Dolores Aveiro said she tried to terminate her pregnancy when she was carrying Cristiano 29 years ago.
The footballer's mum made the admission in a tell-all biography, titled Mother Courage, which has been released in Portugal.
Dolores told how she tried to end the pregnancy herself by drinking warm beer and running 'till she dropped', after doctors refused to help her.
Speaking at the launch of her book, she said Ronaldo knew of her secret and the pair now reflected on the decision with some humour.
According to the Inedpendent.IE, she said: "He told me when he found out, 'Look mum, you wanted to abort and now I'm the one who's pulling the purse-strings in the house."
Dolores, whose late husband Dinis died from liver damage caused by alcoholism, also spoke about Ronaldo's son Cristiano Jr, three.
The footballer has kept the identify of the child's mother a secret, but his mother revealed that she had picked up the child from a clinic in Florida while Ronaldo was playing in the 2010 World Cup in South Africa.
She also said that the Portugal and Real Madrid player had asked her to help raise and educate his son.
In another revelation, Cristiano's sister Elma said Cristiano Jnr, dubbed Cristianinho, still has no idea who his mum is – four years after his birth.
She said he has stopped asking about the woman, believed to be a surrogate mother.
She added: "Once we told him she was in Heaven, but my brother and mum asked us not to repeat it. "We say now his mum went travelling . He doesn't ask anymore."