The trim star was quickly inundated with compliments from friends and fans.
'Have you even had a baby? You look 18,' Today Extra host David Campbell said.
Sister-in-law Jasmine Stefanovic wrote: 'Awwww love this.'
Keep It Cleaner founder Laura Henshaw added, ‘gorgeous’.
Sylvia and husband Peter Stefanovic welcomed their first child together, a son in February.
"Our beautiful boy, Oscar Hamilton Stefanovic 💙 3.4kg, deep blue eyes and perfectly healthy. He’s absolute magic. 💫" Sylvia captioned the gorgeous photo showing their newborn son sleeping in her arms.
Moments later, Karl Stefanovic's brother Peter— who made his TV comeback as co-host of Sky News' First Edition last year— told his Instagram followers that their life is "fantastic" since their bub safely arrived.
"We’ve got our own Oscar! I’m so unbelievably proud of @sylviajeffreys and our new best mate. Love is strong. Life is fantastic 💕," he wrote alongside a photo showing his wife staring adorning at their tiny bub.
The couple shared they were expecting their first child together on August 19, taking to their respective Instagram accounts to also reveal the gender of their unborn baby.
"We’re having a baby boy and we’re totally over the moon," Sylvia wrote, sharing a photo of herself with her husband.