A blastocyst is an embryo that is cultured until it develops into two cell types: the trophectoderm, or outer layer, which will eventually become the placenta, and an inner cell mass that will differentiate into the fetus. Blastocysts usually form by day 5-7 of incubation.

In an IVF cycle, eggs are combined with sperm to form embryos, which are then placed in incubators. The embryos are cultured for different lengths of time dependent upon their development.  Most embryos are ready for transfer by day 3. Embryos that are cultured beyond day 3 may become blastocysts.

Blastocyst embryos provide several advantages.  As embryos are cultured, some will perish, and only the stronger ones will survive extended culture (5-7 days).  By day 5-6, the remaining blastocysts are heartier and more likely to implant and develop into a fetus than day 3 embryos.

By waiting until the blastocysts form, fewer embryos need to be transferred, leading to a lower incidence of multiple births and higher implantation rates. There is an upper limit of how many embryos can be transferred. This limit is based on many factors, including the causes of infertility, the treatment history, the female’s age and more. The physician and embryologist work closely with each couple to decide how many embryos will be transferred.

Blastocyst transfer is not an option in all cases, as a couple must have a minimum number of embryos on day 3 to be eligible for extended culture.  Baystate Reproductive Medicine use the following guidelines to recommend blastocyst transfer:

• Women under age 35 must have four or more top grade embryos on day 3.
• Women 35 and older must have six top grade embryos on day 3. 

If a couple only has three embryos available on day 3, culturing to the blastocyst stage may mean that only one or even no embryos will be available for transfer, thus decreasing the chances of successful conception or causing cycle cancellation.