The endometrium is 12mm thick and it is not sure if you are pregnant. The endometrium changes gradually during the whole menstrual cycle, when the period is just clean, it is about 3-5mm, and after ovulation, the normal thickness of the endometrium is between 8-15mm before menstruation. Therefore, the endometrium of 12mm is the normal thickness of the endometrium. If you are pregnant, the endometrium will gradually thicken, and the endometrium will start to thicken around the time of luteal phase. The pregnancy will be confirmed when a gestational sac is seen in the uterine cavity during an ultrasound.