Not all patients have embryos that survive to the blastocyst stage. Embryos may have arrested development in the first few days after fertilization – some embryos just arrest-no matter what the laboratory conditions are- and some embryos just were not destined to become babies because of inherent problems-such as genetic defects The window of implantation of human embryos is between 7 to 10 days after ovulation. After 10 days, if the embryo does not at least start to implant, the endometrial lining may start to fall apart and not be able to sustain the embryo. Therefore, any early stage embryos placed in the uterus on Day 2 or 3 must depend on the uterus for a few more days until they develop to the blastocyst stage. Only the blastocyst stage embryo can implant in the uterus.

Embryos talk to the uterus!

SET may be better than just one transfer for many patients for many reasons: First, embryos communicate with the uterus using hormones called cytokines. The presence of these cytokines can help the uterus prepare for implantation. The exact makeup of fluids in the oviduct and uterus of each patient may be impossible to determine. Perhaps the embryo can tell the uterus what it needs to implant and develop. The presence of one group of embryos on Day 3 may help the implantation of the second group of embryos by getting the uterus properly prepared. SET also provides a better method of selecting the best of the extra embryos to transfer and decreases the need to cryopreserve (freeze) extra embryos that may not be survivors. Obviously, the ability to decide which embryos are the best increases the chance of implantation and pregnancy. Blastocysts not transferred fresh can still be cryopreserved and thawed later for a frozen embryo transfer (FET).