Human blastocyst culture in IVF: current laboratory applications in reproductive medicine practice

For fertility patients undergoing in vitro fertilization (IVF), blastocyst culture brings a number of potential advantages over laboratory techniques leading to traditional cleavage-stage embryo transfer. Because day 2-3 embryos normally should transit the oviduct only, their direct exposure to an intrauterine microenvironment is physiologically inappropriate. This mismatch is obviated by blastocyst transfer. Moreover, the nutritional milieu inside the fallopian tube is not the same as within the endometrial compartment, a feature possibly antagonistic to implantation when a day 2-3 embryo is placed directly within the uterus. Delaying transfer to day 5-6 may also improve reproductive outcome by reducing risk of embryo expulsion, given increased myometrial pulsatility measured at day 2-3. However, rigid reliance on a blastocyst culture approach will more often result in treatment cancellation due to embryo loss (no transfer), or having fewer embryos for cryopreservation. The development of sequential media to support embryos in extended in vitro culture was a significant laboratory refinement, since it enabled direct observation of embryos to improve transfer selection bias. This approach, in tandem with blastocyst cryopreservation, leads to fewer embryos being transferred and reducing multiple gestation rate. This review discusses key features of human blastocyst culture and its application in clinical reproductive medicine practice.