"Plus size women face so much stigma and doubt over what our bodies are capable of," she wrote.
"It's something I feel doesn't get enough media attention, but I hope that the next few months of my pregnancy will help change the conversation. All bodies are amazing!
"Thank you for the continued support in this exciting time in our lives."
When a fan tweeted to say they couldn't wait to see her maternity fashion, Holliday replied:
"I'm 20 weeks so you've already seen some!"
And she also shared that her baby is due in June.
This will be Holliday's first child with her fiancé Nick Holliday, but she also has a son called Rylee from a previous relationship.
Holliday's pregnancy announcement came a week after Rylee recently celebrated his 10th birthday and the proud mum marked the occasion by sharing a throwback photo of him.
She wrote in the photo caption: "It's hard to think of a time in my life before my son.
"I was young and didn't have a clue what I was doing, but like all mum's... We figure it out.
"He has been by my side through my darkest days, always making me laugh regardless of how bleak things seemed. On the happiest days, my little cheerleader ready to celebrate by my side.
"He is emotional, silly, artsy, and often serious, and always has been. We've been through so much together and I wouldn't change a moment.
"To my son, Happy 10th Birthday. You have taught me how to love, to fight for my dreams and that farts will always be funny. You will always be my little boy."
After making the pregnancy announcement Holliday spoke to blogger Plussizebirth.com about her experience of being plus size and pregnant.
"I think it’s really important to talk about the realities of being plus size and pregnant," she said. "The options for nursing bras, maternity, belly bands and books are so minuscule that it’s insulting.
"I knew it was frustrating but had no idea until I found myself pregnant and searching for things that didn’t exist.
"Thankfully my doctor is amazing and hasn’t shamed me once. We searched all over for a doctor who supported all bodies and he has been a life saver."