Oligohydramnios refers to an abnormally low volume of amniotic fluid. Causes of oligohydramnios from the mid second trimester onward include the following:

Oligohydramnios, if severe and prolonged, can lead to a number of fetal deformities from uterine pressure on the developing fetus, including pulmonary hypoplasia, abnormal facies, and clubfeet. These abnormalities are referred to as Potter syndrome if seen in the setting of bilateral renal agenesis, and are sometimes termed “Potter sequence” when due to other causes of severe, prolonged oligohydramnios.

Oligohydramnios can be present at any stage of pregnancy, though it is relatively infrequent in the first trimester. First trimester oligohydramnios can be established by either subjective or quantitative means. The subjective approach involves visual inspection of the fluid on representative images or a sweep through the gestational sac, and then judgment as to whether the fluid volume is lower than that expected for the gestational age (Figure 21.1.1). A proposed quantitative criterion for first trimester oligohydramnios is to take the mean sac diameter minus the crown-rump length, and diagnose oligohydramnios if the difference is less than 5 mm.

In the second and third trimesters, oligohydramnios can be diagnosed sonographically by subjective assessment (Figure 21.1.2) of the amniotic fluid volume or by using semiquantitative criteria. The latter include measurement of the deepest pocket of amniotic fluid (Figure 21.1.3) or the amniotic fluid index (AFI) (Figure 21.1.4), the sum of the deepest pockets in the four quadrants as determined by sagittal and transverse lines through the uterus. A deepest pocket measurement less than 2 cm or an AFI less than 5 cm is generally considered diagnostic of oligohydramnios. When oligohydramnios is diagnosed, an effort should be made to ascertain its cause. The fetal urinary tract should be assessed, and fetal measurements should be obtained to evaluate for intrauterine growth restriction. If the oligohydramnios is severe and prolonged, measurement of the fetal thoracic diameter may help to assess for pulmonary hypoplasia.