Thirty-one weeks pregnant fitness model Sarah Stage, who earned the nickname "six-pack mom", took to Instagram to flaunt her toned belly, leaving fans wonderstruck.

Carrying her two-year-old son James, she is wearing a white bikini top with blue ripped jeans whose buttons are open. She shows off a nearly flat belly and captioned the post, "We're excited !!! #31weeks (: @von_jackson glam: @makeupbybrittneynicole )"

Instagram users are bamboozled at the toned eight months pregnant belly. A user asked, "You are pregnant? Really?" while another sarcastically wrote, "If she is 8 months pregnant then I am Tom Cruise".

A fan wrote, "I think it's amazing that you have maintained a healthy lifestyle but I just can't get my head around how your baby can be a healthy weight when you cannot see a bump what's so ever? How can your baby grow? I had a huge bump! #confused"

"I don't believe she is pregnant if so practically where is the baby?" asked another Instagram user. "She's 8 months pregnant. Where is her baby I don't see a big baby bump," reads another comment.

While most users could not believe she is 8 months pregnant, some came in support of the soon to be mum-of two.

"She'll never lie because she really is pregnant, she has an ultrasound to prove and her first pregnancy was like this as well," said a user defending the model.

Another fan of hers said, "People need to leave her alone I had no bump until 6 months and I don't work out then and my bump measured 7 weeks behind but they did constant scans with my little girl and she was born 7 15 was just carrying her at the back some people do. You can see her tummy coming out just toned."

Sarah Stage
Model Sarah Stage attends the 2016 MAXIM Hot 100 Party at the Hollywood Palladium on July 30, 2016 in Los Angeles, California. Getty