Fetal Development
RC 290
The Placenta
Placental Structure

Purple and pancake
shaped
 6 inch diameter, 1 inch
thick
 13-16 square meter
surface area

Attaches to upper 1/3
of uterus
 Maternal side:
Decidua Basalis
 Fetal side: Chorion
(attaches to amniotic
membrane)
Placental Functions





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Primary function is to act as organ of respiration
and to be the pathway for exchange of nutrients
and waste products between mother and fetus
Physical protection
Hormone synthesis
Iron and Glycogen storage
Some metabolic functions
Antibodies
Maternal-Fetal Circulation
Note: Maternal and fetal blood do not come into direct contact!
Maternal-Fetal Circulation

Blood comes to IV spaces (maternal side)
from uterine arteries
 Blood in villi (fetal side) comes from
umbilical arteries
 After exchange of O2 and CO2, the “fresh
blood” returns to the fetus via the umbilical
vein
Umbilical Cord

2 arteries
 1 vein
 Surrounded by
Wharton’s jelly
Amniotic Sac

The outer layer of the umbilical cord forms the
amniotic sac around the fetus
 Fetus is in amniotic sac containing amniotic fluid
– Amniotic fluid made up of maternal serum and fetal
urine and fetal lung fluid

3 Functions of amniotic fluid:
– Shock absorption, temperature stability, sterile
environment
Blood Gas Values
Hence, fetal PO2 is 25-30 mmhg!
Why is fetal PO2 low?

Placental O2 consumption
 Admixture and shunting
– Like the lung, the IV space and the villi are not
perfectly matched

Fetal hemoglobin (HbF)
– More of it (when compared to adult blood) and has
greater affinity for O2
– Concentration is 17-18 grams%
– Made up of alpha and gamma chains of amino acids
– HbF is present for up to two months after birth

Fetal O2 consumption is about twice what an
adults is
Fetal Circulation
OOOPS! This is fecal circulation!
Fetal Circulation
Fetal Circulation



UV goes into liver and
portal circulation
Most of blood shunts
across the Ductus Venosus
into the inferior vena cava
As inferior vena cava
empties into right atrium,
most blood shunts across
the Foramen Ovale into
the left atrium where it
enters the systemic
circulation from the left
ventricle
– The heart and brain get the
best blood first!

Blood from superior vena
cava goes into right
ventricle and then to
pulmonary artery
 Most of this blood flow
shunts across the Ductus
Arteriosus into the aorta
– DA is in the descending
aorta after the arch

Pre-ductal blood may have
a higher PO2!
– Hence, only 3-10% of fetal
cardiac out put perfuses the
fetal lings
Summary: Fetal Circulation

3 Shunts: Ductus Venosus, Foramen Ovale,
and Ductus Arteriosus
 Placental Circulation
 Only 3-10% of fetal cardiac output actually
perfuses the fetal lungs
Fetal Lung Development

Lung buds appear at 3.5 weeks
 Tracheo-bronchial tree formed by 16 weeks
 Alveoli start forming at 20-24 weeks
– Surfactant also starts to form

Capillary networks starts forming and
approaching alveoli at 26-28 weeks
 Lungs and surfactant mature at 35 weeks
 Normal term is 40 weeks
Surfactant

Reduces surface
tension so alveoli
don’t collapse with
each exhalation
 Is a phospolipid made
up of dipalmityllecithin and
sphingomyelin

Produced through two
enzymatic pathways:
– Methyl transferase
– Phosphocholine
transferase


This is the major
pathway and has a
longer half-life
Matures last
Respiratory Problems at Birth!

Immature lung architecture if born
premature
 And/or:
 Surfactant levels may not be adequate if:
– Premature birth
– Production inhibited if significant hypoxia,
hypothermia, or hypoglycemia