ABSTRACT
The development of cryopreservation techniques eventually resulted in the fi rst baby being born in 1983 following freezing-and-thawing of human embryos (1). Today this technique accounts for an increasing proportion of all in vitro fertilization (IVF) treatment cycles. ICMART, the International Committee Monitoring ART, in its latest World Report (2) noticed a worldwide increase of the proportion of frozen-and-thawed embryo replacements (FER) or transfers (FET), from 8% in 1991 to 20% in 2004 ( Fig. 1 ). In Europe in 2004, 25% of all embryo transfers were FER (3). ICMART estimates that, worldwide in 2008, altogether about 3.5 million babies have been born after IVF. Possibly, some 15% to 20% of these come from frozen-and-thawed embryos, corresponding to about 500,000 babies. In recent years, cryopreservation of blastocysts and of oocytes has also been introduced in clinics, although cryopreservation of cleavage stage embryos continues to dominate.
