The star sat down with hosts Phillip Schofield and Holly Willoughby to address false information printed in the press, including a report that the termination took place in 2009, and that it was behind Simon's back.
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Confirming that the incident happened in the 1980s when both she and Simon were "very young", Sinitta chose not to discuss the direct circumstances that lead to the decision.
She did, however, want to clear up how the story "came to be", clarifying that she did not approach the papers herself: "It's not something that I want to promote."
The story escalated last week when Sinitta attempted to address the allegations on Twitter. She admitted that decision had probably made the situation worse, but said she was trying to defend herself and had wanted to share the facts straight "from the horse's mouth".
Sinitta, who has been very open about her fertility problems in the past, became a mum when she adopted two children with her ex-husband Andy Willner, Magdalena, eight and Zac, seven.
During her time on This Morning, she reiterated that she was happy for Simon and his girlfriend Lauren Silverman, who are expecting a baby together next year. And she said this latest incident has not changed her relationship with Simon.
"It's been embarrassing and exhausting," she said of her attempts to defend herself on Twitter. "It has not affected Simon and I; it has just affected me."
She went on to address the bizarre reports that her lifestyle is funded by the X Factor mogul. "I am not on Simon's payroll… he doesn't pay me an allowance… In the end it does get to you," she said of the unsubstantiated stories.
The aim of her appearance, she said, was to put an end to all the negative stories, and tell the truth. "I hope it will stop," she said. "It may be naive, but these are the facts."