She gave forewarning to expect a half-vampire birth. Obviously Bella isn't going to give birth to a child herself but if she does in fact use the eggs Rosalie harvested then the child presents the same risk of being misunderstood. If the Volturi have forewarning it will not be necessary to call witnesses to prove that Edward and Bella's surrogate offspring is not an immortal child.

I had wondered whether Bella would want her children to be (biologically) fathered by a sperm donor or by Edward. While a half-vampire pregnancy does seem problematic the alternative would be to have a child that is vulnerable to death for 17 years and then must endure 3 days of living hell. I assume Bella would prefer the half vampire child if a suitable surrogate volunteered.

On the other hand Bella man not want to put a human through a traumatic pregnancy and prefer a human child that will later be turned. In that case it would be time for Edward and Bella to tour the world, with Edward scanning every male they encounter, looking for the most powerful man-witch they can find!