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Traditionally, the pre-embryo transfer has been performed after two or three days of culture. As more experience has been gained in the prolonged culture of pre-embryos and as better culture media has become available, we have been able to culture pre-embryos to the final pre-implantation stage which is called the blastocyst stage. At this stage, pre-embryos of varying quality are more easily distinguished. This allows for the transfer of fewer pre-embryos with an excellent chance of conception and a remote possibility of a triplet or higher order pregnancy. Blastocyst transfer is usually offered if there is a large number (usually greater than eight) of pre-embryos in culture. The final decision to proceed with blastocyst transfer may not be made until the third day of pre-embryo culture. Blastocyst transfer has resulted in pregnancy rates over 70% with only a 15-20% risk of twins and no higher order pregnancies (triplets, quads, etc.) in patients younger than 35 years old.
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