ABSTRACT
The conventional strategy in in vitro fertilization (IVF) has been to perform embryo transfer at the conclusion of a fresh treatment cycle involving controlled ovarian stimulation of the ovaries. In the absence of effective methods of freezing and thawing embryos, this was initially a strategy born of necessity, as well as an intuitive belief that freezing would compromise embryo quality. Since 1983, when surplus embryos were first frozen with the intention of replacing them within the uterus in a nonstimulated cycle at a future date (1), technological advances have led to increasing numbers of embryos being successfully cryopreserved. Yet, the default position has always been to replace the best embryos in the first fresh cycle in the hope of maximizing the chance of pregnancy. In the last couple of decades, greater understanding of cryobiology has led to growing confidence in freezing, which is reflected in the increasing numbers of embryos frozen each year, as well as success rates that appear to be almost comparable to those associated with fresh embryo transfer (2). Embryo cryopreservation has now become an integral part of IVF treatment across the world and underpins fertility preservation and embryo donation programs, as well as strategies to combat the risks of ovarian hyperstimulation syndrome (OHSS) and multiple birth (3).
