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.

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 95th percentile for the corresponding gestational age.

Amniotic fluid volume is the result of a balance between flow into and out of 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 passivelyin response (Brace and Resnik, 1999). In the second half of pregnancy, the majority of amniotic fluid is a result of fetal micturition. Another major source of amniotic fluid is secretion from the fetal respiratory tract. The average amniotic fluid volume is 30 mL at 10 weeks, rising to 780 mL at 32 to 35 weeks, following which time a natural decrease in volume occurs (Brace and Resnik, 1999). The amniotic fluid volume is not stagnant, but is completely turned over at least once daily. Fetal urine first appears at 8 to 10 weeks of gestation, and reaches a production rate of 700 to 900 mL per day near term (Brace and Resnik, 1999).

Polyhydramnios can occur because of increased production of fluid by the fetus, as in the case of hydrops, or it can be due to an obstruction to fetal swallowing, as in the case of congenital gastrointestinal obstruction. Polyhydramnios is idiopathic in greater than 50% of cases (Mann et al., 2006). Maternal diabetes mellitus accounts for 15% of cases, fetal malformations for 13%, multiple gestations for 5%, and other causes for 1% (Hill and Breckle, 1987). However, the proportion of cases of polyhydramnios secondary to fetal malformation increases significantly as the severity of polyhydramnios increases. As the definition of polyhydramnios varies, so too does the reported incidence of associated fetal malformations. Two recent series have reported a 58% to 63% incidence of structural fetal malformations in pregnancies complicated by polyhydramnios (Damato et al., 1993; Many et al., 1996).

Fetal malformations associated with polyhydramnios include neural tube defects (such as anencephaly, Chapter 7), holoprosencephaly (see Chapter 14), cardiac anomalies (such as truncus arteriosus, Chapter 54), gastrointestinal atresias or stenoses (see Chapters 71–73), chest or abdominal masses such as cystic adenomatoid malformation (see Chapter 35) or diaphragmatic hernia (see Chapter 37), skeletal dysplasias, neuromuscular disorders (such as myotonic dystrophy), infections (such as parvovirus), metabolic disorders (such as Gaucher disease), chromosomal abnormalities (such as trisomy 18, Chapter 130), tumors (such as sacrococcygeal teratoma, Chapter 115), and genetic syndromes (such as Beckwith–Wiedemann syndrome).

The reported incidence of polyhydramnios varies, reflecting differences in definition. Polyhydramnios is reported in 0.1% to 3% of pregnancies when clinical methods are used to make the diagnosis (Kramer, 1966). In one series of 9,189 patients who had sonographic assessment of amniotic fluid volume by measuring the MVP, the overall incidence of polyhydramnios was 0.9% (Hill and Breckle, 1987). In this study, mild polyhydramnios was diagnosed when the MVP measured between 8 cm and 11 cm, moderate polyhydramnios was defined as an MVP of 12 cm to 15 cm, and severe polyhydramnios was diagnosed when the MVP exceeded 16 cm. Mild polyhydramnios accounted for 79% of cases, moderate polyhydramnios 16%, and severe polyhydramnios accounted for 5% (Hill and Breckle, 1987). In another series, using an AFI cutoff of greater than 24 cm, mild polyhydramnios was diagnosed in 8% of patients (Smith et al., 1992).

Most diagnoses of polyhydramnios are now based on sonographic findings. The two most common methods of objectively measuring the amniotic fluid volume are the maximum vertical pocket (MVP) and the amniotic fluid index (AFI). The MVP involves surveying the entire uterus and measuring the depth of the deepest pocket of amniotic fluid in centimeters. Only amniotic fluid pockets free of fetal parts and umbilical cord are measured. The most commonly used cutoff for polyhydramnios using the MVP technique is a depth greater than 8 cm. The AFI involves summing the MVPs from each of the four quadrants of the uterus. In a report of sonographic AFI measurements in 197 patients, the mean AFI rose from 7 cm at 12 weeks’ gestation to 20 cm at 26 weeks’ gestation, and then plateaued for the remainder of gestation at approximately 16 cm (Phelan et al., 1987). The 95th percentile value for AFI at 37 weeks is 24 cm, but decreases to 19 cm at 41 weeks, reflecting the normal decrease in amniotic fluid production at term (Moore and Cayle, 1990). It is important to measure the AFI with the patient supine, to orient the transducer in the maternal sagittal plane, to measure the sonographic planes perpendicular to the floor, to measure fluid pockets free from umbilical cord or fetal extremity, and to use the umbilicus and linea nigra as landmarks for dividing the uterus into four quadrants (Phelan et al., 1987).

There is no agreement in the obstetric literature as to which method of sonographic measurement of amniotic fluid volume is best. In one study comparing MVP with AFI, the correlation coefficient was 0.51, and the MVP was associated with a lower sensitivity (Moore, 1990). Others have found good correlation between the MVP and AFI methods, with the MVP being better (Magann et al., 1994). In another study comparing 13 different sonographic methods of amniotic fluid volume assessment with a dye dilution technique, the AFI underestimated the actual amniotic fluid volume by as much as 52% in cases of polyhydramnios (Dildy et al., 1992). When compared with the AFI, several of the other 12 methods of ultrasound prediction of amniotic fluid volume produced lower mean errors as compared with true amniotic fluid volume. However, the investigators concluded that the minimal improvement in accuracy offered by other ultrasound measurements was not sufficient to warrant replacement of the AFI (Dildy et al., 1992).

Sonographic assessment of polyhydramnios should include a careful survey of the fetal anatomy to rule out the presence of structural malformations. Because of the possibility of underlying fetal neuromuscular disorders, fetal swallowing, muscle tone, and fetal movement should be observed. In a study of 41 fetuses with a diagnosis of idiopathic polyhydramnios who were tested for the presence of a myotonic dystrophy mutation, 4 (9.7%) were shown to be affected. Three of the four fetuses had a positive family history of myotonic dystrophy (Esplin et al., 1998).