Amniotic Fluid (AF)

Amniotic fluid is the fluid that surrounds the fetus within the amniotic sac. Early in gestation, amniotic fluid composition resembles protein-free fetal serum, a result of fluid transudation across the fetal skin or the maternal decidua. [1]

After approximately 11 weeks' gestation, amniotic fluid is nearly all fetal urine, with a smaller contribution from fetal lung fluid. Amniotic fluid volume appears to be determined by a balance between inflows (primarily fetal urine) and outflows from fetal swallowing and intramembranous water flow from the amniotic cavity across the amnion into the fetal-placental vasculature. [2,3]

The amniotic fluid volume accumulates throughout the first and second trimesters and reaches a peak at approximately 33–34 weeks' gestation. [4]

Some causes of polyhydramnios (too much amniotic fluid) [5]

• Idiopathic (unknown) — most cases
• Gastrointestinal abnormalities
  • Esophageal atresia
  • Intestinal obstruction
• Central nervous system abnormalities
  • Anencephaly
• Chromosomal abnormalities
• Nonimmune hydrops
• Skeletal dysplasias
• Diabetes
• Twin-to-twin transfusion

Some causes of oligohydramnios (too little amniotic fluid) [6,7]

• Ruptured membranes
• Congenital abnormalities
  • Bilateral renal agenesis or cystic dysplasia
  • Obstruction of the urinary tract
  • Meckel–Gruber syndrome
  • VACTERL (vertebral, anal, cardiac, tracheoesophageal, renal, limb) association
  • Sirenomelia
  • Sacral agenesis
• Fetal growth restriction (placental insufficiency)
• Postterm pregnancy
• Drugs
  • Angiotensin-converting enzyme (ACE) inhibitors
  • Prostaglandin synthase inhibitors
• Twin-to-twin transfusion
• TRAP (twin reverse arterial perfusion) sequence
• Fetal demise
• Idiopathic

Ultrasound can be used to qualitatively assess the amount of amniotic fluid. One method is to measure the depth of the largest visible pocket of fluid surrounding the fetus, called the deepest vertical pocket or maximum vertical pocket. In singleton gestations, the normal range for the deepest vertical pocket is approximately 2 cm to 8 cm. [8–10]

In twin gestations, the normal range for the single deepest pocket appears to be 2.2 cm to 7.5 cm. [11]

Another method for assessing amniotic fluid is the Amniotic Fluid Index (AFI), which sums the deepest vertical pocket in each of four uterine quadrants. The commonly used normal range for the AFI is 5–25 cm. [12,13]

Both the single deepest vertical pocket and AFI are qualitative estimates of amniotic fluid volume and may be a poor reflection of the actual amniotic fluid volume. [14]