Obstetrics Simplified - Diaa
The Foetal Physiology
THE FOETAL CIRCULATION
The foetal circulation differs mainly from the adult one by the presence
of 3 major vascular shunts:
- Ductus venosus: between the umbilical vein and inferior vena cava.
- Foramen ovale: between the right and left atrium.
- Ductus arteriosus: between the pulmonary artery and descending aorta.
The rationale of these shunts is to divert the oxygenated blood from
the less functioning organs as lungs, liver, kidney and intestine as placenta
carries their functions, to the brain, heart and other parts of the body.
The circulation is as follow:
- Oxygen and nutrients are carried from the placenta to the foetus
in a single large umbilical vein.
- The oxygen saturation in the blood of the umbilical vein is reduced
from 95% in the maternal arterial blood to 80% due to its consumption
by the placental metabolism.
- As the umbilical vein entering the foetal body most of its oxygenated
blood passes to the inferior vena cava (IVC) through the ductus venosus.
While the remainder communicates the portal vein to supply the liver.
- The liver drains into the IVC through the hepatic veins.
- The blood in the IVC is a mixture of the oxygenated blood from the
umbilical vein and the desaturated blood from the lower limbs and abdominal
organs including the liver so its O2 saturation is reduced to 65% when
it enters the right atrium.
- Most of the blood is directed to the left atrium through the foramen
ovale and from it to the left ventricle and descending aorta.
- The remainder of the blood in the right atrium passes with that
coming from the head and upper limbs via the superior vena cava to the
right ventricle → pulmonary artery where
most of it passes also to the aorta through the ductus arteriosus because
of the high resistance of the unexpanded foetal lungs.
- The blood passes finally from the aorta to the hypogastric arteries
→ umbilical arteries
Changes At Birth:
- With clamping of the umbilical cord, the pressure in the ductus
venosus drops leading to its closure to form the ligamentum venosum.
- The initiation of respiration creates a negative intrathoracic pressure
which is transmitted to the right ventricle and atrium, while the pressure
in the left atrium is increased due to returning blood from the lungs
this leads to closure of the foramen ovale.
- With diversion of most of the blood into the lungs, no further blood
passes through the ductus arteriosus so it is closed.
- The umbilical vein is obliterated to form the ligamentum teres in
the falciform ligament of the liver.
- The hypogastric arteries are obliterated to form the hypogastric
THE FOETAL BLOOD
- Site of haematopoiesis (formation of blood cells): first in the
yolk sac then foetal liver and lastly in the bone marrow.
- Erythrocytes: are all nucleated but at term only about 5-10% of
them are nucleated. The number is 6 millions/ mm3.
- Concentration at term is 15-20 gm/dl.
- 10-45% of it is adult haemoglobin (HbA), the remainder is foetal
haemoglobin (HbF). At age of one year, less than 2% is HbF.
- HbF has more affinity to oxygen than HbA as it contains less
2,3 diphosphoglycerate (2,3 DPG) than that in HbA. 2,3 DPG competes
for oxygen binding sites in the haemoglobin molecule, so the less
2,3 DPG contents the more affinity to oxygen.
- Serum iron: At term it is 150 m g/dl.
- Leucocytic count: At term it is 2-3 times the adult one.
- The Rhesus factor: can be detected in the foetal blood from the
- Anti-A and Anti-B: appear in the foetal blood at about 4-8 months
after birth. Those present at birth are acquired form the maternal blood
and usually disappear 2 weeks after birth.
- Foetal respiratory movement can be detected by ultrasound as early
as 11 weeks as a chest wall movement. From the beginning of the 4th
month, this respiratory movement is sufficient to move the amniotic
fluid in and out the respiratory tract.
- Pulmonary surfactant:
- It is formed by type II pneumocytes that line the alveoli.
- This starts at the 20th week and level increases gradually up
- Detection of lecithin/ sphingomyelin (L/S) ratio of 2 or more
or detection of phosphatidyl glycerol in the amniotic fluid indicates
- Pulmonary surfactant facilitates distension of the alveoli thus
preventing the development of neonatal respiratory distress syndrome.
- Initiation of respiration:
- During intrauterine life, the foetal respiratory centre in the
medulla is inhibited by cortical impulses. Anoxia at birth affects
the cortical centres leads to release of the respiratory centre
from its inhibition and becomes sensitive to cutaneous stimuli,
muscle stretch and biochemical changes in foetal blood as CO2 concentration.
The small intestine undergoes peristalsis by the 11th week of gestation.
Starting from the 2nd trimester, the foetus swallows and absorbs amniotic
It consists of undigested debris from the swallowed amniotic fluid, secretions
and desquamation from the gastrointestinal tract.
- Glucuronidation: i.e. conjugation of free bilirubin is limited.
- Glycogen: appears in low concentration in foetal liver during
the 2nd trimester but near term it is 2-3 times those in adult liver.
- Clotting factors: fibrinogen, factors II, VII, IX, XI and
XII are produced by the liver in a low level at birth .
- Vitamin K stores in the liver are deficient at birth as vitamin
K is formed by bacteria in the intestine.
- Gall bladder: It secretes bile from the 3rd month of gestation.
The foetal pancreas responds to hyperglycaemia by increasing insulin
secretion. However, the alpha cells of pancreas do not respond to hypoglycaemia
by secreting glucagon.
- By the end of the first trimester, the kidneys can excrete urine
which is hypotonic due to low electrolytes concentrations.
- The full foetal bladder, seen by ultrasound, indicates functioning
CENTRAL NERVOUS SYSTEM
- At full term is partially developed and functioning.
- By the end of the first year of life the brain doubles its weight
and triples it by the end of the fifth year.
Before the end of the 17th week, the foetal pituitary is able to synthesise
and store all pituitary hormones.
Thyroid gland and parathyroid glands
They are capable of function by the end of the first trimester.
- The outer (adult) zone of the foetal adrenal cortex produces cortisol.
- The inner (foetal) zone produces dehydroepiandrosterone, the precursor
- The adrenal medulla produces small amount of catecholamines.
- Testosterone is synthesised by the foetal testis from progesterone
and pregnenolone by 10 weeks of gestation.
- Oestrogen is synthesised by the foetal ovaries but it is not required
for female phenotypic development.
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Edited by Aldo Campana,