In conventional IVF the embryo transfer is done in the uterus after 48 hours at
4-8 cell stage. In natural conception the embryo reaches the uterine cavity only
on day FIVE. With the availability of new extended culture medium, we can now culture
the embryos in the laboratory till the blastocyst stage (i.e. day 5-6). Thereby
increasing the chances of success dramatically to 50%.
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 when the embryo has 50-200 cells. 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.
Blastocyst transfer is not an option for all IVF patients. The technique is most
successful with patients who are younger and have a large number of eggs available
at retrieval. On average, patients with six or more high quality embryos on day
three are the best candidates for blastocyst culture because there is a better chance
of more embryos growing successfully to day five.