To achieve a successful implantation, simultaneous development of blastocyst and the endometrium is essential. In Frozen embryo transfer the timing of blastocyst warming with endometrium is important. In a frozen embryo transfer cycle timing in a natural cycle is taken into consideration. Warming and transfer of a day 5 blastocyst is timed to 5 days after suspected ovulation. Even so, the optimal day of transfer in FET is not fixed.
In humans, implantation of a fertilised ovum is most likely to occur around 6 – 9 days after ovulation. The reception-ready phase of the endometrium of the uterus is usually termed the “implantation window” and lasts about 4 days.
Initially the newly hatched blastocyst loosely adheres to the endometrial epithelium. Then it rolls over the endometrium to find a suitable place for implantation where it gets attached firmly.