Pregnancies with nonanomalous stillbirths were analyzed separately and also are presented in Table 2. For dichorionic pregnancies, the prevalence of stillbirth was 0.9% (95% confidence interval 0.1–3.1%), with no stillborn fetuses among pregnancies with either moderate or severe hydramnios. For monochorionic pregnancies, the prevalence of stillbirth was 3%, and although relatively few pregnancies were complicated by severe hydramnios, stillbirth occurred in 3 of 11 pregnancies (27%, P<.001). On review of these three cases, none met criteria for twin–twin transfusion syndrome at the last ultrasound examination performed before stillbirth occurred; therefore, they were not excluded. However, in retrospect, it is not possible to definitively exclude twin–twin transfusion syndrome or twin-anemia-polycythemia sequence. In the first case, mild hydramnios was first identified at 27 weeks, with normal fluid surrounding the co-twin, and when stillbirth was noted at 31 weeks, the surviving twin had severe hydramnios and the stillborn fetus had anhydramnios. In the second case, severe hydramnios was identified at 25 weeks, which had resolved at follow-up imaging, and when stillbirth was noted at 31 weeks, the surviving twin had normal amniotic fluid and the stillborn fetus had anhydramnios. After delivery 1 month later, autopsy was performed but was noncontributory. In the third case, both twins were found to have severe hydramnios at 35 weeks, with 8% weight discordance, and 3 days later both were found to be stillborn. Autopsy described organ congestion, cardiomegaly, and ascites in the suspected recipient twin, and pale organs concerning for anemia were found in the suspected donor twin, suggesting twin-anemia-polycythemia sequence. However, both fetuses also had evidence of Klebsiella bacteremia.
Pregnancy and neonatal outcomes of nonanomaous liveborn neonates are shown in Table 4. For dichorionic twins, the mean birth weight increased significantly with degree of hydramnios (P<.001), such that dichorionic neonates from pregnancies with moderate or severe hydramnios were more than 200 g heavier than neonates from pregnancies without hydramnios. In addition, dichorionic twins with hydramnios were more likely to be large for gestational age (higher than the 90th percentile; P<.001). There was no association between birth weight and hydramnios in monochorionic twins. For both dichorionic and monochorionic twins, the presence of hydramnios did not affect the proportion of pregnancies, resulting in a preterm birth at or before 36 weeks, 34 weeks, or 32 weeks (Table 4). Similarly, hydramnios was not associated with birth weight discordance of 25% or more, small-for-gestational-age neonates (less than the 10th percentile), need for admission to the neonatal intensive care unit, or neonatal death in either dichorionic or monochorionic pregnancies (all P>.05).
There are four main findings from our series of nearly 2,000 twin pregnancies. First, hydramnios is common, identified in one in six twin gestations, independent of chorionicity, and most often diagnosed at its most severe in the late second or early third trimester. Second, the prevalence of anomalies increased significantly with greater degrees of hydramnios. Third, although stillbirths were fortunately infrequent, they were significantly more common in monochorionic pregnancies complicated by severe hydramnios. And, finally, pregnancies with hydramnios–in the absence of a fetal anomaly–were not more likely to be delivered preterm, were not complicated by small-for-gestational-age status or birth weight discordance, and did not include neonatal morbidity or mortality.
It is difficult to compare our prevalence of hydramnios with that reported in other series. As in singletons, a largest vertical pocket of amniotic fluid of 8 cm has been used to define hydramnios in monochorionic twins for the purpose of diagnosing twin–twin transfusion syndrome.14 However, there are no established criteria for degrees of hydramnios in twins. Our definitions are based on amniotic fluid index criteria that we and others have used to categorize hydramnios in singletons,8,16 along with the general principle that the single deepest pocket of amniotic fluid is approximately one-third the total amniotic fluid index.17 We did not adjust for gestational age. Magann et al9 performed serial sonography in nearly 300 diamniotic twin pairs between 17 and 37 weeks and found that the single deepest pocket of amniotic fluid did not vary with gestational age. Chau et al18 also reported that the single deepest pocket of amniotic fluid was stable across gestation in twin pregnancies. A potential limitation of our series is that pregnancies were considered to have hydramnios based on the largest measurement of amniotic fluid at any point in gestation. We reasoned that this type of analysis might be more useful for counseling. Not knowing if the hydramnios resolved later in the pregnancy, however, could certainly affect our overall prevalence rate.
The finding that selected pregnancy outcomes were more common in the setting of severe hydramnios would support use of our categorization. Specifically, severe hydramnios, defined as a single deepest pocket of fluid of at least 12 cm, was associated with a nearly 20% risk of major anomalies or aneuploidy, regardless of chorionicity. Our group and others have previously reported that the severity of hydramnios correlates with anomaly prevalence of singletons.8,16 We also found that monochorionic gestations with severe hydramnios were at increased risk for stillbirth. Although none of these cases met criteria for twin–twin transfusion syndrome before the occurrence of stillbirth, we realized on investigating them that it would be impossible to definitively exclude acute twin–twin transfusion syndrome or twin-anemia-polycythemia sequence as an etiology, even after autopsy was performed. This illustrates that monochorionic twins have a unique risk compared with dichorionic twins—a risk that should be acknowledged in the setting of severe hydramnios. We suggest that frequent antepartum surveillance of monochorionic gestations may be warranted in the setting of severe hydramnios.
An unexpected finding was that hydramnios was not associated with preterm birth, growth impairment, or adverse neonatal outcome. This was particularly true for dichorionic pregnancies, in which the rates of these outcomes were virtually identical in pregnancies with and without hydramnios, regardless of degree. This is in contrast to a study by Orhan et al,3 in which twins with hydramnios were at increased risk for preterm birth, small for gestational age status, and neonatal death. In that series, hydramnios was defined as an amniotic fluid index of more than 25 cm and occurred in only 1.5% of twins, and pregnancies with complications such as twin–twin transfusion syndrome were not excluded, such that their cohort may have been enriched with at-risk pregnancies.3 A limitation of our series is that even though we included nearly 2,000 sets of twins, monochorionic pregnancies with severe hydramnios were infrequent, such that with greater numbers we may have identified an increase in preterm birth with severe hydramnios.
There are several strengths of this study. First, at our institution, prenatal care is not modified by the diagnosis of hydramnios. Therefore, our finding that adverse pregnancy outcomes are not more common in the setting of hydramnios represents the true natural history of this finding, Second, because we estimated the effect of hydramnios separately in dichorionic and monochorionic pregnancies, providers may modify their counseling based on the specific type of twin gestation when faced with this common finding in practice. Further, we stratified the findings by severity of hydramnios. This is important in practice as well, because pregnancies complicated by mild hydramnios do not appear to be at the same risk as those complicated by severe hydramnios. Additional limitations of our study also should be mentioned. Relatively few pregnancies were complicated by severe hydramnios, and this is a population in which risks may be increased, particularly in monochorionic twins. In addition, because we evaluated outcomes according to the greatest degree of hydramnios present during gestation, thereby including each patient only once, we were not able to evaluate the course of hydramnios across gestation. It is conceivable that risks may be related to whether hydramnios progresses or resolves, and prospective study of the course of hydramnios in twins may clarify this issue.
In conclusion, this study helps us better-evaluate the antenatal and neonatal risks associated with the common finding of hydramnios in twin gestations. Our results suggest that when severe hydramnios is identified in either dichorionic or monochorionic twin gestations, specialized sonography is warranted because of the association between this finding and fetal abnormalities. Furthermore, fetal surveillance should be considered for monochorionic gestations with severe hydramnios after viability because of the increased risk for stillbirth in this cohort. In contrast, common adverse outcomes such as preterm birth and growth restriction did not appear to be not more frequent in the setting of hydramnios in either dichorionic or monochorionic twins.
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