Pitfalls in the diagnosis of nonconjoined monoamniotic twins

The morbidity and mortality of monoamniotic twins are high. Despite recent advances in imaging, the definitive diagnosis of monoamniotic twins remains elusive. When monoamniotic twins are suspected, the optimal antepartum management is uncertain. A false-positive diagnosis of monoamniotic twins by computed tomographic amniography was investigated. By a Medline search the literature on monoamnionicity from 1966 to January 1994 was reviewed. All reports including diagnostic methods and management of monoamniotic twins were reviewed. All five previous case reports of computed tomographic amniography correctly identified amnionicity in cases in which amnionicity was unclear. Four cases confirmed monoamnionicity and one case excluded monoamnionicity. Four recent series that used ultrasonography revealed a low predictive accuracy (9% to 25%) for the establishment of monoamnionicity when intervening membranes were absent. One recent prospective series revealed a positive predictive value of 80% among five monoamniotic twin pairs with the use of first-trimester ultrasonography. The methods available to diagnose monoamniotic twins are improving. The complications that result from a false-positive diagnosis are discussed and the management and diagnosis of monoamniotic twins are reviewed.