u can do your birthing plan whenever u feel u know what it is u want for your birth, u may find that what u may want now may change closer to the birth, i personally never had any kind of birthing plan as i knew i was having a c-section and theres only 1 way of doing that but with my 1st born i had a labor before the emergany section and never had a birth plan i just went with what was happening at the time and decided then and there what pain killers and methods of labor i would go for.
its ur choice when u do it hun x