Adding a low-quality blastocyst to a high-quality blastocyst for a double embryo transfer does not decrease pregnancy and live birth rate.

When there is one HQB available, transferring an additional low-quality blastocyst would only slightly increase the LBR, but significantly increase the twin rate, therefore SET should be recommended. When two or more HQBs are available, SET-H would have a reasonably good chance of success without the very high twin rate associated with DET-HH. DET-LL when compared with SET-L, would increase the LBR, but increase the risk of multiple gestation.