I registered by 32 weeks. In not sure about reserving a birthing room though since you never really know when you will have the baby
My hospital wants me to register by 30 weeks. I'm 27 now, so that's what I'm going to be doing in the next couple weeks.
I mean to ask my ob the same question today. From what I can remember from my first pregnancy, u can either call to pre-register or go online by at least 36wks or 37. I'm 36wks and need to do that soon.