There are 2 key hormones that regulation of menstrual cycle: Estrogen (also called Estrodiol) and Progesterone. (There are many inter-regulatory hormones but these are the ones most people are referring to, and are the components of birth control) While both hormones are continuously produced by your body, typically one will be at a much higher concentration at a specific point in your cycle. When under estrodiol domination (high Estrogen) the mucus of the cervix is caused to become like train tracks effectively helping sperm to travel up into the uterus. When under progesterone domination (high progesterone) the body has switched into the next phase of the cycle. It is hoping that sperm has already been introduced to the egg and is now ready to begin creating a fetus. As such, the body wants to close off any outside contact and start to provide a good environment for the fetus to grow. However, the formation of the appropriate environment in the uterus is largely dependant on chemicals secreted by a sperm/egg combination. Without these chemicals the lining (endometrium) of the uterus begins to break down and initials menstruation. Overall, the thickened mucus helps to provide an appropriate internal environment for a developing fetus. In a sense it provides a plug to seal off the inside of the uterus from the outside world.