Previous reports have described that female mice deficient in cyclooxygenase-2 (COX2) are largely infertile because of failure to ovulate, poor fertilization, and defective implantation and decidualization. In the present study, we reinvestigated reproduction in these mice and found they do show a reduction in the numbers of ovulated and fertilized eggs. However, we did not observe any substantial effect on embryo implantation frequencies or an inability of COX2-deficient females to support embryo development to weaning. Pseudopregnant COX2-null recipients do not show any alteration in the timing of implantation following blastocyst transfer, but they do show a delay in the initial rate of decidual growth after implantation that lags by approximately 24 h compared to that in heterozygous or wild-type recipients. These results support previous findings that COX2 has a role in mediating the initial uterine decidual response but is not essential to sustaining decidual growth and embryo development throughout the remainder of pregnancy.