OBJECTIVE: To determine if pregnancy losses after midtrimester amniocentesis are increased in twin pregnancies. STUDY DESIGN: We utilized oui prenatal diagnosis database from 1988-1995 to compare the pregnancy loss rates in three groups of women: 1) Twin pregnancies who had midtrimester (14 to 22 weeks') amniocentesis 2) Twir pregnancies who underwent counseling and ultrasound examination, but did not have amniocentesis 3) Singleton pregnancies who had midtrimester amniocentesis. A fetal loss was defined as spontaneous abortion, fetal death, stillbirth or neonatal death and was characterized as early, ≤ 30 days, and late, > 30 days after the date of the amniocentesis or initial counseling and ultrasound examination. Exclusion criteria were fetal structural abnormalities, aneuploidy, and monamnionic twins. RESULTS: Follow-up was available in 200/247 (81%) of the twin pregnancies seen for counseling and consideration of midtrimester amniocentesis. There were 184 that met the inclusion criteria, 97 with amniocentesis, and 87 without. These were compared to 8091 .singleton pregnancies undergoing midtrimester amniocentesis. Women with twin pregnancies with amniocentesis compared to those without amniocentesis were older (36.0 ± 3.1 vs 28.8 ± 6.2 years), p < .001, had lower serum AFP levels (2.5 ± .9 vs 3.4 ± 1.8). p = .002, and were seen earlier in pregnancy (16.2 ± 1.5 vs 17.4 ± 2.0 weeks'), p < .001. Early losses were similar in the twin pregnancies with or without amniocentesis. 2/97 (2%) vs 1/87 (1.1%), and not significantly different from singletons. 2/97 (2.1%) vs 66/8091 (0.8%), p = .19. Late losses were similar in twins with or without amniocentesis, 5/97 (5.2%) vs 5/87 (5.8%) and were significantly greater than the late losses in singleton pregnancies (5/97 (5.2%) vs 106/8091 (1.3%,), p = .01. Total losses were higher in twin pregnancies with amniocentesis than in singletons, 7/97 (7.2%) vs 172/8091 (2.1%,), RR 3.1 (1.6, 7.0). Total losses were no different in twin pregnancies with or without amniocentesis. 7/97 (7.2%) vs 6/87 (6.9%). CONCLUSION: Twin pregnancies have higher fetal losses than singleton pregnancies whether or not they undergo midtrimester amniocentesis. These losses are most likely attributable to complications of twin pregnancies not to midtrimester amniocentesis.