Carvel Bennett, aged 74, is due to be sentenced on Tuesday after being found guilty of rape following a three-day trial at Birmingham Crown Court.
Bennett raped an underage girl while she was babysitting in 1975, West Midlands Police said.
The teenage girl was spoken to by police at the time but did not want to make a complaint, when approached again in 2014, she remained unwilling to report the crime.
But in 2019 she changed her mind and an investigation resulted in the arrest and charge of Bennett, from Hansons Bridge Road in Erdington.
The crucial evidence was DNA from the victim's child which proved he is the father.
Superintendent Wendy Bailey, from West Midlands Police, said: "We remain committed to seeking justice for rape victims, and survivors may only feel ready to talk about abuse they suffered many years after the incidents occurred.
"Whenever they do feel ready, the police and wider support services are here for them, as this case demonstrates.
"We have specially trained officers to deal sensitively and compassionately with victims of sexual offences.
"They are able to support and guide them every step of the way from initial report to court conclusion."