Since when?
He ran on it.
Ban late abortions; exceptions for rape, incest or health
What does Donald Trump believe? Social Issues: Abortion should be limited.
Describing himself as “pro-life,” Trump told Bloomberg News in January that he believes abortion should be banned at some point in pregnancy, with exceptions for rape, incest or life of the mother. In 2011 he explained to the Christian Broadcasting Network that he had changed his mind on the issue. In “The America We Deserve,” Trump then wrote that he supported a woman’s right to choose.
Source: PBS News Hour “2016 Candidate Stands” series , Jun 16, 2015