Bumping case up. I think the letter was written by the teenage mother, and I wonder if she was a victim of rape or incest, because she was so desperate for the father to remain unidentfied. It doesn't sound like she felt much of a bond to the baby. She either hid the pregnancy well, or her family pretended not to notice it. JMO.
ETA: And the blunt force trauma and fractured ribs of the baby could have been from someone who was present at the birth and resented the baby -- not the teen mom or a female helper friend...