approximately two thirds of cases; moderate hydramnios accounts for about 20 percent; and severe hydramnios for approximately 15 percent. Using the single deepest pocket of amnionic fluid, mild hydramnios is defined as 8 to 9.9 cm, moderate as 10 to 11.9 cm, and severe hydramnios as 12 cm or more 11-3). In general, severe hydramnios is far more likely to have an underlying etiology and to have consequences for the pregnancy than mild hydramnios, which is frequently idiopathic and benign.
Underlying causes ofhydramnios include fetal anomalies-either structural abnormalities or genetic syndromes-in approximately 15 percent, and diabetes in 15 to 20 percent (Table 11-2). Congenital infection, red blood cell alloimmunization, and placental chorioangioma are less frequent etiologies. Infections that may present with hydramnios include cytomegalovirus, toxoplasmosis, syphilis, and parvovirus. Hydramnios is oten a component of hydrops etalis, and several of the above causes-selected anomalies, infections, and alloimmunization-may result in a hydropic fetus and placenta. he underlying pathophysiology in such cases is complex but is frequently related to a high cardiacoutput state. Severe fetal anemia is a classic example. Because the etiologies of hydramnios are so varied, hydramnios treatment also difers and is tailored in most cases to the underlying cause.
Selected anomalies and the likely mechanism by which they cause hydramnios are shown in Table 11-3. Many of these abnormalities are depicted and discussed in Chapter 10. Because of this association, targeted sonography is indicated whenever hydramnios is identiied. If a fetal abnormality is encountered concurrent with hydramnios, amniocentesis with chromosomal microarray analysis should be ofered, because the aneuploidy risk is signiicantly elevated (Dashe, 2002; Pri-Paz, 2012).
Importantly, the degree of hydramnios correlates with the likelihood of an anomalous infant (Lazebnik, 1999; Pri-Paz, 2012). At Parkland Hospital, the prevalence of an anomalous neonate was approximately 8 percent with mild hydramnios, 12 percent with moderate hydramnios, and more than 30 percent with severe hydramnios (Dashe, 2002). Even if no abnormality was detected with targeted sonography, the likelihood of a major anomaly identified at birth was 1 to 2 percent if hydramnios was mild or moderate and 10 percent if hydramnios was severe. he overall reported risk that an underlying anomaly will be discovered ater delivery has ranged from 9 percent in the neonatal period to 28 percent among infants followed to 1 year of age (Abele, 2012; Dorleijn, 2009). he anomaly risk is particularly high with hydramnios coexistent with fetal-growth restriction (Lazebnik, 1999).
Although amnionic luid volume abnormalities are associated with fetal malformations, the converse is not usually the case. In the Spanish Collaborative Study of Congenital Malformations that included more than 27,000 anomalous infants, only 4 percent of pregnancies were complicated by hydramnios, and another 3 percent with oligohydramnios (Martinez-Frias, 1999).
he amnionic luid glucose concentration is higher in diabetic women than in those without diabetes, and the AFI may
TABLE "e-2. Hydramnios: Prevalence and Associated Etiologies-Values in Percent aA signiicant correlation was identified between severity of hydramnios and likelihood of an anomalous infant.
TABLE 11-3. Selected Anomalies and Mechanism for Hydramnios
Cystic adenomatoid malformation?