TR056/PG1007 Lecture 12 Growth of The Foetus and the Role of the Placenta. Dr. Neil Docherty My Teaching Objec/ves • To describe the changes in growth from week 9 to birth
(the foetal period)
• To revise the development of the trophoblast through the
embryonic period
• To outline the functions of the interface between
foetal and maternal blood and outline the sequence of events
leading to the the maturation of the placenta and amniotic cavity
• To contrast the arrangement of embryos and foetal membranes in
monozygotic and dizygotic twins
The Foetal Period
• The period from the 9th week to birth is referred to as the
FOETAL PERIOD
• During this period growth dynamics occur as follows;
1.  Growth of the head slows
2.  The foetus initially increases more in length than in
weight
3.  Finally the foetus gains weight. N.B. Rapidly toward the
end of pregnancy
Gestational Time and Survival
D.O.B. estimates are most accurately inicatd as being 266 days or 38 weeks after
Fertilisation
Most foetuses are born within 10 to 14 days of this
SURVIVAL
> Early part of the 6th month- highly unlikely
6.5-7 months-90% survival
DISCUSSION-Possible Reasons???
Development of The Placenta
Mirrors Growth of the Foetus
• As the foetus grows, its metabolic demands increase
• This drives maturation of the placenta
Mainly characterised by
•  An increase in the surface area over which
exchange of material occurs
Functions of the Mature Placenta
THE MATURE PLACENTA
Nutrient and waste exchange
-Gas exchange
-Nutrient and electrolytes
Other
-Passive Immunity (maternal antibody)
-Hormone production (progesterone,estradiol)
Selectivity implies the presence of a PLACENTAL BARRIER
Trophoblast Development (day 4-14)
4.5-6 days
9 days
12 days
14 days
Trophoblast Development (day 14-21)
Formation of secondary villi
-mesodermal migration
Formation of tertiary villi
-mesodermal vascular
differentiation
Complete connection of
Embryonic and trophoblastic
circulation
Formation of the Mature Placenta
• By 8 weeks the trophoblast is characterised by a large number
of secondary and tertiary villi
Transverse section
STEM VILLI STRUCTURE
Placental Maturation Months 2-4
• Expansion of the Intervillous space
• Formation of free villi
• Thinning of the placental
barrier
Chorion and Decidua-1
Early in development villous structures form around the entire
circumference of the chorion
Later they become assymetrically limited to the embryonic pole
CHORION FRONDOSUM (bushy)
The degeneration of villi at the abembryonic pole of the chrion
Causes generation of the CHORION LAEVE (smooth)
Parallel changes in the adjoining uterine wall yield the
DECIDUA BASALIS and DECIDUA CAPSULARIS
Chorion and Decidua-2
(8 WEEKS)
Chorion and Decidua-3
(12 WEEKS)
MEMBRANE FUSIONS
FORM THE MATURE
AMNIOTIC CAVITY
1.  Decidua capsularis
and decidua parietalis
2. Amnion and chorion fuse
Placental Structure
Decidual septa
form divisions
in the placenta
COTYLEDONS
Full Term Placenta
• Discoid
• 15-25 cm in diameter
• 3 cm thick
• 500-600g
At the end of pregnancy a number of changes have occurred which
limit interchange between maternal and foetal circulations
• Fibrosis in and on villi and in the chorionic plate
• Basement membrane thickening in capillaries
• Obliteration of capillary lumens
RE: DISCUSSION-Why does this happen? What organ(s) are
Required as a pre-requisite to shutting down the placenta?.
Foetal Membranes in Twins (Dizygotic)
Fertilisation of two ova by
individual spermatazoa
11:1000
Foetal Membranes in Twins
(Monozygotic)
Develop from a single
Fertilised ovum
3:1000
A= Split at 2 cell stage
B= Split of inner cell mass
(early)
C) Split of inner cell mass
(late)
Today’s Learning Objec/ves Your learning from lecture 12 should focus on being able
to;
1) Describe the changes in growth from week 9 to birth
(the foetal period).
2) Detail the maturation structure and function of the placenta.
3) Track changes in the chorionic and amniotic cavities during
the early foetal period.
4) Contrast the arrangement of developing embryos
and foetal membranes in monozygotic and dizygotic twins.