What is learned about Feyre and Rhysand's baby in Chapter 31, and why is that a problem?

It is learned that Feyre and Rhysand's baby has wings. Illyrian females have a pelvis shaped specifically for children with wings to pass through. High-Fae females do not. When a child has wings, they can get stuck during labor. Most High Fae females die and the babies with them. Magic cannot help other than fracturing a female's pelvis to make it wider. Some survive, but the labor is so terrible that many nearly die or cannot have another child.