SN6 SHORT NOTE 5M ★ High-yield

Describe the amnion and amniotic fluid with their functions and disorders of volume.

Anatomy · 1st Professional — Pre-Clinical

The amnion is the innermost fetal membrane, enclosing the amniotic cavity and its fluid, in which the fetus develops.

Appears in the second week as a cavity within the epiblast
It is avascular and lined by a single layer of amniotic epithelium on a thin connective tissue layer
As it expands it obliterates the chorionic cavity and fuses with the chorion by about the 12th week

Stage	Volume	Chief source
12 weeks	50 mL	Maternal plasma and amniotic secretion
20 weeks	400 mL	Fetal urine begins to contribute
36 weeks	1000 mL — the maximum	Chiefly fetal urine
Term	About 800 mL	Fetal urine

Composition — 99% water, with electrolytes, protein, urea, creatinine, and desquamated fetal cells, lanugo and vernix
Removed by fetal swallowing (about 400 mL a day) and by absorption across the membranes
Turnover is complete about every 3 hours

Mechanical protection — cushions the fetus against injury
Permits fetal movement, which is essential for musculoskeletal and lung development
Maintains a constant temperature
Prevents adhesion between the fetus and the amnion
Allows symmetrical growth
In labour — the bag of forewaters helps to dilate the cervix, and the fluid lubricates the birth canal and dilutes organisms

Condition	Definition	Causes	Consequences
Oligohydramnios	Less than 200 mL; amniotic fluid index below 5 cm	Renal agenesis, posterior urethral valves, ruptured membranes, placental insufficiency, post-maturity	POTTER sequence — pulmonary hypoplasia (the cause of death), flattened facies, limb deformity, club foot
Polyhydramnios	More than 2000 mL	Oesophageal or duodenal atresia, anencephaly (impaired swallowing); maternal diabetes; twin-to-twin transfusion; fetal hydrops	Preterm labour, malpresentation, cord prolapse, postpartum haemorrhage

Amniocentesis at 15–18 weeks under ultrasound guidance; used for karyotyping, AFP, and assessment of lung maturity by the lecithin:sphingomyelin ratio. Miscarriage risk about 0.5%
Meconium-stained liquor suggests fetal distress and carries the risk of meconium aspiration syndrome
Premature rupture of the membranes risks ascending infection (chorioamnionitis) and cord prolapse
Amniotic fluid embolism is rare but often fatal, causing sudden collapse with disseminated intravascular coagulation
Amniotic band syndrome — strands of ruptured amnion encircle fetal parts and cause constriction rings or amputation
Ultrasound assessment of liquor volume is a routine part of fetal wellbeing assessment, and its reduction is an early sign of placental insufficiency

All questions in General Embryology