OBJECTIVE To study the frequency and obstetric outcome of monochorionic multiple pregnancies in a population referred for fetal reduction. METHODS Data charts of all patients with multifetal (> or =3) pregnancies referred for fetal reduction over the last 10 years were reviewed for the presence of monochorionic twin pairs or triplets. RESULTS Twenty-nine of 239 high-order multiple pregnancies contained a monochorionic component (12.1%), eight of which were monochorionic triplets. Half of all naturally conceived pregnancies contained a monochorionic component. High-order multiple pregnancies with a monochorionic component resulted significantly more frequently from natural conceptions (7 of 29) than multichorionic pregnancies (7 of 210) (P =.001). Fetal reduction of the monochorionic twin pair in 21 pregnancies resulted in eight twin and 13 singleton pregnancies; mean gestational age at delivery was, respectively, 34.3 +/- 2.9 and 39.2 +/- 1.4 weeks. Pregnancy loss rate was one of 21 (4.8%). In the remaining eight multiple pregnancies with a monochorionic triplet present, three were complicated by a twin reversed arterial perfusion sequence, and two couples requested a first trimester termination of pregnancy. Fetal reduction of the monochorionic triplet in a dichorionic quadruplet pregnancy resulted in a normal pregnancy outcome. In two monochorionic triplet pregnancies, fetal reduction to monochorionic twin pregnancies with bipolar coagulation of the umbilical cord resulted in a favorable pregnancy outcome. CONCLUSIONS Monochorionic twins or triplets are frequently part of naturally conceived high-order multiple pregnancies. Reduction of the monochorionic twin pairs improves pregnancy outcome. Monochorionic triplet pregnancies show a high complication rate, but may benefit from fetal reduction by cord coagulation.