There are lots of problems with any kind of surgical procedure on someone like Nessie. It’s just a lot of work trying to figure out some way to get it right so that whatever solution you come up with seems believable. The more complex, the greater the likelihood you end up leaving a gaping hole in logic somewhere that was not immediately obvious, and then all that work you did trying to come up with a believable solution goes down the drain and it makes things look even worse because you spotlighted the whole thing. Believe me, I gave a lot of thought to your question, and there are just so many pitfalls with the procedure itself and Nessie’s hardened skin, her regenerative abilities, and the whole menstruation thing. I don’t see her regenerating ova, and I don’t think anyone wants to curse her with an eternity of periods. Once she hits her period she has a maximum of somewhere around 40 years or so to get pregnant, and then that is it, unless you want to fool around with the frequency of her cycles, and that might be your best bet.
How about a simpler solution that gives Nessie and Jacob a lot more flexibility in the timing of childbearing? Let’s say that Nessie doesn’t have any periods whatsoever; that gets you through some potential weird issues when she is still growing up and having periods at that time that are going haywire due to her accelerated growth. Maybe she thinks she is infertile, but she discovers at some point that she can have periods as long as she maintains either a strict all-human food diet (maybe even a pure vegan diet), or an all-vampire diet of blood (all animal, all human, or it doesn’t matter). That also enables her to turn it on at will, as long as that diet is strictly maintained for a certain period of time (going off the diet would then stop the ovulation). The diet might also have to be maintained throughout her pregnancy or else she has miscarriages, who knows, there’s a lot of room to play around in that area. Throw in some artistic license, that the ova do not degenerate over time, and Nessie would be able to potentially bear children centuries from now.
"It will take an amazing amount of control,” she mused. “More even than Carlisle has. He may be just strong enough…the only thing he’s not strong enough to do is stay away from her. That’s a lost cause.”