Her sister, fellow former Bachelorette Elly Miles, left an adoring comment for her niece.
"Gorgeous little darling," she commented on the post.
Becky announced her pregnancy in January, in an interview with the Daily Telegraph.
At 18 weeks pregnant, she told the publication, "I can't wait to be a mum. I have always wanted to be a mother, and I guess I have had some close friends that have had some issues falling pregnant in the past.
"I was given some advice from a friend who said if I was dreaming of being a mum and getting pregnant, maybe I should just let nature do its thing. And so, we did."
While Becky was unlucky in love on reality TV, she fell for her partner James almost two years ago after meeting in a very modern way: on the dating app, Hinge.
"We have been dreaming of those next steps and talking about engagement and having a baby and now that has all become a reality," Becky said of their growing family.
"Although it seems a bit surreal still, it is really exciting, and I can't wait to be a mum."
Congratulations to the happy couple on the birth of baby Billie!