It mentioned at the beginig, and Nat taking birth-control too.The problem with the impregnation idea for me is that there have been zero foundation or build up to it so far. Something that big needs to be planted early so the idea can grow, preferably from the very first update. Correct me if I'm wrong, but there have been zero mention of them even wanting to have kids or trying to have kids. For that to suddenly become a thing will feel very out of place, basically just a kink being forced into the story because people want it. Which is fine but at least make it make a tiny bit of sense. And don't get me wrong, I'm all for Nat being bred. Hell the entire neighbourhood can pump her entire womb full. Maybe Nat develops a breeding fetish. Whatever it is, it needs to have some thought put into it before it happens.
Maybe it will be an accident? People saying Nat is on birth-control, but I can't recall that has ever been mentioned or shown at all. Other than using condoms obviously. So if you want someone to plant a seed in Nat and have it really grow, we should probably have some seeds planted in the story to give it some credibility. I think people are really jumping the gun on this.