s 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. he technique is similar to that for genetic amniocentesis, described in Chapter 14 (p. 292). One diference 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 Approximately 1000 to 2000 mL of luid is slowly withdrawn over 20 to 30 minutes, depending on the severity of hydramnios and gestational age. he 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 oten 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. he 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 deliveY in 4 percent and ruptured membranes in 1 percent. here was no instance of chorioamnionitis, placental abruption, or bradycardia requiring delivery (Dickinson, 2014).
his is an abnormally decreased amount of amnionic luid. Oligohydramnios complicates approximately 1 to 2 percent of pregnancies (Casey, 2000; Petrozella, 2011). When no measurable pocket of amnionic fluid is identiied, the term anhydramnios may be used. Unlike hydramnios, which is often mild and often confers a benign prognosis in the absence of an underlying etiolo)i, oligohydramnios is always a cause for concern, as discussed on page 231.
The sonographic diagnosis of oligohydramnios is usualy based on an API less than 5 cm or a single deepest pocket of amnionic luid below 2 cm (American College of Obstetricians and Gynecologists, 2016). Using the Moore nomogram, an API threshold of 5 em is below the 2.5th percentile throughout the second and third trimesters (see Fig. 11-1). Either criterion is considered acceptable. However, use of API rather than single deepest pocket will identiy more pregnancies as having oligohydramnios, without evidence of improvement in pregnancy outcomes (Kehl, 2016; Nabhan, 2010). When evaluating multifetal pregnancies for TTTS, a single deepest pocket below 2 em is used to define oligohydramnios (Society for Maternal-Fetal Medicine, 2013).
Pregnancies complicated by oligohydramnios include those in which the amnionic luid volume has been severely diminished since the early second trimester and those in which the fluid volume was normal until near-term or even full-term. The prognosis depends heavily on the underlying cause and thus varies. Whenever oligohydramnios is diagnosed, it becomes an important consideration in clinical management.
When amnionic fluid volume is abnormally decreased from the early second trimester, it may reflect a fetal abnormality that precludes normal urination, or it may represent a placental abnormality suiciently severe to impair perfusion. In either circumstance, the prognosis is poor. Ruptured membranes should be excluded, and targeted sonography is performed to assess for fetal and placental abnormalities.