Prenatal nutrition consists of 2 major phases: - the trophoblastic phase, where the
trophoblast digests and derives nutrition from
endometrial tissue, from week 2 to week 8, - and the placental phase from week 9 onward.
There is a long transition period, from week 4 to 12, where the placenta gradually
takes over the role of feeding the embryo. The placenta is a hybrid organ containing
both maternal and fetal components. It develops from the deeper layer of the
endometrium and the trophoblast of the embryo. Basically, in week 2, the superficial layer
of the trophoblast - the syncytiotrophoblast - invades and digests the nearby endometrial tissue,
while maternal capillaries at the implantation site begin to dilate, forming sinusoids. As
these events continue, maternal blood vessels surrounding the embryo are slowly dissolved.
The deeper layer of the trophoblast - the cytotrophoblast - then invades the
syncytiotrophoblast to form the primary chorionic villi.
In week 3, extraembryonic mesoderm cells fill the villi and differentiate into fetal blood vessels,
including the three umbilical vessels that connect the embryo to the placenta. The now secondary
villi are submerged in pools of maternal blood. Fetal blood travels to the placenta via 2
umbilical arteries. The exchange between maternal and fetal blood takes place in the capillaries
within chorionic villi. Fetal waste such as carbon dioxide moves into maternal blood, while nutrients
and oxygen move into fetal blood and travel back to the fetus via the single umbilical vein.
The placenta is also permeable to lipid-soluble fetotoxic substances,
which, if consumed by the mother during pregnancy, may cause harm to fetal development.
Note that maternal and fetal blood do not mix because blood cells cannot move across
the placenta. This separation protects the fetus – a genetically-distinct and
hence a “foreign” organism - from being attacked by the maternal immune system.
Fetal circulation is markedly different from neonatal circulation due to the
placental-umbilical circuit and presence of 3 fetal shunts.
The umbilical vein carries oxygen- and nutrient-rich blood from the placenta to
the liver. Because fetal liver is not yet ready for its blood filtering function,
most of the blood bypasses the liver via a shunt called the “ductus venosus”,
which drains into the inferior vena cava. In the inferior vena cava, placental blood
mixes with venous blood of the fetus, and travels to the heart’s right atrium.
While the right heart normally pumps blood to the lungs after birth,
this step is skipped in the fetus because the fetal lungs are not yet functional. Fetal blood
bypasses the pulmonary circuit in 2 ways: - First, part of the blood passes directly
from the right atrium to the left atrium via an opening in the interatrial septum, called
the “foramen ovale”. It then travels through the left ventricle to coronary arteries and the aorta,
which branches to provide blood to the brain. - Second, part of the blood flows to the right
ventricle and is pumped to the pulmonary trunk, but most of this blood is immediately
shunted into the aorta via the so-called “ductus arteriosus”. This happens because
the collapsed fetal lungs put up a much higher resistance than the blood pressure in the aorta.
Blood then enters the systemic circulation. After nourishing the fetus, it returns to the
placenta via 2 umbilical arteries, which are branches of the fetal internal iliac arteries.
At birth, as the lungs expand with air, pulmonary vascular resistance drops rapidly.
At the same time, clamping of the umbilical cord increases systemic vascular resistance.
These 2 changes enable the right heart to pump blood to the now lower-resistant lungs.
The increased systemic resistance also raises the pressure in the left atrium to
a higher value than the right atrium, forcing the foramen ovale to close.
Pressure changes, together with presence of oxygen and decreased prostaglandin levels,
cause the fetal shunts to close and become ligaments.