from the fertility booklet The A.R.T. of Fertility: A patient guide
by the Fertility & IVF Center of Miami

Blastocysts are embryos that have developed for 5 to 6 days after fertilization. A healthy blastocyst is ready to hatch from its outer shell by the end of the 6th day and implant into the endometrial lining within 24 hours. Currently only about 20-40% of embryos mature into blastocysts, but those that do survive have a better chance to implant and develop into a baby. Why transfer blastocysts? Blastocyst culture can provide a natural selection of the best embryos instead of random selection among good quality embryos on "day 3". Blastocysts are a viable option whenever a good number and quality of embryos exist. When only a limited number of embryos are available on the third day, there is no advantage to continue culture to a blastocyst.

Since blastocysts have a higher potential for implantation, fewer of them are transferred, generally two. Transferring fewer embryos translates into a lower risk for multiple pregnancy, and this is paramount for those couples that want to avoid selective reduction.