Yeah, but I thought her body was almost "frozen in time" while she keeps phasing, but then when she finally stops phasing her body will start working again--and she'll get her monthly friend again, allowing her to have children. Or atleast that was what I always assumed as I was reading the books.

That's the difference between her situation and say Roaslie's. Rosalie's body can never work like a human's again, but Leah's can if she stops phasing long enough for it to jump start again.