Givemore Gezi, 40, changed his plea on the second day of his trial at Exeter Crown Court, admitting sexual activity with a child and sexual activity by a carer with a person with a mental disorder.
Charges of rape and perverting the course of justice were dropped.
The judge adjourned sentencing until Wednesday and remanded him in custody.
Gezi was a senior nurse in Devon when he started treating the girl in 2017.
She was in foster care and had a history of depression, self-harm and had been diagnosed as being emotionally unstable with post-traumatic stress disorder (PTSD).
The court heard Gezi exploited her vulnerability by grooming her, starting a sexual relationship when she was 15 and having sex with her when she turned 16.
She became pregnant and he persuaded her to have an abortion.
He then told her he was in love with her and convinced her they were boyfriend and girlfriend.
Their relationship only came to light when Gezi’s wife became suspicious, found the girl’s number on his phone and called her to tell her he was married with two children, the court was told.
She tried to kill herself after arriving at school in tears and revealing what had been going on to a teacher.
Gezi, of Maidenway Road, Paignton, stopped being her carer when he was demoted in February.
But he carried on seeing her while police were investigating him and tried to stop her cooperating.
He was suspended by the Nursing and Midwifery Council at a hearing in London on May 30.-BBC