The formulation of new laboratory culture media - the liquid in which the embryo is grown in vitro - has made it possible to "grow" embryos in vitro beyond the typical 2 to 3 day state of development , till they become blastocysts. A blastocyst is the final stage of the embryo’s development before it hatches out of its shell (zona pellucida) and implants in the uterine wall.

Initial studies suggest that transfer of the embryo on day 5, at the blastocyst stage, may yield higher pregnancy rates. There may be two possible reasons for this. Firstly, transfer of the blastocyst to the uterus may be more physiologically appropriate , since this mimics nature more closely, so that the implantation rate may be higher. Also, waiting till the blastocyst stage allows the doctor to select the "best " embryos, since unhealthy embryos are likely to die ( arrest) before they reach this stage.

Blastocyst transfer also significantly reduces the possibility of potentially dangerous high-order multiple births, such as triplets. Higher implantation rates allows doctors to transfer fewer blastocysts - perhaps only one - reducing or avoiding multiple births and their associated problems. Supernumerary blastocysts can also be successfully cryopreserved using vitrification so that pregnancies can be achieved after thawing.

While blastocyst transfer is a very promising advance for patients who grow lots of eggs ( good ovarian responders), its utility for the difficult patient - the poor ovarian responder - is still debatable. This is because if there are few eggs, there is a very real risk that none of them may develop to the blastocyst stage. All of them may "arrest", so that there are no embryos available for transfer. Every patient needs to balance these risks and benefits , depending upon the clinic’s experience and success rate.

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A beautiful blastocyst on Day 5.