An informed decision between cleavage-stage and blastocyst-stage transfer in IVF requires data on the transfers of frozen–thawed embryos

Sir, We read with interest the report by Yang et al. (2018) comparing the use of two IVF strategies in a good prognosis patient group: IVF with time-lapse embryo selection and transfer at the cleavage stage and IVF with conventional morphological embryo selection and transfer at the blastocyst stage (Yang et al., 2018). A study like this is much needed, as there is currently insufficient evidence to decide whether to opt for cleavage-stage or blastocyst-stage embryo transfer in IVF (Glujovsky et al., 2016). Unfortunately, essential data are missing from the report to come to a firm conclusion. In deciding between cleavage-stage and blastocyst-stage embryo transfer in IVF, couples that opt for IVF weigh multiple variables, such as the chance of pregnancy, the time to pregnancy, the safety of the treatment, its burden and the cost involved. Prior to this study, it was already established that blastocyst-stage transfer is associated with higher implantation rates per transferred embryo, and thus with higher pregnancy rates per first transfer than cleavage-stage transfer (Glujovsky et al., 2016). Conversely, cleavage-stage transfer is associated with a higher number of embryos available for fresh or frozen–thawed embryo transfer than blastocyst-stage transfer (Glujovsky et al., 2016). This higher number of embryos available in cleavage-stage transfer leads to more embryo transfers per started cycle and thus, potentially, to a higher cumulative live birth rate, i.e. the chance of a live birth per started cycle. There is less burden, and in many IVF programs less cost, for a woman undergoing a frozen–thawed embryo transfer than undergoing an additional new IVF cycle. The result and contribution of frozen–thawed embryo transfers to IVF programs clearly increased in recent years (Wong et al., 2014). For all these reasons, an informed decision between cleavage-stage or blastocyst-stage embryo transfer needs information on the cumulative live birth rate per started cycle, and in addition information on the embryo utilization rate, the number and result of frozen–thawed embryo transfers within a certain time span, and the number of embryos still in the freezer for each arm of the presented study (Maheshwari et al., 2015; Glujovsky et al., 2016). We kindly ask the authors to provide this information. On a separate note, it is unclear from the report whether time-lapse observation of embryos in the cleavage-stage arm of the study was used for ranking of all embryos for transfer or for deselection, i.e. the discarding of embryos, and subsequent ranking of embryos for transfer. This is important as deselection of embryos lowers the number of embryos available for transfer and cryopreservation, which makes this information important for correct interpretation and comparison of data between studies. Deselection could also lower the pregnancy rate in case the deselection method does not have a 100% specificity when identifying non-viable embryos (Mastenbroek et al., 2011; Armstrong et al., 2015a,b; Kaser et al., 2017). None declared.