ABSTRACT
Extended embryo culture to blastocyst stage is being used frequently in many in vitro fertilization (IVF) centers. The purpose was to transfer the best available single embryo, to prevent multiple pregnancies, which satisfies the regulatory bodies. Blastocyst transfer mimics the normal physiology in natural pregnancy, where it reaches the uterine cavity on days 5–6, and it guarantees the activations of the embryonic genome. Culture media differ in composition, and incubators differ in their O2 and CO2 tensions, and prolonged culture may create genetic and epigenetic changes in the embryo. A recent Cochrane review showed a significantly higher live birth rate after blastocyst transfer and similar cumulative pregnancy rate when compared to cleaved embryos transfer. However, differences were considered low-quality evidence. The most recent meta-analysis showed no significant difference in live birth rate or cumulative pregnancy rate between blastocyst and cleaved embryos. Blastocyst stage might not be reached after extended culture of embryos, and transfer is canceled. There are also significantly more embryos for cryopreservation after transfer of cleaved-stage embryos. Transfer at the blastocyst stage was associated with higher risk of perinatal morbidity, as well as increased risk of premature labor and monozygotic twins. A possible explanation is genetic and epigenetic changes caused by extended culture. IVF clinicians and scientists may decide after careful evaluation of new data to go back again for cleaved-stage embryo transfer.
Comparing the outcome of IVF/intracytoplasmic sperm injection between cleaved embryo transfer and blastocyst transfer showed several controversies. In this chapter, we present different aspects of opposing opinions.
