Sonographic detection of monoamniotic twins.

The true incidence of monoamniotic twinning has been almost impossible to determine accurately because it requires closer inspection of the membranes and placenta at delivery than is usually performed. Sonography of 3440 patients presenting for genetic amniocentesis identified 39 twin gestations, one of which was monoamniotic (0.026%). Prospectively with ultrasound, four of the twin gestations had been thought to be monoamniotic. A more recent case of suspected monoamniotic twinning revealed two sacs when newer computer-based real-time equipment with variable focusing capability and improved spatial resolution was used. Sonographic diagnosis of monoamniotic twinning must be accurate since it identifies patients at higher risk for cord accidents. These patients need obstetrical care appropriate for a pregnancy at high risk. In addition, failure to identify a second sac that could harbor a chromosomally abnormal fetus has both medical and legal implications. State-of-the-art ultrasound equipment and attention to detail is required.