First, let's review the parts of your body that are involved with pregnancy and labor.
Your uterus, also called your "womb," is a pear-shaped organ that rests above the vagina, or birth canal. During your pregnancy, your uterus stretches and expands, along with the growing baby. Although, your baby's position will vary throughout your pregnancy, almost all babies are in a head down position as your due date gets closer.
Your baby grows inside your uterus, in a balloon-like sac called the amniotic sac. This sac is filled with amniotic fluid, which is mostly water.
The placenta, which is also known as the "afterbirth," is a sponge-like layer between the amniotic sac and the inside of the uterus. It contains 2 sets of blood vessels, one set from the mother and one from the growing baby. These vessels are close enough that food and oxygen from the mother's blood can easily move to the baby's blood, and provide essential nutrients for the baby to grow. The nutrients travel from the placenta to the baby, through a blood vessel in the umbilical cord which enters the baby's navel.
As the baby grows, waste products that are naturally made, need to be cleared from the baby's blood. These waste products travel through the baby's blood vessels to the placenta, and are transferred to the mother's blood. The waste products are then eliminated from the mother's body.
The cervix is the lower part of your uterus. It is round and shaped like a tube. It has a small hole in the middle of it. During pregnancy, a protective mucus plug forms inside this hole. Usually, the cervix is almost completely closed, until labor, when it dilates. This provides room for your baby to move from the uterus into the birth canal, and then out into the world.