Previously in IVF, all embryos were transferred around two to three days after fertilisation, when they reached the ‘four to eight cell’ stage. Recent developments mean they can now be grown in the lab for a little longer and allow them to reach the blastocyst stage at which point they have around 100 cells (five to six days after fertilisation). By extending the number of days that the embryos are observed in the lab we have a greater chance of selecting the embryo with the best potential for pregnancy.