OBJECTIVE: To evaluate the impact of number of anomalous fetuses on the prevalence of very preterm births (< 32 weeks), very low birth weight (< 1,500 g), and small for gestational age (SGA) infants among their normal cotriplets. METHODS: Cross-sectional study of United States triplet births from 1995 to 2000 using Matched Multiple Birth Files, comparing outcomes for triplet sets with no anomalous, 1 anomalous, and 2 anomalous fetuses. Chi-square test for trend and multiple logistic regression analyses, applying generalized estimating equations to account for correlation among cotriplets, were implemented. RESULTS: Of 34,881 matched triplets, 34,104 (97.8%) were nonanomalous (2.2% prevalence of fetal anomaly). Prevalence of very preterm birth (P < .001) and very low birth weight (P < .001), but not SGA (P = .52), among nonanomalous births increased significantly with number of anomalous cotriplets. The prevalence of very preterm birth was 44.1%, 54.8%, and 68.8%, respectively, as number of anomalous triplets per set increased from none to two. The prevalence of very low birth weight was 32.3%, 39.6%, and 62.0%, respectively. These trends remained unchanged after adjustment for correlation and potential confounding. CONCLUSION: Prevalence of triplet births with gestation less than 32 weeks and birth weight less than 1,500 g, but not SGA, among nonanomalous triplets increase linearly with number of anomalous members within triplet sets. © 2006 by The American College of Obstetricians and Gynecologists. Published by Lippincott Williams & Wilkins.