I've been thinking about all the uncertainty and unknown facts about how the Hydro Dragon might have kids because I just know that Neuvillette never brings up the topic because it scares him. It scares him to think that maybe he can't have kids with a human. That maybe he can, but it will be most certainly end in death. That maybe the fact that he doesn't know if it's possible it's enough sign that he shouldn't find out.
But with Natsu he can't help but wonder and ponder on all the what ifs. Because he wants to make her happy, and the last thing he wants is to deprive her of anything that she might want. But the thought that maybe a pregnancy like that might kill her just—it won't let him sleep.
So when Natsu does get pregnant, Neuvi is writing a very frantic and panicky letter to Sigewinne, asking her to please come stay with him because he needs her to look over Natsu. And Neuvi is already very overprotective because, in his mind, Natsu being less powerful than him and thus weaker, makes her also infinitely more fragile normally—her being pregnant makes her fragility a thousand times worse. He's not letting her go to work (even though it's in HIS office). He's not letting her get out of bed. He's just losing his mind.
Sige, save him from himself.