ABSTRACT
Transfer of human embryos to the uterus at the blastocyst stage has several advantages. There may be an improvement in success rates of pregnancy due to better synchronization of the uterine and embryonic development, self-selection of embryos for transfer along with the possibility of reducing the number of embryos transferred and the risk of multiple pregnancies without altering the overall pregnancy rate would be decreased. However, embryos are transferred to the uterus on day 2 or 3 after insemination before the blastocyst stage is reached in many cases. This may be a reflection of sub-optimal culture conditions although inherent abnormalities like chromosomal anomalies will also contribute to the loss.1~8 Physiologically, the human endometrium prepares itself to its optimum approximately on days 5~7 after ovulation so as to receive a cavitated blastocyst from the fallopian tube for successful implantation.1,9,10 However, conventionally, in most programs offering in vitro fertilization (iVF), 4~8 cell stage embryos are replaced to the uterus.1,9,11 This asynchrony of embryonic stage and preparation of endometrium may be one major contributory cause of increased abortion and low take-home baby rates in infertility patients.1,10~12 Leaving all embryos in extended culture until they develop to the blastocyst stage might result in cancellation of the embryo transfer (ET) procedure if none of the embryos reach that stage. This could have a major adverse psychological impact on the patient, and also denies her the possibility of implantation of early embryos that did not develop to the blastocyst stage in vitro but might have done so in vivo.2 To avoid such events while explore the feasibility of blastocyst transfer, subsequent ET (SET) of early embryos and blastocysts was applied on day 2~3 and 5.2,6,7,13,14 The current study was conducted to investigate the effectiveness of attempted SET of blastocysts on day 5~7 following initial multi-cell embryos transfer on day 2, 3 or 4 in iVF cycles.