Which statement concerning polyhydramnios is true?

Polyhydramnios is an excessive quantity of amniotic fluid. The frequency of diagnosis varies, but poly hydramnios sufficient to cause clinical symptoms probably occurs in 1 of 1000 pregnancies, exclusive of twins. The incidence of associated malformations is about 20%, with CNS and GI abnormalities being paicularly common. For example, polyhydramnios accompanies about half of cases of anencephaly and nearly all cases of esophageal atresia. Edema of the lower extremities, vulva, and abdominal wall results from compression of major venous systems. Acute hydramnios tends to occur early in pregnancy and, as a rule, leads to labor before the 28th week. The most frequent maternal complications are placental abruption, uterine dysfunction, and postpaum hemorrhage.