blastocyst implants in the uterus in order to become an embryo.
in what organ does the blastocyst implant, in order to become an embryo?
uterus
implantation is critical to the survival and development of the early human embryo. it establishes a connection between the mother and the early embryo which will continue through the remainder of the pregnancy. implantation is made possible through structural changes in both the blastocyst and endometrial wall. the zona pellucida surrounding the blastocyst breaches, referred to as hatching. this removes the constraint on the physical size of the embryonic mass and exposes the outer cells of the blastocyst to the interior of the uterus. furthermore, hormonal changes in the mother, specifically a peak in luteinizing hormone (lh), prepare the endometrium to receive and envelop the blastocyst. the immune system is also modulated to allow for the invasion of the foreign embryonic cells. once bound to the extracellular matrix of the endometrium, trophoblast cells secrete enzymes and other factors to embed the blastocyst into the uterine wall. the enzymes released degrade the endometrial lining, while autocrine growth factors such as human chorionic gonadotropin (hcg) and insulin-like growth factor (igf) allow the blastocyst to further invade the endometrium.
blastocyst
the technique of selecting only one embryo to transfer to the woman is called elective-single embryo transfer (e-set) or, when embryos are at the blastocyst stage, it can also be called "elective single blastocyst transfer (esbt)". it significantly lowers the risk of multiple pregnancies, compared with e.g. double embryo transfer (det) or "double blastocyst transfer" (2bt), with a twinning rate of approximately 3.5% in set compared with approximately 38% in det, or 2% in esbt compared with approximately 25% in 2bt. at the same time, pregnancy rates is not significantly less with esbt than with 2bt. that is, the cumulative live birth rate associated with single fresh embryo transfer followed by a single frozen and thawed embryo transfer is comparable with that after one cycle of double fresh embryo transfer. furthermore, set has better outcomes in terms of mean gestational age at delivery, mode of delivery, birthweight, and risk of neonatal intensive care unit necessity than det. e-set of embryos at the cleavage stage reduces the likelihood of live birth by 38% and multiple birth by 94%. evidence from randomized, controlled trials suggests that increasing the number of e-set attempts (fresh and/or frozen) results in a cumulative live birth rate similar to that of det.
embryo transfer