Blastocyst transfer

Reproductive technology is advancing by strides. Conventionally human preimplantation embryos conceived through IVF are routinely transferred to the uterus on day 2 or 3, around the 4- to 8-cell stage of development. A recent availability of co-culture system has greatly increased the abilities to extend the duration of embryo culture to 5 days, corresponding with the blastocyst stage of development, largely improve the embryo-endometrium synchronhy. In normal pregnancy, the oocyte is fertilized by the sperm and develops in the oviduct. While the embryo moves toward the uterus, the endometrium is also got prepared for implantation. The embryo enters the uterus at approximately the blastocyst stage, and the blastocyst undergoes implantation, embedding in the endometrium of the uterine wall, and is so called successful pregnancy.

Complete development of human embryos from fertilized eggs to blastocyst in vitro is still challenging due to the fact that human embryo usually suffer developmental blocks arrest at 8-cell stage in in vitroculture, which in turn limited the window of embryo transfer to the 4- to 8-cell stage, at the time the endometrium is not ready for implantation. Various co-culture systems have been emerged to overcome the developmental arrest. For example, co-culture with green monkey kidney cells (VERO) as feeder cells and with sequential culture medium may facilitate embryo developmentfrom the 8-cell stage to morula, in this system, the feedersmay secrete the growth factors promoting embryonic development, and remove metabolic products and chelation of heavy metal ions from the media that may affect embryo development. Approximately half of the embryos could develop into the blastocyst stage at the end of co-culture.

One of the advantages of blastocyst embryo transfer is improved embryo selection. In most cases that two embryos will be selected, the number of birth is at most as twins, multiple pregnancy rates are significantly decreased, so as the abortion or premature birth. The other advantage is the embryo-endometrium synchrony. A better implantation rate and pregnancy rate has been shown with blastocyst transfer. Embryonic block at the 8-cell stage has been overcome, thus improved embryo selection can be conducted, allowing the best embryo to be transferred as well as largely eliminating the suffering from pregnancy failure. The excess blastocysts can be cryopreserved, and better cryopreservation results have been demonstrated with vitrification of blastocysts, which increases pregnancy rate with the thawed embryos.

In addition, extended embryo culture to day 5 facilitates the time-consuming preimplantation genetic diagnosis which results with a day 3 biopsy of 2 blastomeres from the 8-cell stage embryos.

Albeit blastocyst transfer largely improves the pregnancy rate and reduces the risk of multiple pregnancies, it requires optimal laboratory conditions. A certain scale of the IVF lab as well as a high pregnancy rates are the prerequisites for an IVF lab to move onto blastocyst culture. The blastocyst culture technology can therefore serve as an indicator of an IVF institution.