Synchrony between embryo competency and uterine receptivity is essential for a successful implantation. Mice with ablation of COUP-TFII in the uterus (PRCre/+;COUP-TFIIflox/flox), exhibit implantation defects and increased ER activity in the luminal epithelium, suggesting the high ER activity may disrupt the window of uterine receptivity. In order to determine if the increased ER activity in PRCre/+;COUP-TFIIflox/flox mutant is the cause of the defective implantation, we inhibited of ER activity in order to rescue the implantation defect in mutant mice. ICI 182,780 (ICI), a pure ER antagonist, was administered to PRCre/+;COUP-TFIIflox/flox mutant and COUP-TFIIflox/flox control mice during receptive period and the number of implantation sites were examined. COUP-TFIIflox/flox control mice treated with oil or ICI showed the normal number of implantation sites. As expected no implantation sites were observed in PRCre/+;COUP-TFIIflox/flox mutant mice treated with oil, consistent with previous observation. However, implantation sites were detected, albeit at a reduced number in comparison to the control in PRCre/+;COUP-TFIIflox/flox mutant mice upon ICI treatment.. ICI treatment was also able to rescue the expression of WNT4 and BMP2, genes important for endometrial decidualization in the PRCre/+;COUP-TFIIflox/flox mutant mice. To ensure the rescue of embryo attachment and decidualization is a consequence of a reduction of estrogen receptor activity with ICI treatment of the mutants, we examined the expression of ER target gene, such as lactoferrin, in PRCre/+;COUP-TFIIflox/flox mutant mice. Having shown that ICI could rescue the implantation and decidualization defects of the PRCre/+;COUP-TFIIflox/flox mutant mice, the ability of ICI treatment to rescue pregnancy in these mice was assayed. While mice were born in COUP-TFIIflox/flox control mice given ICI, no pups were born in the PRCre/+;COUP-TFIIflox/flox mutant mice, with the loss in pregnancy in the PRCre/+;COUP-TFIIflox/flox mutant mice treated with ICI being due to defects in placentation. These results demonstrate that during the peri implantation period, COUP-TFIIs role in regulating embryo attachment and decidualiton is through the reduction of ER activity. However COUP-TFII expression is still required in the post implantation period to facilitate placentation.