They don't shed the Uterus... rather the placenta!
Um, you sure about that? I seem to "shed" my uterus every 28 days. Think they call it a menstrual cycle. Look it up. I might be wrong!
No, the point was that you don't shed the uterus. You shed the Placenta. If you shed your uterus, you'd only ever have one period in your life because then you wouldn't have anything to bleed from to have a child grow in.
Menstruation is bleeding caused by the breakdown of the endometrium (the endometrium being the deepest part of the uterus) placenta develops when an egg is fertilized and actually embeds itself into the endometrium; ultimately via hormones preventing the endometrium from breaking down while the egg-foetus-baby develops seriously dude