• Initially secreted by the amniotic cells contained in the amnion (a membrane enclosing the embryo)
• ~10 wks the amniotic fluid consists mainly of transudate fluid from foetal & maternal serum, passing out via the umbilical cord and skin
• As the foetus matures, its skin and umbilical cord becomes impermeable to water and the transudation of fluid slowly ceases
• Amniotic fluid is then produced and increases through a small imbalance between kidney production and foetal swallowing
• From around the 2nd trimester amniotic fluid is predominantly produced from foetal urine
• The volume of amniotic fluid increases steadily until term, then begins to decrease rapidly:-

 

        Amniotic Fluid during pregnancy - taken from Campbell & Lees (1)

10 wks	30 mls
20 wks	300 mls
30 wks	600 mls
38 wks	1000 mls
40 wks	800 mls
42 wks	350 mls

 

Functions of amniotic fluid

• Protection for the developing baby
• Permits maturation of the lungs - absence can lead to pulmonary hypoplasia
• Allows movement of the foetus & normal development of the limbs

 

Problems associated with amniotic fluid

• Polyhydramnios and oligohydramnios
• Potential infections secondary to premature rupture of membranes
• Amniotic fluid embolus
• Please see specific notes

 

Estimation of the amniotic fluid index

• The Amniotic Fluid Index (AFI) provides an estimation of the quantity of amniotic fluid present
• It is derived using Ultra Sonography - adding together the vertical depth of amniotic pools in the four main quadrants of the abdomen (measured in cm)
• The AFI can then be calculated and compared
• (Note there is some degree of error in this method and can lead to variable results)