BACKGROUND: Young adults born preterm at very low birth weight start families later. Whether less severe immaturity affects adult social outcomes is poorly known. METHODS: The study "Preterm birth and early life programming of adult health and disease" (ESTER, 2009-2011) identified adults born early preterm (<34 weeks' gestation, N = 149), late preterm (? 34 to <37 weeks' gestation, N = 248), and at term (? 37 weeks' gestation, N = 356) from the Northern Finland Birth Cohort 1986 and the Finnish Medical Birth Register (1987-1989), with perinatal data, medical and family history, socioeconomic status, and lifestyle from routine visits or questionnaires. Cox, logistic, and ordinal regressions estimated the hazard and odds ratios (HR and OR) with 95% confidence intervals of outcomes related to preterm birth. RESULTS: Compared with term-born subjects, those born early and late preterm were less likely to have cohabited with a romantic partner (HR, 0.79; [0.61-1.03] and HR, 0.80; [0.65-0.99], respectively) or experienced sexual intercourse (HR, 0.83; [0.66-1.05] and HR, 0.76; [0.63-0.92], respectively) by young adulthood. They also had higher odds of obtaining a 1-point lower score in a visual analog scale of self-perceived sexual attractiveness (OR, 1.45; [1.09-1.98] and OR, 1.44; [1.06-1.97] for early and late preterm birth, respectively). No difference was observed in the likelihood of departing from childhood home and number of individuals having their own families. CONCLUSIONS: Young adults born preterm experience more social challenges, which may affect their romantic relationships and future family planning.