I don't know of an ethical way to absolutely prove whether or not the embryo is rejected. (just as I can't prove that the thinned uterine lining of an anovulatory nursing mother doesn't increase her lost embryos)
But, this study came very close: Surgically sterilized ovulating women who had serial ultrasounds, urinary and serum hormone levels and even uterine biopsies.
Not only that, but all the methods appear to have been designed to avoid any unethical practices, such as one of the studies which included women with IUD's.
This lead investigator did a similar study in Cereus monkeys and found no effect on implantation or uterine lining.
"Cereus" should be "Cebus" monkeys.