She added: “It’s going to be a busy Christmas!” with the hashtags “pregnant” and “baby on the way”. She also tagged her husband.
Thornton, who played John Barton in Emmerdale, reshared the same photo to his Instagram and wrote: “My wonderful wife @iamjoannapage is pregnant and doesn’t she look amazing!!!
“We are having baby number 4!”, he added, including the hashtags “almost a five aside team” and “no sleep”.
The couple have a daughter Eva, who was born in February 2013, and sons, Kit born in April 2015 and Noah born in December 2016.
Friends and famous faces offered their congratulations, including fellow Emmerdale star Charley Webb, who commented: “Oh wow, lovely news” with a heart emoji and actress and TV presenter Denise Van Outen replied: “Aw, congratulations”.