Sue and Noel Radford planned to have three children. They're now expecting their 21st.

Earlier this week, the parents of Britain’s largest family, Sue and Noel Radford, posted a video to their YouTube account.

“We’re just on the way home from having a very special, lovely appointment. And I’m just wondering if anybody can guess where we’ve been,” Sue, 42, said to the camera.

“I bet you can.”

If you guessed the obstetrician, snaps for you. Because yes, the Radfords are expecting another child – their 21st.

Sue and her husband, 46, later shared a sonogram to Instagram, with a message to their tenth daughter: “Can’t wait to welcome you into the family, precious little one”.

We can’t wait to welcome you into the family precious little one ????????if you missed the vlog yesterday head over to watch it the link is in our bio ????#soblessed

A post shared by Radford Family (@theradfordfamily) on May 29, 2018 at 2:27am PDT

The announcement comes just months after the Lancashire couple declared that 20 was plenty, following the birth of their youngest boy, Archie, in September.

“We’re happy to be finishing on a nice even number,” Sue told The Sun at the time. “It’s strange knowing you’re not going to have any more.”

But eight months later, here we are… Archie will have a little sister. As will Chris, Sophie, Chloe, Jack, Daniel, Luke, Millie, Katie, James, Ellie, Aimee, Josh, Max, Tillie, Oscar, Casper, Hallie and Phoebe.

Sue and Noel also had a son, Alfie, who was stillborn at 23 weeks in 2014.

“We’re addicted to having all these kids.”

The eldest, Chris, was born in 1989 when Sue was just 14 years old. Despite her young age, she and Noel decided to raise the boy, as both parents had themselves been given up for adoption at birth.

Their second child, Sophie – who now has three children of her own – followed three years later.

“We set out to maybe have three children but enjoyed having the kids around so just carried on,” Noel previously told The Sun. “We’re addicted to having all these kids. We just love it.”

Noel did undergo a vasectomy while his wife was pregnant with child number nine, but the couple ultimately decided their family wasn’t complete and had the procedure reversed.

“Having it reversed is worse than going in the first time. I won’t be going back again,” Noel told The Sun last year. “We’ll work something out.”

A post shared by Radford Family (@theradfordfamily) on Feb 2, 2018 at 5:50am PST

The family, who were the subject of British TV documentary series 20 Kids and Counting, lives in 10-bedroom converted care home in Morecambe in England’s northwest.

According to The Sun spend the equivalent of AU$440 a week on food shopping and consume 8.5 litres of milk, three litres of juice and three boxes of cereal every day.

The couple have owned a bakery since 1999 and have told media they’ve never claimed welfare benefits.

“It’s important that our children see us working,” Sue told The Daily Mail in 2015.

“Big families will always be tarred with the same brush. People will say ‘They must be on benefits’, but there are lots of very hard-working large families who support themselves.”