“Ten tiny fingers, ten tiny toes, but who are you? Nobody knows,” the mum to three-year-old Alaia-Mai wrote in the caption.
She shared a tender black and white snap of herself cradling her pregnant belly.
Marvin Humes also shared the announcement with his followers, by posting a collage of photos from his wife’s maternity shoot.
“Absolutely delighted to announce that we’re extending our family,” he wrote in the caption.
“Rochelle and I are over the moon! Baby Humes no2 is on the way.”
The collage captured four different shots of Rochelle baring her pregnant belly.
The couple married in 2012 and welcomed their first child together in 2013.
Rochelle has previously admitted she felt “pressure” to have another child - which came from her own daughter.
“I’m feeling it from Alaia, for sure! She’s already quizzing me and saying that her friends all have brothers or sisters,” she told The Sun’s Fabulous magazine on 6 November.
Congrats to the couple!