In the early days of IVF, embryos were routinely transferred into the uterus on Day 3 (cleavage stage, 6 to 8 cells). Today, advances in sequential culture media allow embryos to be safely incubated to Day 5 or Day 6, reaching the blastocyst stage.
The Power of Natural Selection
A blastocyst contains over 100 differentiated cells with an inner cell mass that becomes the baby and an outer trophectoderm layer that forms the placenta. Culturing to Day 5 allows natural physiological selection of the most metabolically competent embryos, increasing pregnancy rates to over 65% per transfer while preventing multiple pregnancies through single-embryo transfer (eSET).