A birth plan (or proposal if you prefer) stops you from going in blind, or at the whim of others. It doesn't need to be set in stone but can act as a learning experience, a guide or a steel beam to your beliefs and hopes around the birth you wish to have.
Do you want to find out the sex of your baby? Because there is a fair chance that your sonographer will be able to tell you at this point. As one rather amusing sonographer said to me once...hamburger or snail?