It seems that religions are the same as any organism. They evolve the same way. The strongest survives and spreads, and continually changes to promote it's own existance.

For instance Mormons were encouraged to have multiple wives, and often had well over a dozen children. The more babies, the more followers of their religion. I think that's part of pro-life too, although as an order from higher up, not at the individual follower level.

Or certainly more appropriate, come to think of it, is religions which refuse the use of contraceptives. Sex is fun, there's really no preventing it. So refuse contraceptives for some dumb reason, and they're guaranteed to have a bunch of children. Voi-la, the religion lives on.