Embryo freezing does not adversely affect perinatal outcome in terms of prematurity, low birthweight and being small for gestational age versus the fresh embryo transfer and the outcome is similar or even better, particularly regarding fetal growth. Our study, which is one of the largest on FET pregnancies, provides further evidence on the safety of FET.

This study was supported by a research grant from the Academy of Finland. The authors declare no conflicts of interest.

University Hospital of Oulu and Helsinki, Finland. THL covered the data linkages and the work of Annukka Ritvanen and Mika Gissler. There are no competing interests to be reported.

This study was funded by the University Hospital of Oulu and Helsinki, Finland. The National Institute for Health and Welfare (THL) covered the data linkages and the work of Mika Gissler. There are no competing interests to be reported.