In the past, physicians recommended transferring multiple embryos to improve the chances of fertility since knowing which embryo was viable was hard to determine. This sometimes led to multiple births.
However, new advances like extended blastocyst culture and PGS/CCS have made this less necessary. In fact, due to the potential health risks related to multiple births, most fertility specialists advise against implanting more than one embryo at a time unless indicated. Instead, viable embryos can be frozen and stored for future use.