The truth is Rosemary was born normally. She had some learning difficulties, but could do math - long division and algebra.
Her main problem was that she was not athletically coordinated and became frustrated when she could not keep up with all the other athletic Kennedy's.
Her father saw her inadequacies as imperfections that he could not bear.
So, he got a doctor to "fix" the problem, which left her worse off and unable to care for herself.
If you want to know the truth about Rosemary then read "Sins of the Father" by Ronald Kessler.
In typical liberal fashion, “It isn’t broken, let’s fix it.”