A blastocyst is transformed when an embryo reaches the five to six day development stage. At this point in development, the embryo has between 60 and 100 cells distributed in two areas: an outer embryo lining (which will later form the placenta), and an inner mass (which will later become the fetus). During a natural cycle, the embryo develops into the blastocyst stage as it is leaving the fallopian tubes and entering the uterus. An embryo needs to have entered into the blastocyst stage once it arrives in the uterus to ensure proper implantation.
Blastocyst transfer is becoming extremely popular amongst reproductive endocrinologists and infertile couples. A blastocyst transter is associated with a number of advantages over the typical Day 3 transfer procedure.
Blastocyst transfer is considered a more “natural” type of transfer than the Day 3 transfer procedure. This is because the blastocyst embryo is implanted into the uterus at almost the same time that it would have entered the uterus should the pregnancy have been a natural one. Alternatively, during Day 3 transfers, embryos are placed inside the uterus at a time when they should normally still be in the fallopian tubes. It is believed that by waiting to transfer a blastocyst embryo, a woman may be more likely to achieve a successful pregnancy. Blastocyst embryo transfers have also been associated with better IVF success rates. By allowing embryos to culture for a longer period of time, embryologists are better able to choose healthy embryos to transfer into your uterus.
Not all embryos are strong enough to reach the blastocyst stage of development; in fact, approximately 50% of all embryos die soon after the third day of development. By holding transfer off until the blastocyst stage, your embryologist will be better able to ensure that your embryos are healthy and capable of further development. However, blastocyst transfer is associated with almost a 50% success rate, compared to the usual 35%.