hasn't it been several months? why doesn't Ru look pregnant? long time because vampire, or doesn't know how to draw halfway between not pregnant and 9 months?
I wouldn't be too surprised at this point of the manga, if she laid a tiny egg that will grow to the size of a (human) baby in two days and will then hatch. But for now I just go with author-forgot-about it/can't draw-middle-stages-of-pregnancy-excuse.
@MeLikeChoco Even if we call them Lizardmen, they have no relation to lizards from Earth. They may not be cold blooded at all. They might be more related to dragons if anything, and well. That's a full on fantasy race right there, anything like cold blooded or warm blooded would be up to each particular author writing about them for their settings.