Establishment of pregnancy is a critical event, and failure of embryo implantation and stromal decidualization in the uterus contribute to significant numbers of pregnancy losses in women. Glands of the uterus are essential for establishment of pregnancy in mice and likely in humans. Forkhead box a2 (FOXA2) is a transcription factor expressed specifically in the glands of the uterus and is a critical regulator of postnatal uterine gland differentiation in mice. In this study, we conditionally deleted FOXA2 in the adult mouse uterus using the lactotransferrin Cre (Ltf-Cre) model and in the neonatal mouse uterus using the progesterone receptor Cre (Pgr-Cre) model. The uteri of adult FOXA2-deleted mice were morphologically normal and contained glands, whereas the uteri of neonatal FOXA2-deleted mice were completely aglandular. Notably, adult FOXA2-deleted mice are completely infertile because of defects in blastocyst implantation and stromal cell decidualization. Leukemia inhibitory factor (LIF), a critical implantation factor of uterine gland origin, was not expressed during early pregnancy in adult FOXA2-deleted mice. Intriguingly, i.p. injections of LIF initiated blastocyst implantation in the uteri of both gland-containing and glandless adult FOXA2-deleted mice. Although pregnancy was rescued by LIF and was maintained to term in uterine gland-containing adult FOXA2-deleted mice, pregnancy failed by day 10 in neonatal FOXA2-deleted mice lacking uterine glands. These studies reveal a previously unrecognized role for FOXA2 in regulation of adult uterine function and fertility and provide original evidence that uterine glands and, by inference, their secretions play important roles in blastocyst implantation and stromal cell decidualization.T he uterus is comprised of two tissue compartments, the endometrium and the myometrium. The endometrium contains three cell types, luminal epithelium (LE), glandular epithelium (GE), and stroma. In mice, the uterus becomes receptive to blastocyst implantation on gestational day (GD) 4 (with the observation of a postcoital vaginal plug designated GD 0.5); it is prereceptive on GD 1-3, and by the afternoon of GD 5 it becomes nonreceptive (refractory) (1-3). Dynamic changes in ovarian estrogen and progesterone production act via the uterus to regulate uterine receptivity, blastocyst implantation, and stromal cell decidualization necessary for the establishment of pregnancy (4-6). The implantation process, which is initiated by the attachment of the blastocyst trophectoderm to the receptive LE, occurs before or right after midnight in the evening of GD 4 and becomes more prominent by the morning of GD 5. By GD 6, the trophectoderm begins to contact directly stromal cells that then begin to differentiate into decidual cells, which are required for successful pregnancy because they regulate placental development and local maternal immune responses (7,8).The infertility observed in leukemia inhibitory factor (Lif)-null mice and uterine gland-knockout (UGKO) mice and sheep established ...