so, disregarding the question of whether or not reneesme and and jacob can have a kid /together/, biologically - should nessie be able to have kids AT ALL once she stops aging? sure, she still has a heartbeat so she’s not really undead like the vampires, but if she’s frozen at a certain age, then wouldn't that mean that her body is no longer changing (just like the vampires), so she wouldn’t have a menstrual cycle and therefor ability to conceive?
That would make sense to me, anyway. If the logic is that female vampires can’t reproduce sexually because their bodies are unchanging and thus don’t have menstrual cycles and can’t change to accommodate a growing fetus, then it stands to reason the same rule applies to female hybrids who have reached maturity and are no longer growing/changing themselves.
I doubt SM is going to go that route, though–I fully suspect that Nessie won’t have to face the infertility issues that Esme, Rosalie and even Leah do. I think the loophole will be “well she’s half human and has a heartbeat so it’s different” but personally that just seems so unfair, that Nessie gets to have her cake and eat it too, you know? There should be a trade-off. If your body is not aging/growing/changing anymore, then you shouldn’t be able to get pregnant.
The other issue is what Renesmee’s cycle would look like, if she had one. Is she on a human monthly schedule? Or is it faster, given her lightning-quick development? Was she born with all the eggs she’ll ever have like a human female is? Or can she produce more to keep herself fertile for much longer in her lifespan? Will she experience menopause–and how would that work if she body is ‘frozen?’
A lot of unanswered questions there. But at the end of the day motherhood is so important to SM I can’t imagine she’ll let Nessie suffer what Esme and Rosalie (and possibly Leah, although I still think once she stops phasing her cycle will come back) have suffered and fully intends to Nessie and Jacob to have children–powerful special amazing unbelievable children–and that SM wouldn’t force it into a tiny window where Nessie is fertile but not yet frozen.