Yes, yes, and yes. The chances of all these occuring are several times higher than in a sexually mature adult. One of the main reasons is that mothers-to-be who are still growing must split their nutritional intake with the baby, resulting in the baby (and the girl, to a lesser extent) becoming significantly malnourished. Often times the pelvic area is not fully formed which can cause complications, too. Things get even worse when the mother is in a third world country where proper nutrition and medical care is unavailable.