What is a Blastocyst?
A blastocyst is formed when an embryo reaches the five to seven-day development stage. At this point in development, the embryo has between 60 and 100 cells distributed in two areas: an outer embryo lining (which will later form the placenta), and an inner mass (which will later become the fetus). During a natural cycle, the embryo develops into the blastocyst stage as it is leaving the fallopian tubes and entering the uterus. An embryo needs to have entered into the blastocyst stage once it arrives in the uterus to ensure proper implantation.
Recent advances in blastocyst culture and transfer have resulted in improved IVF pregnancy rates and reduced multiple pregnancy rates. Traditionally, embryos are transferred to the uterus on day three (called Day 3 transfer) after fertilization and it is not uncommon to transfer three or four embryos. However, it is now possible to grow embryos in the laboratory to the blastocyst stage of development, which occurs on day five after fertilization.. Typically, the strongest, healthiest embryos make it to blastocyst stage as they have survived key growth and division processes and have a better chance of implanting once transferred. The selection of potentially more viable embryos allows the embryologist to transfer fewer embryos, often one or two, lowering the risk of high order multiples while maintaining high pregnancy rates.
Before you and your partner decide on a blastocyst transfer, it is important to consider whether or not you are suited for the procedure. Blastocyst transfer does have its disadvantages and may not be appropriate for all couples.