break up into small branches which enter
the stems of the chorionic villi.
• Each in turn divides into primary, secondary and tertiary
vessels of the corresponding villi.
• The blood flows into the corresponding venous channels
either through the terminal capillary networks or
through the shunts.
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29. Fetal circulation contd…
• Maternal and fetal blood streams flow side by
side, but in opposite direction.
• This counter current flow facilitates material
exchange between the mother and fetus.
• The villus capillary pressure varies from 20-40 mm
Hg.
• The fetal blood flow through the placenta is about
400 ml per minute.
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30. Fetal circulation contd…
• Placental membrane: Inspite of close proximity, there is no
mixing of the maternal and fetal blood. The two are separated
by tissues called placental membrane or barrier, consisting of
the following. In the early pregnancy, it consists of
– Syncytiotrophoblast
– Cytotrophoblast
– Basement membrane
– Stromal tissue and
– Endothelium of the fetal capillary wall with its basement
membrane. It is about 0.025mm thick.
• Near term, there is attenuation of the syncytial layer. Sparse
cytotrophoblast and distended fetal capillaries almost fill the
villus. The specialized zones of the villi where the
syncytiotrophoblast is thin and anuclear known as Vasculo-
Syncytial membrane.
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31. 1. Intervillous space (with maternal blood)
1. Intervillous space 2. Placental barrier of a terminal villus
2. Syncytiotrophoblast 3. Fetal capillaries
3. Cytotrophoblast 4. Merged basal membranes of the fetal
4. Villus mesenchyma capillary and of the syncythiothrophoblast
5. Fetal capillaries 5. Endothelial cells
6. Hofbauer macrophage 6. Rare cytotrophoblast cells
7. Basal membrane of the capillaries
8. Basal membrane of the trophoblast portion
9. Syncytiotrophoblast with proliferation knots
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(nuclei rich region) 31

32. Placental ageing
• As the placenta has got a life span, it is likely to undergo
degenerative changes as a mark of senescence. The
ageing process involves both the fetal and maternal
components.
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33. Placental ageing contd…
Villi changes: The following changes are observed:
• Decreasing thickness of the syncytium and appearance of
syncytial knots.
• Partial disappearance of Langhan's cells.
• Decrease in the stromal tissue including Hofbauer cells.
• Obliteration of some vessels and marked dilatation of the
capillaries.
• Thickening of the basement layer of the fetal endothelium
and the cytotrophoblast.
• Deposition of fibrin on the surface of the villi.
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34. Placental ageing contd…
• Decidual changes: There is an area of fibrinoid
degeneration, where trophoblast cells meet the
decidua. This zone is known as Nitabuch layer. The
membrane is absent in placenta accreta.
• Intervillous space: The syncytium, covering the villi
and extending into the decidua or intervillous
space, undergoes fibrinoid degeneration and form a
mass entangling variable number of villi. These are
called white infarcts. Calcification or even cyst
formation may occur on it. There may be
inconsistent deposition of fibrin called Rohr's stria at
the bottom of the intervillous space and surrounding
the fastening villi
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35. The fetal membranes
• It consists of two layers
• Outer chorion and the inner amnion.
Chorion:
• Represents the remnant of chorion laeve.
• Thicker than amnion
• Friable and shaggy on both sides.
• Internally, it is attached to the amnion by loose areolar
tissue and remnant of primitive mesenchyme.
• Externally, it is covered by vestiges of trophoblastic layer and
the decidual cells of fused decidua capsularis and parietalis .
• Contains no vessels or nerves.
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36. The fetal membranes contd…
Amnion:
• Inner layer of the fetal membranes.
• Internal surface is smooth and shiny and is in contact with
liquor amnii.
• The outer surface consists of a layer of connective tissue and
is apposed to the similar tissue on the inner aspect of the
chorion from which it can be peeled off.
• The amnion can be peeled off except at the insertion of the
umbilical cord.
• Offers support to amniotic fluid and also produces the fluid.
• In addition, it produces a phospholipid that initiates the
formation of prostaglandins.
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37. Functions of placenta
• Transfer of nutrients and waste products
• Enzymatic function
• Barrier function
• Immunological function
• Storage
• Endocrinal function
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38. Functions of placenta
1. Transfer of nutrients and waste products
between the mother and fetus.
• The mechanism involved in the transfer of
substances across the placenta are:
– Simple diffusion
– Facilitated diffusion (Carrier mediated)
– Active transfer (against concentration gradient)
– Endocytosis
– Exocytosis
– Leakage
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39. Transfer function contd…
Respiratory function:
• Although, the fetal respiratory movements are observed
as early as 11 weeks, there is no gaseous exchange.
• Intake of oxygen and output of carbon dioxide take place
by simple diffusion.
• Partial pressure gradient is the driving force for exchange
of gases.
• The oxygen supply to the fetus is at the rate of 8
ml/kg/min and this is achieved with cord blood flow of
165-330ml/min.
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40. Transfer function contd…
Excretory function
• Waste products like urea, uric acid and creatinine are
excreted to the maternal blood by simple diffusion.
• The main substance excreted from the fetus is
carbon dioxide.
• Bilirubin will also be excreted.
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41. Transfer function contd…
Nutritive function
a. Glucose:
• Principle source of energy
• Transferred to the fetus by facilitated diffusion
• Transfer rates decreases as maternal glucose concentration
increases.
b. Lipids
• For fetal growth and development
• Triglycerides and fatty acids are directly transported from
the mother to the fetus in early pregnancy but are
synthesized in the fetus later in the pregnancy.
• EFA are transferred more than non EFA
• Cholesterol is capable of direct transfer.
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42. Transfer function contd…
c. Amino acids
• Transferred by active transport through enzymatic
mechanism (ATPase).
• Some proteins (IgG), cross by the process of
endocytosis.
• Fetal proteins are synthesized from the transferred
amino acids and the level is lower than in mother.
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43. Functions of placenta contd…
d. Water and electrolytes
• Sodium, potassium and chloride cross by simple
diffusion, whereas calcium, phosphorus and iron cross by
active transport.
• Water soluble vitamins are transferred by active transport
but the fat soluble vitamins are transferred slowly.
e. Hormones-
• Insulin, steroids from the adrenals, thyroid, chorionic
gonadotrophin or placental lactogen cross the placenta at
very slow rate.
• Neither parathormone nor calcitonin crosses the placenta.
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44. Functions of placenta contd…
2. Enzymatic function:
• Diamine oxidase which inactivates the circulatory
pressure amines
• Oxytocinase which neutralizes the oxytocin
• Phospholipase A2 which synthesizes arachidonic acid
etc
3. Barrier function:
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45. Functions of placenta contd…
4. Immunological function
• Placental hormones, proteins (SP1), early pregnancy factor
(EPF), PAPP-A, steroids and chorionic gonadotrophin, have
got some immunosuppressive effect.
• Villous trophoblasts do not express HLA Class I and class II
molecules. Extravillous trophoblasts only express HLA class
I molecules and no HLA class II molecules.
• Though anti HLA antibodies and sentitised T cells against
paternal antigens have been detected in the maternal
serum, they have no significant effects on pregnancy.
• There is shift of maternal response from cell mediated (T
helper I) to humoral (T helper 2) immunity, which may be
beneficial to pregnancy.
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46. Functions of placenta contd…
• Decidual natural killer (NK) cells and trophoblast
(extravillous) HLA Class I molecules interact.
• The immunological response of implantation and that
of organ transplantation are different and not
comparable.
• Syncytiotrophoblast has got trophoblast lymphocytes
cross reactive antigen (TLX). Consequently there is
production of antibodies by the mother in response to
this TLX. These blocking antibodies protect the fetus
from rejection.
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47. Functions of placenta contd…
5. Storage
• The placenta metabolizes glucose, stores it in the form of
glycogen and reconverts it to glucose as required.
• It can also store iron and the fat soluble vitamins.
6. Endocrine Function
• Human Chorionic Gonadotropin
• Estrogen
• Progesterone
• Human Placental Lactogen
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48. The Umbilical Cord
• The umbilical cord is formed from the fetal membranes
• Provides a circulatory pathway that connects the embryo
to the chorionic villi of the placenta.
• Its function is to transport oxygen and nutrients to the
fetus from the placenta and to return waste products
from the fetus to the placenta.
• Measurement: It is about 53 cm (21 in) or 30-100 cm of
variation in length at term and about 2 cm (3⁄4 in) thick.
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49. Development of umbilical cord
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50. The umbilical cord contd…
• The bulk of the cord is a gelatinous mucopolysaccharide
called Wharton’s jelly, which gives the cord body and
prevents pressure on the vein and arteries that pass
through it.
• The outer surface is covered with amniotic membrane.
• An umbilical cord contains only one vein but two arteries.
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51. The umbilical cord contd…
• The number of veins and arteries in the cord is always
assessed and recorded at birth because about 1% to 5%
of infants are born with a cord that contains only a single
vein and artery.
• From 15% to 20% of these infants are found to have
accompanying chromosomal disorders or congenital
anomalies, particularly of the kidney and heart.
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52. The umbilical cord contd…
• The rate of blood flow is 350 mL/min at term.
• Blood can be withdrawn from the umbilical vein or transfused
into the vein during intrauterine life for fetal assessment or
treatment (termed percutaneous umbilical blood sampling
[PUBS]).
• The cord is unlikely to twist or knot to interfere with the fetal
oxygen supply.
• In about 20% of all births, a loose loop of cord is found around
the fetal neck at birth.
• The walls of the arteries are lined with smooth muscle.
• It contains no nerve supply.
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53. The umbilical cord contd…
• Remnant of the umbilical vesicle (yolk sac) and its
vitelline duct: May found as a small yellow body near
the attachment of the cord to the placenta or on rare
occasion, the proximal part of the duct persists as
Meckel's diverticulum.
• Allantois: A blind tubular structure may occasionally
present near the fetal end which is continuous inside the
fetus with its urachus and bladder.
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54. The umbilical cord contd…
Obliterated extra embryonic coelom: In the early
period, intraembryonic coelom is continuous with
extraembryonic coelom along with herniation of coils of
intestine. The condition may persists as a congenital
umbilical hernia or exomphalos.
Attachment:
• Early period- Cord is attached to the ventral surface of
the embryo close to the caudal extremity
• The point of attachment is moved permanently to the
center of the abdomen at fourth month.
• Unlike the fetal attachment, the placental attachment is
inconsistent, either centre or eccentric.
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55. Amniotic cavity, amnion and amniotic fluid
Development:
• Fluid accumulates slowly, at first, but ultimately the
fluid filled cavity obliterates the chorionic cavity.
• Initially the cavity is located on the dorsal surface of
the embryonic disc.
• With the formation of the head, tail and lateral
folds, it comes to surround the fetus.
• Its two growing margins merge into the body stalk.
• Thus, the liquor amnii surrounds the fetus
everywhere except at its attachment with the body
stalk.
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58. Structure of amnion
• Fully formed amnion is 0.02-0.5 mm in thickness.
• From within outwards the layers are:
– Single layer of cuboidal epithelium
– Basement membranes
– Compact layer of reticular structure
– Fibroblastic layer
– Spongy layer
• No blood supply, nerve supply nor any lymphatic
supply.
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59. Amniotic fluid
• Origin of amniotic fluid:
Mixed maternal and fetal origin. The following are the
speculative theories:
a. As a transudate across the umbilical cord or from fetal
circulation in the placenta or secretion from the
amniotic epithelium.
b. Transudate of fetal plasma through the highly
permeable fetal skin before it is keratinized at 20th
week.
c. Contribution from the fetus- Fetal daily urine output at
term is about 400-1200 ml. The fetal swallows about
200-500 ml of liquor every day at term.
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60. The origin and circulation of amniotic fluid
Maternal circulation amniotic epithelium
(Transudation)
Secretes
Placenta Amniotic fluid Fetal Skin
Fetal Urine
Exchange with Ingestion by the fetus
respiratory tract
Intestinal absorption of water
Fetal Circulation
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61. Amniotic fluid contd…
Circulation: The water in the amniotic fluid is completely
changed and replaced in every 3 hours. The presence of
lanugo and epithelial scales in the meconium shows that the
fluid is swallowed by the fetus and some of it passes from the
gut into the fetal plasma.
Volume:
• 50ml at 12 weeks
• 400 ml at 20 weeks and
• reaches its peak of 1 litre at 36-38 weeks.
• Thereafter the amount diminishes, till at term it measures
about 600-800 ml.
• As the pregnancy continues post term, further reduction
occurs to the extent of about 200 ml at 43 weeks.
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62. Amniotic fluid contd…
Physical features:
• Faintly alkaline with a pH of about 7.2 with low specific gravity of
0.010.
• It becomes highly hypotonic to maternal serum at term pregnancy.
• Osmolarity of 250 m Osmol/liter (fetal maturity).
• Checking the pH of the fluid at the time of rupture helps to
differentiate it from urine, which is acidic (pH 5.0–5.5).
Colour: In early pregnancy, it is colourless but near term, it becomes
pale straw colored due to presence of exfoliated lanugo and
epidermal cells from the fetal skin. It may look turbid due to
presence of vernix caseosa.
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63. Abnormal colour of amniotic fluid
• Meconium stained (green) is suggestive of fetal distress in
presentation other than breech or transverse. Depending
upon the degree and duration of the distress, it may be thin
or thick or pea souped.
• Golden colour in Rh incompatibility is due to excessive
haemolysis of the fetal RBC and production of excess
bilirubin.
• Greenish yellow (saffron) in post maturity.
• Dark colored in accidental hemorrhage is due to
contamination of blood.
• Dark brown (tobacco juice) amniotic fluid is found in IUD.
The dark colour is due to frequent presence of old HbA.
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64. Amniotic fluid contd…
Composition:
• first half of pregnancy, the composition of fluid is almost
identical to a transudate of plasma.
• late pregnancy, the composition is altered mainly due to
contamination of fetal urinary metabolites.
• The composition includes- water (98-99%) and solid (1-2%).
• The following are the solid constituents:
Organic-
Protein- 0.3 mg%
Uric acid- 4 mg%
Glucose- 20 mg%
Creatinine- 2 mg%
Urea- 30 mg%
Total lipids- 50 mg%
NPN- 30 mg%
Hormones (Prolactin, insulin
and renin)
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65. Amniotic fluid contd…
Inorganic
• The concentration of the sodium, chloride and potassium
is almost the same as that found in maternal blood.
Suspended particles: lanugo, exfoliated squamous
epithelial cells from the fetal skin, vernix caseosa, cast off
amniotic cells and cells from the respiratory tract, urinary
bladder and vagina of the fetus.
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66. Function of amniotic fluid
• Its main function is to protect the fetus.
During pregnancy:
• It acts as a shock absorber.
• Maintains an even temperature
• The fluid distends the amniotic sac and thereby allows
for growth and free movement of fetus and prevents
adhesion between fetal parts and amniotic sac.
• Its nutritive value is negligible however, water supply to
the fetus in quite adequate.
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67. Function of amniotic fluid contd…
During labour:
• The amnion and chorion are combined to form a
hydrostatic wedge which helps in dilatation of cervix.
• It prevents marked interference with the placental
circulation during uterine contraction.
• It flushes the birth canal at the end of first stage of
labour and by its aseptic and bactericidal action protects
the fetus and prevents ascending infection to the uterine
cavity.
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68. Clinical importance of amniotic fluid
• Provides useful information about the well being and
also maturity of the fetus.
• Intra amniotic instillation of chemicals is used as method
of induction of abortion.
• Excess or less volume of liquor amnii is assessed by
amniotic fluid index (AFI). It is used to diagnose the
clinical condition of polyhydramnios or oligohydramnios
respectively.
• Rupture of the membranes with drainage of liquor is a
helpful method in induction of labour.
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69. References
• Jacob A.A comprehensive textbook of midwifery and
gynecological nursing.3rd edition.New
Delhi:Jaypee;2012.
• Fraser DM, Cooper MA.Myles textbook for
midwives.15th edition. Philadelphia:churchill
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• Dutta DC.Textbook of obstetrics. 6th
edition.Calcutta:New central book agency;2004
• Pillitteri A. Maternal and Child health nursing: Care of
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Lippicott Williams and Wilkins : Philadelphia; 2010
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72. • The process of spiral artery remodeling during pregnancy. In early
pregnancy, two types of extravillous trophoblasts are found outside the
villous, endovascular and interstitial trophoblasts. Endovascular
trophoblasts invade and transform spiral arteries during pregnancy to
create low-resistance blood flow that is characteristic of the placenta.
Interstitial trophoblasts invade the decidua and surround spiral arteries.
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