"We waited so long that we decided to double up!" the couple said to PEOPLE. "We are very grateful for the blessing of twins."
The photo showed a sonogram with an unborn child giving a thumbs up.
"It's a thumbs up," Johnson captioned the post. She also added emojis of a thumbs up, a pregnant woman and pink hearts.
The couple met in 2015 when they were partners on Dancing With the Stars. They married in July 2016, and these upcoming twins will be their first children together.
Herjavec is also a father to three children from a previous marriage, but this will be Johnson's first.