OBJECTIVE: This was a population-based medical birth registry investigation designed to study whether a preterm twin delivery portends a shorter gestation in a subsequent pregnancy. STUDY DESIGN: Data on all pregnancies and deliveries from 1973 to 1993 inclusive, stored at the Medical Birth Registry, the National Board of Health and Welfare, Stockholm, were analyzed. Information on gestational duration, estimated during pregnancy with or without ultrasonography in the second trimester, was analyzed for 2979 twin births followed by a singleton delivery. RESULTS: Gestation for women with a singleton pregnancy after a preterm twin birth in the twenty-eighth to thirty-sixth gestational week lasted about 39 ± 0.5 weeks (mean ± SEM). When the twin birth had taken place at term (37 to 42 weeks) the corresponding singleton gestation lasted about a week longer. When the twin birth occurred in gestational weeks 28 to 30, 31 to 33, or 34 to 36, the corresponding proportions of women who gave birth earlier than 37 weeks (20 to 27 weeks, only live births) in the subsequent singleton pregnancy were 9.8%, 7.0%, and 5.8%, respectively (χ² = 3.4, p > 0.05). CONCLUSIONS: It would appear that a preterm twin birth does not to any significant extent affect the gestational duration of a subsequent singleton pregnancy. (Am J Obstet Gynecol 1998;178:136-9.)