Embryos which have been cultured for five or six days of development often form blastocysts (also known as extended culture). A blastocyst has a pocket of fluid which contributes to the development of the waters around the baby and an inner cell mass which forms the baby. The cells around the edge of the blastocyst form the placenta. A blastocyst is therefore a much more complicated and developed embryo than the embryo seen at two or three days of development.
What is the success rates?
Approximately 40% of blastocysts are genetically normal and therefore potentially able to form a baby. Not all embryos form a blastocyst – some stop dividing at an earlier stage of development, which can be because of genetic abnormality of the embryo.
Why is blastocyst used?
Extended culture is often used as a way of selecting the best embryo for transfer or freezing.