In order to systematically analyze the maternal, i.e. the endometrial, changes in the equine endometrium underlying the complex embryo-maternal dialogue during early pregnancy, a transcriptome study of endometrium samples from mares at day 16 of pregnancy and day 12 cyclic mares was performed. Results were compared to a previous study of samples from day 12 of pregnancy and day 12 cyclic controls. Endometrial biopsies were taken from six wormblood mares at day 16 of early pregnancy after embryo recovery and day 12 cyclic controls. 8 samples were analyzed: one pregnant sample and the corresponding control sample of every mare each at a time.