Polyhydramnios (also known as hydramnios) refers to an excessive volume of amniotic fluid. It has been associated with an increased risk of various adverse pregnancy outcomes, including preterm birth, placental abruption, and fetal anomalies [1-3]. Polyhydramnios should be suspected clinically when uterine size is large for gestational age. The diagnosis is made prenatally by ultrasound examination using a noninvasive qualitative or quantitative approach. (See "Assessment of amniotic fluid volume".)
The incidence of polyhydramnios in a general obstetric population generally ranges from 1 to 2 percent [4-8]. Reported rates are influenced by variations in diagnostic criteria, the population studied (low or high risk), the subjective volume of fluid where polyhydramnios is diagnosed (eg, mild, moderate, or severe), and the gestational age (preterm, term, or postterm) at examination. In one series of 93,332 singleton pregnancies delivering at a single hospital from 1991 to 1997, polyhydramnios was diagnosed during antepartum sonography in 708 pregnancies (0.7 percent of deliveries); mild, moderate, and severe disease occurred in 66, 22, and 12 percent of cases, respectively .
The volume of amniotic fluid reflects the balance between fluid production and movement of fluid out of the amniotic sac; the regulation of this process is incompletely understood. (See "Physiology of amniotic fluid volume regulation".) In late gestation, the primary sources of amniotic fluid production are fetal urination and secretion of lung fluid; oral and nasal secretions make minimal contributions. The main routes of amniotic fluid removal are fetal swallowing and absorption via the intramembranous pathway. Even a relatively minor increase in daily fetal urine production or decrease in fetal swallowing can result in a marked increase in amniotic fluid volume (AFV) [9-11].
The most common cause of severe polyhydramnios are fetal anomalies (often associated with an underlying genetic diagnosis), while maternal diabetes, multiple gestation, and idiopathic factors are more often associated with milder cases. In one series of 272 singleton pregnancies with polyhydramnios, approximately one-third were associated with a congenital anomaly and one-quarter were associated with maternal diabetes; the remaining 40 percent were considered idiopathic . After birth, an abnormality is diagnosed in up to 25 percent of cases considered idiopathic prenatally [12-15]. Fetal infection, Bartter syndrome, anemia, and neuromuscular disorders account for some of these cases and should be considered in the differential diagnosis if a structural abnormality and maternal diabetes are excluded, although Bartter syndrome and neuromuscular diseases are quite rare.
Polyhydramnios has been associated with fetal anomalies in most organ systems. The most common structural anomalies associated with polyhydramnios are those that interfere with fetal swallowing and/or absorption of fluid [16,17]. Decreased swallowing may be due to a primary gastrointestinal obstruction (eg, duodenal, esophageal, or intestinal atresia), neuromuscular disorders (eg, anencephaly), or to secondary obstruction of the gastrointestinal tract (eg, massive unilateral dysplastic kidneys).