John Lennon was forced to persuade his wife Yoko Ono to keep the baby when she discovered she was pregnant in 1975, because she was ready to have an abortion.
Ono, 74, admits she had just reconciled her relationship with the Imagine singer, and was worried a child would complicate things.
She later gave birth to baby Sean Taro Ono Lennon on 9 October 1975.
She says, "I thought I should let John decide whether to keep it or not. We'd just got back together and I became pregnant very soon.
"I didn't know if it was the right moment to have a child. I just didn't want to burden him with something he didn't want.
"He said, 'Of course we're going to keep it' as if he was very upset by the remark."