['Myotonic dystrophy, the most common form of muscular dystrophy seen in pregnant women, may be a significant cause of middle trimester polyhydramnios. Our purpose was to determine the prevalence of myotonic dystrophy in women with idiopathic polyhydramnios and to characterize the ultrasonographic findings associated with cases.', 'We examined the cases of 67 patients who were delivered of infants at the University of Utah between 1992 and 1996 with a diagnosis of idiopathic polyhydramnios (amniotic fluid index >25). Women with diabetes mellitus, hydrops, or fetal anomalies known to cause polyhydramnios were excluded from the study. Amniotic fluid samples or cord blood samples were obtained from 41 patients, and polymerase chain reaction amplification and Southern blot analysis were performed to detect the presence of the myotonic dystrophy mutation. Ultrasonographic findings, prenatal course, and neonatal outcomes were reviewed in all cases.', 'Four of the 41 patients tested had the myotonic dystrophy mutation, yielding a prevalence in our population of 9.7%. Three of the 4 patients reported a family history of myotonic dystrophy. Ultrasonographic findings associated with a positive result included abnormal posturing of extremities (3/4) and unilateral clubbed foot (3/4). No other structural or growth abnormalities were seen. Two of the patients were delivered before term, 1 at 26 weeks and 1 at 32 weeks. Three of the 4 infants were severely affected, necessitating admission to the intensive care unit, and 1 died on day 11 after birth. One infant, whose myotonic dystrophy mutation consisted of between 800 and 900 triplet repeats, did not require admission to the intensive care unit.'] Is myotonic dystrophy a significant cause of idiopathic polyhydramnios?