the uterus or womb is part of the reproductive system of the female body. the uterus is the place a baby grows for nine months during pregnancy. it is a pear-shaped organ inside a woman. it is behind the bladder and in front of the rectum. the uterus weighs 70 grams. the endometrium is the tissue that lines the uterus. a hormone called estrogen makes the endometrium thick with blood and fluid. this uterine lining gives the growing baby what it needs to grow. the endometrium leaves the uterus as the monthly flow of blood (menstruation). the endometrium will form again. this happens every 28 days. the number of days can be different for each woman. the fertilized egg will move through the fallopian tube and into the uterus. it will then attach to the endometrium.