The amniotic fluid surrounding the fetus has major functions. Among others, it protects the fetus from trauma and major changes in temperature, avoids pressure on the umbilical cord and is bacteriostatic. It also allows for movements and facilitates lung maturation. Its production is strictly regulated and its volume increases during the pregnancy from 30cc around 10 weeks to about 1000cc at approximately 35-36 weeks. It then somewhat decreases until 40 weeks. Normal volume is from about 500cc to 2000cc. From about 20 weeks on, it is mostly composed of fetal urine. Several methods have been described to evaluate this volume, the most commonly utilized in the third trimester being the four-quadrant index (AFI). Anomalies of the volume reflect various complications. When the volume is low (oligohydramnios, under 500cc, AFI<5), rupture of the membranes should be investigated first. Other etiologies include renal anomalies (including agenesis) or decreased production (in intra-uterine growth restriction, for instance). An abnormally high volume (polyhydramnios, above 1500-2000cc, AFI>20) can be seen in cases of macrosomia or can point to the presence of fetal structural anomalies (of the CNS, GI tract, lungs, cleft lip/palate and more). It can also be a manifestation of maternal diabetes but in up to 60% cases remains unexplained. When persistent into the second and third trimester, it has been associated with an increased incidence of chromosomal abnormalities. Investigation for both extremes of volume should be extensive.