By blastocyst culture we refer to keeping the embryos in the incubator until day 5-6 of development, that is, until they reach blastocyst stage. By doing this, we perform a much more thorough embryo selection process. The fact they they are capable of making it to blastocyst means that they have a greater implantation potential, as it was able to stay longer in the lab. Pregnancy success rates depend on age, though.