A blastocyst is an embryo that has developed in culture in the IVF laboratory for at least five days after egg retrieval and has divided into two different cell types. The surface cells are termed the trophectoderm and will eventually become the placenta and the inner cells, termed inner cell mass will become the fetus. A healthy blastocyst should hatch from its shell (zona pellucida) by the end of six days or earlier and is then ready to begin to implant within the lining of the uterus.
Blastocysts have survived an important "survival test". During the first few days, the embryo relies on the egg cell (from the mother) for all its growing nutrients. However, in order to survive post day three or four, the embryo must activate its own genes so that it can carry on growing and dividing. Unfortunately, not all embryos are able to grow past this milestone and in fact, only about one-third of embryos are capable of becoming blastocysts. If an embryo can propel itself into becoming a blastocyst in vitro then it would appear that nature has sent us a message that these embryos are the product of a "survival of the fittest" test.
There is now abundant evidence that transferring blastocyst embryos into the uterus five or six days after egg retrieval, results in higher implantation rates per embryo transferred compared to a day three embryo transfer. An embryo reaching the blastocyst stage has an increased chance of implantation as opposed to a day 3 embryo (all things being equal regarding a healthy uterus). It is believed that the improved implantation rates following a blastocyst transfer are due to selection of the "best" embryos.
Traditionally, in an IVF cycle, embryos were transferred to the uterus on the second or third day of development and initial embryonic cell division. La Jolla IVF’s embryology team has moved systematically towards refining the techniques in the laboratory which now enable many of our patients to avail themselves to transferring blastocysts as opposed to the older more traditional methodology of day 3 transfers. IVF practitioners were aware that day three embryo transfers were too early when compared to what happens in naturally conceived pregnancies. In a naturally conceived pregnancy, the embryo reaches the uterus on day four or five because it grows and divides in the fallopian tube for several days after fertilization.