Mrrzy, there *is* another opening... two, to be exact. The uterine tubes or Fallopian tubes come out the top of the uterus and end in fringed openings. Normally, the ends of the tubes, with their little fingers, sit around the ovaries, ready to tickle that egg on its merry way. But under the right circumstances, things can escape out the tubes and enter the abdominal cavity. If bits of sloughed endometrium do this, you get endometriosis. Even plain menstrual blood can do this (rarely) and cause a lot of pain.
A fertilized egg can get lost and do this, and give the poor woman a condition known as an abdominal pregnancy. Very bad news.