HVI1: Freeze all and transfer later
This study is managed by the fertility clinic at Hvidovre Hospital but is performed in collaboration with the fertility clinics at Rigshospitalet, Holbæk Sygehus, Herlev Hospital and Malmø.
The embryo survival rate following freezing and thawing (FET) is now higher than after fresh embryo transfer. This translates into a higher overall success rate measured by pregnancy rates and number of babies delivered.
The reasons behind this improvement are improved laboratory techniques, but may also be due to the endometrial environment in the FET cycles, which mirrors the natural cycle. In the stimulated cycle supra-physiological levels of estradiol and progesterone are present and may cause impaired endometrial receptivity. Furthermore, obstetric and perinatal outcomes after cryopreservation of embryos have been investigated and follow-up data from children born after FET have shown lower perinatal morbidity compared with fresh embryo transfer, but FET may also give rise to more large-for-gestational age babies. Moreover, FET has the benefits of minimizing the risk of ovarian hyper-stimulation syndrome (OHSS), which is the most severe side effect of assisted reproduction (ART) and potentially life threatening.
Despite the noticeable advantages of embryo cryopreservation, fresh embryo transfer has persistently been the conventional in vitro fertilisation (IVF) procedure as only one in five transfers were made using frozen-thawed embryos in Denmark in 2013. This favour of a fresh embryo transfer strategy is however reflected in other European countries including Finland, Sweden and Iceland where approximately every third ART child is born after FET. Some evidence suggests that IVF outcomes can be further improved with the adaptation of a `freeze-all´ or elective frozen embryo transfer (eFET) strategy with replacement of thawed embryos in natural cycles.