We report two cases of conjoined twins complicating a triplet pregnancy diagnosed by two-dimensional ultrasound in the first trimester and evaluated further by three-dimensional ultrasound. A review of the literature over the last 30 years revealed 11 other cases diagnosed prenatally by ultrasound. Overall, three (23%) of these 13 pregnancies were achieved by assisted reproductive techniques and 10 (77%) were diagnosed before 18 weeks. Four women opted for termination of the whole pregnancy and three were managed expectantly, with two delivering before 32 weeks. Two monochorionic pregnancies underwent selective feticide with intrauterine demise of the non-conjoined fetus in both cases. All four dichorionic pregnancies undergoing selective termination or spontaneous embryo reduction to singleton in the first trimester resulted in term delivery of the non-conjoined fetus. The rare condition of conjoined twins in a triplet pregnancy poses a significant obstetric challenge from both diagnostic and management points of view. Accurate determination of chorionicity in these cases plays a critical role in determining management and outcome. Copyright 2003 ISUOG. Published by John Wiley & Sons, Ltd.
