Somewhat arbitrarily, more than 2000 mL of amnionic fluid is considered excessive and is termed hydramnios and sometimes called polyhydramnios. In rare instances, the uterus may contain an enormous quantity of fluid (15 L). As shown in Figure 11-1, minor-to-moderate degrees of hydramnios—2 to 3 L—are rather common and are identified in about 1 percent of all pregnancies. Sonographically, hydramnios is most commonly defined as an amnionic fluid index (AFI) of greater than 24 or 25 cm—corresponding to greater than either the 95th or 97.5th percentiles (see Appendix B, “Ultrasound Reference Tables”). Hydramnios has also been defined by ultrasound measurement of the deepest vertical pocket of fluid. In this system, severe hydramnios is defined by a free-floating fetus found in pockets of fluid of 16 cm or greater.
FIGURE 11-1 Amnionic fluid indexes in 36,796 pregnancies studied sonographically at 20 weeks or greater. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010. Data from Biggio JR Jr, Wenstrom KD, Dubard MB, Cliver SP: Hydramnios prediction of adverse perinatal outcome. Obstet Gynecol 94:773, 1999.)
CAUSES OF HYDRAMNIOS
The degree of hydramnios, as well as its prognosis, is related to the cause. Hydramnios is frequently associated with fetal malformations, especially of the central nervous system or gastrointestinal tract. For example, hydramnios accompanies about half of cases of anencephaly and esophageal atresia. A fetal anomaly is identified in almost half of cases with moderate or severe hydramnios.
Early in pregnancy, the amnionic cavity is filled with fluid very similar in composition to the maternal extracellular fluid. During the first half of pregnancy, transfer of water and other small molecules takes place not only across the amnion but also through the fetal skin. During the second trimester, the fetus begins to urinate, swallow, and inspire amnionic fluid. These processes have a modulating role in the control of amnionic fluid volume. Hydramnios that commonly develops with maternal diabetes during the third trimester remains unexplained. One potential explanation is that maternal hyperglycemia causes fetal hyperglycemia and results in osmotic diuresis leading to excess amnionic fluid.
Major maternal symptoms accompanying hydramnios arise from purely mechanical causes and result principally from pressure exerted within and around the overdistended uterus upon adjacent organs. When uterine distention is excessive, the mother may suffer from dyspnea and, in extreme cases, she may be able to breathe only when upright. Edema, the consequence of compression of major venous systems by the enlarged uterus, is common, especially in the lower extremities, the vulva, and the abdominal wall. Rarely, severe maternal oliguria may result from ureteral obstruction by the enlarged uterus.
The primary clinical finding with hydramnios is uterine enlargement in association with difficulty in palpating fetal small parts. In severe cases, the uterine wall may be so tense that it is impossible to palpate any fetal parts. The differentiation between hydramnios, ascites, or a large ovarian cyst can usually be made by sonographic evaluation.
In general, the more severe the degree of hydramnios, the higher is the perinatal mortality rate (Table 11-1). Even when sonography shows an apparently normal fetus, the prognosis is still guarded, because fetal malformations and chromosomal abnormalities are common. Perinatal mortality is increased by preterm delivery and fetal growth restriction. Other conditions adding to bad outcomes are erythroblastosis, maternal diabetes, umbilical cord prolapse, and placental abruption.
TABLE 11-1. Pregnancy Outcomes with Hydramnios Compared to Women with Normal Amnionic Fluid Volume
The most frequent maternal complications associated with hydramnios are placental abruption, uterine dysfunction, and postpartum hemorrhage. Premature separation of the placenta sometimes follows escape of massive quantities of amnionic fluid because of the decrease in the area of the emptying uterus beneath the placenta (see Chapter 25). Uterine dysfunction and postpartum hemorrhage may also result from uterine atony consequent to overdistention. Abnormal fetal presentations and operative intervention are also more common.
Minor degrees of hydramnios rarely require treatment. Even moderate degrees with some discomfort can usually be managed without intervention until labor ensues or until the membranes rupture spontaneously. If dyspnea or abdominal pain is present, or if ambulation is difficult, hospitalization becomes necessary. Bed rest, diuretics, and water and salt restriction are ineffective.
The principal purpose of amniocentesis is to relieve maternal distress by decompressing the uterus. Unfortunately, the relief is only transient. To remove amnionic fluid, a commercially available plastic catheter that tightly covers an 18-gauge needle is inserted through the locally anesthetized abdominal wall into the amnionic sac, the needle is withdrawn, and an intravenous infusion set is connected to the catheter hub. The opposite end of the tubing is dropped into a graduated cylinder placed at floor level, and the rate of flow of amnionic fluid is controlled with the screw clamp so that about 500 mL/h is withdrawn. After about 1500 to 2000 mL has been collected, the uterus has usually decreased in size sufficiently so that the catheter may be withdrawn from the amnionic sac. At the same time, maternal relief is dramatic and the risk of placental separation from decompression is very low. Using strict aseptic technique, this procedure can be repeated as necessary to make the woman comfortable.
The disadvantages inherent in rupture of the membranes through the cervix are the possibility of cord prolapse and especially of placental abruption. Slow removal of the fluid by abdominal amniocentesis helps obviate these dangers.
Indomethacin impairs lung liquid production or enhances absorption, decreases fetal urine production, and increases fluid movement across fetal membranes. Doses employed by most investigators range from 1.5 to 3 mg/kg per day (based on maternal weight). A major concern with the use of indomethacin is the potential for closure of the fetal ductus arteriosus (see Chapter 8).
For further reading in Williams Obstetrics, 23rd ed.,
see Chapter 21, “Abnormalities of the Fetal Membranes and Amnionic Fluid.”