Obstetrics Simplified - Diaa
The placenta develops from the chorion frondosum (foetal origin) and
decidua basalis (maternal origin).
Anatomy at term:
- Shape: discoid. Diameter: 15-20 cm. Weight: 500 gm.
- Thickness: 2.5 cm at its center and gradually tapers towards the
- Position: in the upper uterine segment (99.5%), either in the posterior
surface (2/3) or the anterior surface (1/3).
- Foetal surface: smooth, glistening and is covered by the amnion
which is reflected on the cord. The umbilical cord is inserted near
or at the center of this surface and its radiating branches can be seen
beneath the amnion.
- Maternal surface: dull greyish red in colour and is divided into
15-20 cotyledons. Each cotyledon is formed of the branches of one main
villus stem covered by decidua basalis.
Functions of the Placenta
O2 and CO2 pass across the placenta by simple diffusion. The foetal haemoglobin
has more affinity and carrying capacity than adult haemoglobin. 2,3 diphosphoglycerate
(2,3-DPG) which competes for oxygen binding sites in the haemoglobin molecule,
is less bounded to the foetal haemoglobin (HbF) and thereby allows a greater
uptake of O2 (O2 affinity). The rate of diffusion depends upon:
- maternal/foetal gases gradient.
- maternal and foetal placental blood flow.
- placental permeability.
- placental surface area.
The transfer of nutrients from the mother to the foetus is achieved by:
- Simple diffusion: e.g. water and electrolytes.
- Facilitated diffusion: e.g. glucose.
- Active diffusion: e.g. aminoacids.
- Pinocytosis: e.g. large protein molecules and cells.
Waste products of the foetus as urea are passed to maternal blood by
simple diffusion through the placenta.
Production of enzymes
E.g. oxytocinase, monoamino oxidase, insulinase, histaminase and heat
stable alkaline phosphatase.
Production of pregnancy associated plasma proteins (PAPP)
E.g. PAPP-A, PAPP-B, PAPP-C, PAPP-D and PP5. The exact function of these
proteins is not defined.
The foetal blood in the chorionic villi is separated from the maternal
blood, in the intervillous spaces, by the placental barrier which is composed
- endothelium of the foetal blood vessels,
- the villous stroma,
- the cytotrophoblast, and
- the syncytiotrophoblast.
However, it is an incomplete barrier. It allows the passage of antibodies
(IgG only), hormones, antibiotics, sedatives, some viruses as rubella and
smallpox and some organisms as treponema pallida. Substances of large molecular
size as heparin and insulin cannot pass the placental barrier.
- Protein hormones:
- Human chorionic gonadotrophin (hCG):
- It is a glycoprotein produced by the syncytiotrophoblast.
- It supports the corpus luteum in the first 10 weeks of pregnancy
to produce oestrogen and progesterone until the syncytiotrophoblast
can produce progesterone.
- HCG molecule is composed of 2 subunits:
- alpha subunit which is similar to that of FSH, LH and
- beta subunit which is specific to hCG.
- HCG rises sharply after implantation, reaches a peak of
100000 mIU/ml about the 60th day of pregnancy then falls sharply
by the day 100 to 30000 mIU/ml and is maintained at this level
- Estimation of beta-hCG is used for:
- diagnosis of early pregnancy.
- diagnosis of ectopic pregnancy.
- diagnosis and flow-up of trophoblastic disease.
- Human placental lactogen (hPL):
- It is a polypeptide hormone produced by the syncytiotrophoblast.
- The supposed actions of hPL include:
- lipolysis: increasing free fatty acids which provide
a source of energy for mother and foetal nutrition.
- inhibition of gluconeogenesis: thus spare both glucose
and protein explaining the anti-insulin effect of hPL.
- somatotrophic: i.e. growth promotion of the foetus due
to increased supply of fatty acids, glucose and amino acids.
- mammotropic and lactogenic effect.
- HPL can be detected by the 5-6th week of pregnancy, rises
steadily until the 36th week to be 6m g/ml.
- Its level is proportional to the placental mass.
- Human chorionic thyrotrophin (hCT):
- No significant role has been established for it, but it
is probably responsible for increased maternal thyroid activity
and promotion of foetal thyroid development.
- Hypothalamic and pituitary like hormones:
- e.g. gonadotropin releasing hormone (GnRH), corticotropin
releasing factor (CRF), ACTH and melanocyte stimulating hormone.
- Others as inhibin, relaxin and beta endorphins.
- Steroid Hormones:
- They are synthetized by syncytiotrophoblast from their precursors
dehydroepiandrosterone sulphate (DHES) or its 16a -hydroxy (16α-OH-DHES).
- Near term, 50% of DHES is drived from the foetal adrenal
gland and 50% from maternal adrenal. It is transformed in the
placenta into oestradiol- 17β (E2).
- On the other hand, 90% of 16α-OH-DHES
is drived from foetal origin after hydroxylation of DHES in
the foetal liver, while only 10% is drived from the mother by
the same way.
- Oestrogens are excreted in the maternal urine as oestriol
(E3), oestradiol (E2) and oestrone (E1). Oestriol (E3) is the
largest portion of them.
- Maternal urinary and serum oestriol level is an important
index for foetal wellbeing as its synthesis depends mainly on
the integrity of the foetal adrenal and liver as well as the
placenta (foeto-placental unit).
- Urinary oestriol increases as pregnancy advances to reach
35-40 mg per 24 hours at full term. Progressive fall in urinary
oestriol indicates that the foetus is jeopardous.
- Oestrogens are responsible with progesterone for the most
of the maternal changes due to pregnancy especially that in
genital tract and breasts.
- It is synthetized by syncytiotrophoblast from the maternal
- Excreted in maternal urine as pregnandiol.
- Increasing gradually during pregnancy to reach a daily production
of 250 mg per day in late normal single pregnancy.
- It provides a precursor for the foetal adrenal to produce
glucocorticoids and mineralocorticoids.
Abnormalities of the Placenta
- Placenta Bilobata: The placenta consists of two equal lobes connected
by placental tissue.
- Placenta Bipartita: The placenta consists of two equal parts connected
by membranes. The umbilical cord is inserted in one lobe and branches
from its vessels cross the membranes to the other lobe. Rarely, the
umbilical cord divides into two branches, each supplies a lobe.
- Placenta Succenturiata: The placenta consists of a large lobe and
a smaller one connecting together by membranes. The umbilical cord is
inserted into the large lobe and branches of its vessels cross the membranes
to the small succenturiate (accessory) lobe. The accessory lobe may
be retained in the uterus after delivery leading to postpartum haemorrhage.
This is suspected if a circular gap is detected in the membranes from
which blood vessels pass towards the edge of the main placenta.
- Placenta Circumvallata: A whitish ring composed of decidua, is seen
around the placenta from its foetal surface. This may result when the
chorion frondosum is two small for the nutrition of the foetus, so the
peripheral villi grow in such a way splitting the decidua basalis into
a superficial layer (the whitish ring) and a deep layer. It can be
a cause of abortion, antepartum haemorrhage, premature labour and intrauterine
- Placenta Fenestrata: A gap is seen in the placenta covered by membranes
giving the appearance of a window.
Placenta membranacea: A great part of the chorion develops into placental
tissue. The placenta is large, thin and may measure 30-40 cm in diameter.
It may encroach on the lower uterine segment i.e. placenta praevia.
The placenta increases in size and weight as in congenital syphilis,
hydrops foetalis and diabetes mellitus.
Placenta Praevia: The placenta is partly or completely attached to the
lower uterine segment.
Placenta Accreta: The chorionic villi penetrate deeply into the uterine
wall to reach the myometrium, due to deficient decidua basalis. When the
villi penetrate deeply into the myometrium, it is called "placenta increta"
and when they reach the peritoneal coat it is called "placenta percreta".
- Placental Infarcts:
- Seen in placenta at term, mainly in hypertensive states with
- White infracts: due to excessive fibrin deposition. Normal
placenta may contain white infracts in which calcium deposition
- Red infarcts: due to haemorrhage from the maternal vessels
of the decidua. Old red infarcts finally become white due to
- Placental Tumour:
- Chorioangioma is a rare benign tumour of the placental blood
vessels which may be associated with hydramnios.
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Edited by Aldo Campana,