The mucous plug is caused by the build up of mucous in the cervical canal. It tends to block the cervix and works to protect the area from bacteria and infection. When you begin to dilate, your plug will come loose and may pass while you are going to the bathroom or out and about. Occasionally the plug may appear pink or reddish in colour because of blood vessels in the cervix that have broken.
Anytime you think you have passed your mucous plug, give your doctor a call. This does not necessarily mean that labour is imminent, but it does mean your body is preparing for childbirth. It could mean you will go into labour right away or that you still have a bit of waiting to do. One of the most frustrating aspects of pregnancy is knowing that your body is getting ready to give birth but having to wait until it decides to get on with it.