So the big question can sometimes be "who do I want present at my birth?" Everyone has a different answer to this question. If you're planning a hospital birth, there is usually a limit of two additional people as the birthing rooms can be space restricted. So your doula may be or may not be allowed to attend your birth depending on several factors. In the event of a c-section, this may also be restricted due to space and sterility factors. Hospitals have their own policy about visitors and guests. If you get a chance to visit your birthing place, find out what their current policy is.
Beyond actual policy, you may have an idea of who you want at your birth. It can be many people, from the father of your child, your current partner, your mother/mother in law, sister, aunt, second-cousin twice removed or a friend. Birth is a joyous event that belongs to you, the birthing mother, and whoever you chose to share it with.
It's common to feel pressure from those in your life to want to attend the birth. Sometimes those who you want to attend feel differently. In my situation, I expected my mother to want to attend, but I knew that I would have the most positive outcome with my husband and my doula. However, my husband worked shift work at the time and also isn't a fan of blood and gore (we could watch ER, but he would keep himself busy in the event of a trauma scene). I knew I would have a doula who would stay with me if he couldn't be there, which was very reassuring. But this is a personal situation, keep in mind that the people in your life should be prepared for anything in a birthing situation. All should be prepared to ask to leave if they are interfering in your birth INCLUDING the doula.
If your doula is disallowed at the birth, several things can happen. Most likely, she would remain in the waiting area and kindly ask for updates but she will not disobey the staff or family's request. Once you are transferred to the mother-baby unit, she will try her best to make a short visit. This, again, is also at staff and family discretion.
Read more of Rhonda's work on her blog: http://birthingutopia.blogspot.com