C.Hydén-Granskog1,2, L.Unkila-Kallio1, M.Halttunen1 and A.Tiitinen1

BACKGROUND: A good strategy to decrease multiple pregnancy rate in assisted reproduction technology (ART) is the use of single embryo transfer (SET). METHODS: This retrospective study analysed 1647 frozen embryo transfers carried out during 1998–2003 in Helsinki University Central Hospital; of these, 872 were double embryo transfers (DETs) and 775 SETs. The SET group included 140 (18.1%) elective SETs (eSETs). RESULTS: The yearly rate of SETs in frozen cycles increased from 28 to 66%. Overall, the clinical pregnancy rate per frozen embryo transfer was 30.7 % and the delivery rate 22.6%. The delivery rate was significantly higher in DET cryocycles than in SET cryocy-cles (25.7 versus 19.2%, respectively; P < 0.01). In DET cryocycles, the multiple delivery rate was 21.9%, 10 times higher than that observed in cryocycles with SET (2.0%) (P < 0.0001). When eSET was applied, no difference in delivery rate was observed when compared with cryocycles with DET (28.6 and 25.7%, respectively). CONCLUSIONS: SET can be used in frozen cycles to reduce multiple delivery rates. Key words: cryopreservation of embryos (human)/frozen embryo transfer/multiple pregnancy rate/re-cryopreservation/single embryo transfe