++
This is an abnormally increased amnionic fluid volume, and it complicates 1 to 2 percent of singleton pregnancies (Dashe, 2002; Khan, 2017; Pri-Paz, 2012). It is more frequently noted in multifetal gestations (Hernandez, 2012). Hydramnios may be suspected if the uterine size exceeds that expected for gestational age. The uterus may feel tense, and palpating fetal small parts or auscultating fetal heart tones may be difficult. An extreme example is shown in Figure 11-2.
++
++
Hydramnios may be further categorized according to degree. Such categorization is primarily used in research studies to stratify risks. Several groups have termed hydramnios as mild if the AFI is 25 to 29.9 cm; moderate, if 30 to 34.9 cm; and severe, if 35 cm or more (Lazebnik, 1999; Luo, 2016; Odibo, 2016; Pri-Paz, 2012). Mild hydramnios is the most common, comprising 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 (Fig. 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 of hydramnios 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 often a component of hydrops fetalis, and several of the above causes—selected anomalies, infections, and alloimmunization—may result in a hydropic fetus and placenta. The underlying pathophysiology in such cases is complex but is frequently related to a high cardiac-output state. Severe fetal anemia is a classic example. Because the etiologies of hydramnios are so varied, hydramnios treatment also differs 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 identified. If a fetal abnormality is encountered concurrent with hydramnios, amniocentesis with chromosomal microarray analysis should be offered, because the aneuploidy risk is significantly 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. The overall reported risk that an underlying anomaly will be discovered after 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). The anomaly risk is particularly high with hydramnios coexistent with fetal-growth restriction (Lazebnik, 1999).
++
Although amnionic fluid 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).
++
The amnionic fluid glucose concentration is higher in diabetic women than in those without diabetes, and the AFI may correlate with the amnionic fluid glucose concentration (Dashe, 2000; Spellacy, 1973; Weiss, 1985). Such findings support the hypothesis that maternal hyperglycemia causes fetal hyperglycemia, with resulting fetal osmotic diuresis into the amnionic fluid compartment. That said, rescreening for gestational diabetes in pregnancies with hydramnios does not appear to be beneficial, provided that the second-trimester glucose tolerance test result was normal (Frank Wolf, 2017).
++
Hydramnios is generally defined in multifetal gestations as a single deepest amnionic fluid pocket measuring 8 cm or more. It may be further characterized as moderate if the single deepest pocket is at least 10 cm and severe if this pocket is at least 12 cm. In a review of nearly 2000 twin gestations, Hernandez and colleagues (2012) identified hydramnios in 18 percent of both monochorionic and dichorionic pregnancies. As in singletons, severe hydramnios was more strongly associated with fetal abnormalities. In monochorionic gestations, hydramnios of one sac and oligohydramnios of the other are diagnostic criteria for twin-twin transfusion syndrome (TTTS), discussed in Chapter 45 (Twin-Twin Transfusion Syndrome). Isolated hydramnios of one sac also may precede the development of this syndrome (Chon, 2014). In the absence of TTTS, hydramnios does not generally raise pregnancy risks in nonanomalous twins (Hernandez, 2012).
+++
Idiopathic Hydramnios
++
This accounts for up to 70 percent of cases of hydramnios and is thus identified in as many as 1 percent of pregnancies (Panting-Kemp, 1999; Pri-Paz, 2012; Wiegand, 2016). Idiopathic hydramnios is rarely identified during midtrimester sonography and is often an incidental finding later in gestation. The gestational age at sonographic detection usually lies between 32 and 35 weeks (Abele, 2012; Odibo, 2016; Wiegand, 2016). Although it is a diagnosis of exclusion, an underlying fetal abnormality may subsequently become apparent with advancing gestation, particularly if the degree of hydramnios becomes severe. In the absence of an etiology, idiopathic hydramnios is mild in approximately 80 percent of cases, and resolution is reported in more than a third of affected pregnancies (Odibo, 2016; Wiegand, 2016). Mild, idiopathic hydramnios is most commonly a benign finding, and associated pregnancy outcomes are usually good.
++
Unless hydramnios is severe or develops rapidly, maternal symptoms are infrequent. With chronic hydramnios, fluid accumulates gradually, and a woman may tolerate excessive abdominal distention with relatively little discomfort. Acute hydramnios, however, tends to develop earlier in pregnancy. It may result in preterm labor before 28 weeks or in symptoms that become so debilitating as to necessitate intervention.
++
Symptoms may arise from pressure exerted within the overdistended uterus and upon adjacent organs. When distention is excessive, such as that shown in Figure 11-2, the mother may suffer dyspnea and orthopnea to such a degree that she may be able to breathe comfortably only when upright. Edema may develop as a consequence of major venous system compression by the enlarged uterus, and it tends to be most pronounced in the lower extremities, vulva, and abdominal wall. Rarely, oliguria may result from ureteral obstruction by the enlarged uterus (Chap. 53, Lower Genital Tract Lesions). Maternal complications such as these are typically associated with severe hydramnios from an underlying etiology.
++
Maternal complications associated with hydramnios include placental abruption, uterine dysfunction during labor, and postpartum hemorrhage. Placental abruption is fortunately infrequent. It may result from the rapid decompression of an overdistended uterus that follows fetal-membrane rupture or therapeutic amnioreduction. With prematurely ruptured membranes, a placental abruption occasionally occurs days or weeks after amniorrhexis. Uterine dysfunction consequent to overdistention may lead to postpartum atony and, in turn, postpartum hemorrhage.
++
Some outcomes more common with hydramnios include birthweight >4000 g, cesarean delivery, and importantly, perinatal mortality. Pregnancies with idiopathic hydramnios are associated with birthweights exceeding 4000 g in nearly 25 percent of cases, and the likelihood appears to be greater if the hydramnios is moderate or severe (Luo, 2016; Odibo, 2016; Wiegand, 2016). A rationale for this association is that larger fetuses have higher urine output, by virtue of their increased volume of distribution, and fetal urine is the largest contributor to amnionic fluid volume. Cesarean delivery rates are also higher in pregnancies with idiopathic hydramnios, with reported rates of 35 to 55 percent (Dorleijn, 2009; Khan, 2017; Odibo, 2016).
++
An unresolved question is whether hydramnios alone raises the risk for perinatal mortality. Some studies have found no increase in stillbirth or neonatal death rates with idiopathic hydramnios, whereas others show a greater risk (Khan, 2017; Pilliod, 2015; Wiegand, 2016). Using birth certificate data from the state of California, Pilliod and coworkers (2015) identified hydramnios in 0.4 percent of singleton, nonanomalous pregnancies, and affected pregnancies had significantly greater stillbirth rates. At 37 weeks, the stillbirth risk was sevenfold higher in pregnancies with hydramnios. By 40 weeks, this risk was more than tenfold higher-66 per 10,000 births compared with 6 per 10,000 without hydramnios.
++
Risks appear to be compounded when a growth-restricted fetus is identified with hydramnios (Erez, 2005). The combination also has a recognized association with trisomy 18. When an underlying cause is identified, degree of hydramnios has been associated with likelihood of preterm delivery, small-for-gestational age newborn, and perinatal mortality (Pri-Paz, 2012). However, idiopathic hydramnios is generally not associated with preterm birth (Magann, 2010; Many, 1995; Panting-Kemp, 1999).
++
As previously noted, treatment is directed to the underlying cause. Occasionally, severe hydramnios may result in early preterm labor or the development of maternal respiratory compromise. In such cases, large-volume amniocentesis—termed amnioreduction—may be needed. The technique is similar to that for genetic amniocentesis, described in Chapter 14 (Technique). One difference is that it is generally done with a larger needle, 18- or 20-gauge, and uses either an evacuated container bottle or a larger syringe. Approximately 1000 to 2000 mL of fluid is slowly withdrawn over 20 to 30 minutes, depending on the severity of hydramnios and gestational age. The goal is to restore amnionic fluid volume to the upper normal range. Hydramnios severe enough to necessitate amnioreduction almost invariably has an underlying cause, and subsequent amnioreduction procedures may be required as often as weekly or even semiweekly.
++
In a review of 138 singleton pregnancies requiring amnioreduction for hydramnios, a fetal GI malformation was identified in 20 percent, a chromosomal abnormality or genetic condition in almost 30 percent, and a neurological abnormality in 8 percent (Dickinson, 2014). In only 20 percent of cases was the hydramnios idiopathic. The initial amnioreduction procedure in this series was performed at 31 weeks’ gestation, and the median gestational age at delivery was 36 weeks. Complications within 48 hours of amnioreduction included delivery in 4 percent and ruptured membranes in 1 percent. There was no instance of chorioamnionitis, placental abruption, or bradycardia requiring delivery (Dickinson, 2014).