Section O

#### Oligohydramnios

##### KEY POINTS

Key Points

• Oligohydramnios is a decrease in the volume of amniotic fluid, with the diagnosis usually being made using ultrasound.

• Causes of oligohydramnios include ruptured membranes, placental insufficiency, fetal anomalies, maternal injestion of medications, complications of a multiple gestation, chromosomal abnormalities, and idiopathic.

• Significant oligohydramnios occurring prior to 22 weeks of gestation is associated with a poor prognosis because of a high likelihood of pulmonary hypoplasia and associated malformations.

• Once oligohydramnios is diagnosed, a careful maternal history should be obtained and a physical examination should be performed to evaluate for preterm premature rupture of membranes.

• Whenever a diagnosis of oligohydramnios is made, a careful sonographic anatomical survey should be performed to evaluate for fetal anomalies such as features of urinary tract obstruction or renal malformation.

• Amnioinfusion may assist sonographic visualization of the fetus when severe oligohydramnios is diagnosed in the midtrimester.

• Management of oligohydramnios secondary to preterm premature rupture of membranes depends on the gestational age and on the fetal and maternal status.

• Long-term outcome will depend on gestational age at diagnosis, etiology of the problem, and gestational age at delivery.

##### CONDITION

Oligohydramnios is a decrease in the volume of amniotic fluid. The diagnosis of oligohydramnios is most frequently made by ultrasound examination. Oligohydramnios was initially defined as a subjective decrease in amniotic fluid volume resulting in fetal crowding as compared with normal values (Crowley et al., 1984). Objective sonographic estimation of amniotic fluid volume involves measuring different dimensions of amniotic fluid pockets. Various definitions of oligohydramnios exist. Oligohydramnios has been defined as a maximal vertical pocket (MVP) of less than 1 cm, but has also been defined as a MVP of less than 2 cm (Manning et al., 1981; Chamberlain et al., 1984). A semiquantitative four-quadrant technique, known as the amniotic fluid index (AFI), is also widely used. Oligohydramnios can be defined as an AFI of less than 5 cm, but has also been defined as an AFI of less than 8 cm (Phelan et al., 1987; Moore, 1993).

Amniotic fluid volume is the result of a balance between inflow and outflow to and from the amniotic cavity. In the first half of pregnancy, the majority of amniotic fluid is a result of active transport of sodium and chloride across the amniotic membrane and fetal skin, with water moving passively in response (Brace and Resnik, 1999). In the second half of pregnancy, the majority of amniotic fluid is a result of fetal micturition (Underwood et al., 2005). Another major source of amniotic fluid is secretion from the respiratory tract. The average amniotic fluid volume is 30 mL at 10 weeks of gestation, rising to 780 mL at 32 to 35 weeks, after which time a natural decrease in volume occurs (Brace and Resnik, 1999). The amniotic fluid volume is not stagnant, ...