We will now proceed to follow the
further development of the placenta
and its relationship to the membranes
that surround the developing fetus.
Once the placenta is in place,
we have a situation where uterine arteries
travel into placenta bringing
oxygen in blood to the embryo.
This maternal oxygen and blood
fills intervillous spaces
and allows gas exchange
to occur across the villi.
Maternal blood interacts
with deoxygenated blood
coming from the umbilical
arteries of the fetus,
and even if though blood
does not directly mingle,
gas exchange and nutrients
exchange occur across the villi.
Thereafter, umbilical veins take
oxygenated blood back to the embryo
and maternal veins take deoxygenated
maternal blood from the intervillous space
back into maternal
circulation to be replenished.
Typically, the blood of the fetus and
the mother should never directly connect.
One problem that arises
with this situation is that
as maternal blood enters
the intervillous space,
it necessarily mixes with
blood that's already there;
meaning that the oxygenated
blood from the mother
is mixing with the deoxygenated blood
already present in the intervillous space
that has not yet left
through maternal vein.
What this means is that the oxygen content
of the blood in the intervillous space
is lower than pure
To compensate for this,
the fetus produces a fetal hemoglobin
which binds to oxygen much more
strongly than mature hemoglobin.
In physiology, we can say that the
O2 P50 value of fetal hemoglobin
is 19 millimeters of mercury,
meaning it's about 50% saturated at
a partial pressure of 19 millimeters
compared with 26.8 millimeters
of mercury for adult hemoglobin.
Long story short, it binds to oxygen
much more strongly than mature hemoglobin
and allows the fetal hemoglobin
to actually take the oxygen
away from the
even though we've got mixture
of maternal oxygenated
and deoxygenated blood in
the intervillous space.
Now, one problem
that can also occur,
is that if fetal blood
does contact maternal blood
due to a rupture of a
villus or another problem,
there can be immune reactions on
the maternal side to fetal blood.
In particular, if the fetal blood produces
the D antigen and is therefore Rh positive,
then it can create an immune reaction
in a mother who is Rh negative.
So she has not got that
antigen on her own blood,
she will produce antibodies against
the D antigen and these IgG antibodies
can cross the placenta and
attack fetal red blood cells.
Anti-D immune globulin prevents
the Alloimmunization in a D-Woman
by interfering with the maternal
immune response provoked by D antigen
positive fetal red cells that have
escaped into the maternal circulation.
Exactly how immunization
is prevented is unknown,
but it may include rapid
clearance of anti
decoded red cells
and or down regulation of antigen specific
B cells before alloimmunization occurs.
All D-women who have a
negative anti-D antibody screen
and who are carrying a
fetus that is or maybe D+
should be given anti-D
The specific circumstances are at 28 weeks
of gestation after delivery of a D+ newborn
or after an anti partum event
associated with an increased risk
of fetal maternal bleeding such as
spontaneous or induced abortion,
threatened abortion, ectopic
pregnancy, blunt abdominal trauma,
amniocentesis or antepartum
D- women who screen positive
for anti-d antibodies
should not receive
anti-d immune globulin.
Because it is not effective and will
not prevent a rise in maternal tighter.
Attacking the fetal red blood
cells causes a condition known as
erythroblastosis fetalis, that's going to
mean that we have an excess of creation
of new red blood cells erythroblast
in the fetus to compensate
for the red blood cells that are being
attacked by the maternal antibodies.
This can progress to a
condition called fetal hydrops
where the tissues
of the fetus swell
in response to the immune attack
from the mothers antibodies
and in less severe but still serious forms
you can have breakdown of fetal blood cells
producing excessive bilirubin leading to
jaundice as development proceeds further.
This response becomes more and more
severe for subsequent pregnancies
because the maternal immune system
is already primed to produce
those antibodies against the D
antigen and RH positive blood.
The placenta initially is going to
form as a spherical object surrounding
the developing embryo in the uterine lining
but as the embryo and placenta inlarge
it starts to to push its way
into the cavity of the uterus.
As this occurs the placenta thins
on one side and will eventually form
more or less a pancake shaped structure
on its attachment to the uterus
and not be present outside
the fetus beyond that.
As the embryo enlarges the chorionic
cavity which contains the yolk sac
will thin and the amniotic cavity
shown here in blue will expand
outward tremendously giving buoyancy
to the fetus and supporting it
as it moves,
but also helping it resist gravity.
The portion of the endometrium,
the lining of the uterus that's
pushed outward as the embryo
grows out of the uterine wall is going
to be known as the decidua capsularis.
And the lining of the
endometrium everywhere else
is called the
At the point where the decidua
parietalis of the uterine wall
meets the decidua capsularis
covering the developing fetus,
we have an area called the decidua
basalis and that's just marking
the subdivisions of the endometrial
lining as the fetus continues to develop.
As it develops,
the fetus becomes larger and larger.
The placenta is more or
less pushed to one side,
and as I said before from is a
relatively pancake shaped structure
most likely on the posterior wall
of the embryos of the uterus.
It's connected to the developing
fetus by the umbilical cord,
which is full of a loose mesenchyme
called Wharton's jelly that surrounds
the vessels that are traveling in
the record to and from the placenta.
The membrane that supports the fetus
consists of the amniotic membrane
and then a smoothly chorion a
very thin layer of the endometrium
that was present when the fetus
grew out of the uterine wall
and then another smooth layer of the
chorion this amniochorionic membrane
is what ruptures when delivery is imminent
and is known as the water breaking
that allows the amniotic
fluid to exit the birth canal
and set the stage for a
hopefully smooth delivery
By the 14th or 15th week of pregnancy,
there's enough amniotic fluid surrounding
the developing fetus that we can safely
sample some of it by a Amniocentesis.
This is generally done in order to check
some genetic issues that may be arising
in the fetus and looking for
markers of neural tube defects
such as alpha-fetoprotein
and other possible issues
that might be anticipated.
the chorionic villi can be sampled
and then taken for
This can be done either
through the anterior body wall
using ultrasound or vaginally to
harvest some of the chorionic villi.