What defines oligohydramnios based on ultrasound?

Oligohydramnios is characterized by a decreased volume of amniotic fluid surrounding the fetus. The most widely accepted method for diagnosing oligohydramnios via ultrasound is the measurement of the single deepest pocket (SDP) of amniotic fluid. When the SDP is less than 2 cm, it is indicative of oligohydramnios, suggesting that the amniotic fluid volume is insufficient to support normal fetal movement and development and can lead to complications during pregnancy.

In contrast, the amniotic fluid index (AFI) provides a broader assessment, but specific values can fall within ranges that do not necessarily confirm oligohydramnios. For instance, an AFI greater than 5 cm may be considered normal or mild, while AFI values between 5 cm and 10 cm indicate a borderline situation rather than definitive oligohydramnios. Similarly, an SDP measurement between 2 cm and 4 cm does not meet the criteria for oligohydramnios, as it remains above the threshold for normal fluid volumes. Thus, the definition primarily hinges on identifying an SDP of less than 2 cm to affirm the diagnosis of oligohydramnios.