Within 24 hours after fertilization, embryos begin to cleave, from 1 to 2 cells and then to 4. In the next 24 hours embryos cleave again to 8 cells. When they reach 32 to 64 cells they become blastocysts. At this stage of development, for the first time, different parts of the embryo are visible. A thin layer of cells encircles a fluid-filled cavity. A small group of cells known as the inner cell mass eventually develops into the fetus itself.

Culture systems capable of supporting blastocyst growth were first developed in Europe. In many European countries, IVF clinics are required, by law, to transfer no more than 2 embryos during each IVF cycle in an effort to limit high-order multiple pregnancies (greater than twins). However, at the 2- to 8-cells stages it is often quite difficult to determine which embryos are most likely to result in a pregnancy. Since many embryos are incapable of developing to blastocysts the ability to culture embryos for a few more days makes the task far easier.

We only recommend blastocyst culture to those patients who respond well to ovarian simulation, have many eggs, good fertilization rates, and many good quality early stage embryos. These patients are most likely to have embryos capable of surviving in culture and developing to blastocysts. They are also at risk of having a high-order multiple pregnancies if more than 2 embryos are transferred.

Patients, who do not respond well to stimulation, have poor quality eggs, and/or less than good quality embryos may not benefit from extended culture. Blastocyst culture is not a solution for poor ovarian function or poor egg and embryo quality.