It was seen as “low class” for a baby to be born with only a nurse present, so:

During her birth, the doctor was not immediately available because of an outbreak of the Spanish influenza epidemic, and the nurse ordered Rose Kennedy to keep her legs closed, forcing the baby’s head to stay in the birth canal for two hours. The action resulted in a harmful loss of oxygen.[3] As Rosemary began to grow, her parents noticed she was not reaching the basic developmental steps normally reached at a certain month or year. At two years old, she had a hard time sitting up, crawling, and learning to walk.

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Her occasionally erratic behavior frustrated her parents; her father was especially worried that Kennedy’s behavior would shame and embarrass the family and damage his and his children’s political careers.[3][18] When Kennedy was 23 years old, doctors told her father that a lobotomy would help calm her mood swings and stop her occasional violent outbursts.[19][20] Joe Sr. decided that Rosemary should have a lobotomy; however, he did not inform his wife of this decision until after the procedure was completed.