<P> In humans, implantation of a fertilized ovum is most likely to occur about 9 days after ovulation, ranging between 6 and 12 days . </P> <P> The reception - ready phase of the endometrium of the uterus is usually termed the "implantation window" and lasts about 4 days . The implantation window follows around 6 days after the peak in luteinizing hormone levels . With some disparity between sources, it has been stated to occur from 7 days after ovulation until 9 days after ovulation, or days 6 - 10 postovulation . On average, it occurs during the 20th to the 23rd day after the last menstrual period . </P> <P> The implantation window is characterized by changes to the endometrium cells, which aid in the absorption of the uterine fluid . These changes are collectively known as the plasma membrane transformation and bring the blastocyst nearer to the endometrium and immobilize it . During this stage the blastocyst can still be eliminated by being flushed out of the uterus . Scientists have hypothesized that the hormones cause a swelling that fills the flattened out uterine cavity just prior to this stage, which may also help press the blastocyst against the endometrium . The implantation window may also be initiated by other preparations in the endometrium of the uterus, both structurally and in the composition of its secretions . </P> <P> To enable implantation, the uterus goes through changes in order to be able to receive the conceptus . </P>

Where does the blastocyst implant to become an embryo