Michael, however, who simply captioned the post "A bit of the day to day #nofilter," neither confirmed nor denied the rumours.
In fact, he created even more mystery by pinning two comments - one that said "it's a girl", and one that said "it's a boy".
Martha and Michael announced they were expecting their first child together in September.
WATCH: Martha Kalifatidis and Michael Brunelli get crafty in lockdown
The news came as a surprise after the couple drastically cut down their social media presence in July during their European holiday. They later revealed that the mum-to-be was seriously ill and had to return home to Australia.
"It's been a minute since we've been online, so we wanted to explain our situation and make a little bit of an announcement, and that is that Martha is pregnant," Michael began on Instagram.
Martha, however, added that her pregnancy had not been the most "exciting, wonderful time".
"I don't want to sound like I am complaining," she said. "We are so excited; we are so happy. It was a complete surprise."
"But at five weeks I was diagnosed with hyperemesis gravidarum, which is basically severe chronic nausea and vomiting with no relief. It is like 24/7," Martha continued.
"I literally did not get out of bed for two months, I was so sick. I could not eat or drink anything. There were days that I did not even get up to pee.
"I feel really bad because I don't really know how to navigate this and talk about this because I feel like there's women who have the best time when they're pregnant. And then there's people who struggle so hard, and they will do anything to be pregnant," she admitted.