The embryo develops into a blastocyst on the fifth day after oocyte aspiration. By culturing to the blastocyst stage, some embryos will stop development before that stage, and this makes it easier to identify the embryo with the best chance of resulting in a viable pregnancy. However, it does not improve the quality of the single embryo. We especially recommend blastocyst culture to young women with good quality embryos who want single embryo replacement. If there are several good quality blastocysts, the others can be frozen stored.