How do vampires age in the Underworld Universe?

Throughout the franchise we see young children created by vampires:

• Viktor’s daughter at young age sees Lucian training and then she is a grown and having a baby
• Amelia’s son David

Now history tells that us vampires don’t age, so how is it possible to have them fully mature?

Like, wouldn’t a fetus last a few hundred years in the stomach if ageing is slowed? Is this a movie mistake? Shouldn’t they remain children if they are vampires?