The amount of amniotic fluid should be estimated. Subjective estimates of amniotic fluid volumes are best left to experienced sonographers, but extremes of polyhydramnios and oligohydramnios may be obvious to the neophyte. Amniotic fluid volume is large compared with fetal volume in normal early pregnancy; this should not be mistaken for polyhydramnios. A single measurement of the length of the deepest pocket of amniotic fluid gives a gross estimate of the volume and may be the best method of measurement for the inexperienced sonographer. A pocket more than 8 cm deep indicates polyhydramnios and a pocket less than 1 cm deep indicates oligohydramnios. A four-quadrant index can also be used to estimate the volume of amniotic fluid. The uterus is divided into four quadrants and the deepest pocket in each quadrant is measured. If the sum of the four measurements is less than 5 cm, oligohydramnios is present. If the sum is greater than 20 cm, polyhydramnios is present. An obstetrician should be consulted immediately when oligohydramnios is found, since it is associated with fetal renal malformation, severe growth retardation, and fetal death. Polyhydramnios may be associated with fetal anomalies, preterm labor, and premature rupture of membranes, but it is generally considered less serious.
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