The 42-year-old mum and her husband Noel, 45, who rose to fame on the TV show ‘16 Kids And Counting’, shared the news on Instagram.
The couple welcomed their 19th child, Phoebe, in July 2016 and have since filmed ‘19 Kids and Counting’ as Britain’s largest family.
The mum shared a photo of a blackboard on Wednesday 29 March that read: “Boys - 10, girls - 9 and BABY makes 20. Arriving Sept 2017.”
Radford’s 17,000 followers immediately sent over their well-wishes for the impending arrival.
“You lot are bloody amazing,” commented one person. “Congratulations - another girl to even out the numbers.”
Another wrote: “I just love this! I fear my body would fall apart with that many pregnancies and births but you were clearly born to do this. Congratulations.”
And another wrote: “This is awesome and so exciting. Hope there’s another documentary coming in the future.”
The couple revealed in July 2016, after welcoming their 19th child, that they didn’t think she would be their last.
“Our friends and family keep saying I should have another one to get an even number – so round it off to an even 20,” the mum told The Sun at the time.
“I can’t rule it out. At the moment I am happy to have Phoebe with us, she is healthy and a little stunner. We are over the moon to have her here.”
Radford said falling pregnant with her 19th child was a bit of a shock. Announcing the news in January 2016, she said it was a “huge surprise”, adding: “We were adamant that we wouldn’t have more.”
The couple are also parents to Chris, 28, Sophie, 22, Chloe, 21, Jack, 19, Daniel, 17, Luke, 15, Millie, 14, Katie, 13, James, 12, Ellie, 11, Aimee, 10, Josh, nine, Max, seven, Tilly, six, Oscar, four, Casper, three and toddler Hallie.
The couple had their first child, now 28, when Sue Radford was just 14. The couple also have three grandchildren.