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Key Points
Polyhydramnios is an increase in the volume of amniotic fluid, the diagnosis of which is usually made using ultrasound.
Causes of polyhydramnios include fetal congenital malformations, fetal neurological anomalies, fetal genetic abnormalities, and maternal issues, although the majority of cases are idiopathic.
Sonographic assessment of polyhydramnios should include a careful survey of the fetal anatomy to rule out the presence of structural abnormalities or fetal growth restriction, and karyotyping should be considered if these are present.
Polyhydramnios has been associated with an increased risk for preterm contractions and preterm delivery, in which case-reduction amniocentesis may be considered.
Delivery at 39 weeks may be reasonable to relieve maternal symptoms as well as to reduce the risk of cord prolapse, should spontaneous rupture of membranes occur.
Pregnancies complicated by polyhydramnios have a higher incidence of inefficient uterine activity leading to prolonged labor, postpartum uterine atony, and postpartum hemorrhage; cesarean delivery should be reserved for standard obstetric indications.
Neonates should be evaluated carefully to assess for anatomical abnormalities that may have caused polyhydramnios.
The long-term outcome for infants following a prenatal diagnosis of polyhydramnios depends on the gestational age at delivery and the presence of associated structural malformations.
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Polyhydramnios, also known simply as hydramnios, is an increase in the volume of amniotic fluid. The diagnosis of polyhydramnios is most frequently made by ultrasound examination, but is often suspected by clinical examination revealing a fundal height greater than that expected for gestational age. Before the advent of prenatal sonography, polyhydramnios was defined as an amniotic fluid volume of more than 2 L. Sonographic diagnosis of polyhydramnios relies on the finding of a maximum vertical pocket of more than 8 cm (Figure 126-1) (Chamberlain et al., 1984). However, due to the asymmetric location of the fetus within the uterus, the use of this maximum vertical pocket (MVP) technique may lead to an overestimation of the amniotic fluid volume.
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The amniotic fluid index (AFI) has been described as a more reliable means of quantifying amniotic fluid volume (Phelan et al., 1987). The AFI involves the summing of the largest vertical pockets from each of the four quadrants of the uterus. A normal amniotic fluid volume is defined as an AFI of between 8 and 18 cm, while polyhydramnios is defined as an AFI of greater than 24 cm. Normal values for the AFI throughout gestation have been described based on sonographic measurements of amniotic fluid volume in 791 uncomplicated pregnancies between 16 and 42 weeks of gestation (Moore and Cayle, 1990). These values are listed in Table 126-1. A reasonable working definition of polyhydramnios using current sonographic criteria is an AFI greater than the ...