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Please cite this paper as: Tandberg A, Bjørge T, Nygård O, Børdahl P, Skjaerven R. Trends in incidence and mortality for triplets in Norway 1967–2006: the influence of assisted reproductive technologies. BJOG 2010; DOI: 10.1111/j.1471-0528.2010.02530.x. Objective To assess changes in incidence rates and outcomes of triplets over 40 years with a particular focus on the influence of assisted reproductive technology (ART). Design Population-based cohort study. Setting The Medical Birth Registry of Norway. Population 2.18 million pregnancies, including 448 sets of triplets and 27 575 twin pairs, covering the years 1967–2006. Since 1988, pregnancies from ART have been available through a separate registry and linked with the birth record. Methods Incidence rates and outcomes for triplets were analysed and compared with those for singletons and twins. Relative risks were estimated between time periods and between ART and non-ART pregnancies. Main outcome measures Incidence rates, birthweight, gestational age and perinatal mortality. Results The total triplet rate per 10 000 pregnancies increased from 1.0 during 1967–71 to 3.5 during 1987–92, followed by a decline to 2.7 during 2002–06. After excluding ART pregnancies, the incidence was more than doubled at the end of the study period. The mean gestational age and birthweight of triplets were significantly lower during 1988–2006 than 1967–87, but similar for ART and non-ART triplets in the last period. The caesarean rate in triplets increased from 47 to 92%. The relative risk of perinatal death in triplets relative to singletons did not change after the introduction of ART [before: relative risk, 8.9 (95% confidence interval, 6.8–11.7); after: relative risk, 10.4 (95% confidence interval, 8.3–13.0)]. Conclusions The triplet incidence rate in Norway has more than doubled during the last 40 years, even after excluding ART pregnancies. The risk of perinatal death in triplets is ten times higher relative to singletons and has not changed during this 40-year period, independent of the introduction of ART. [ABSTRACT FROM AUTHOR]