Mrs Ryan said she always checked the bag to see all of her items were in there but she never thought to open the boxes too before leaving the drive-through.
When she got home she realised it was the drive-through operator's fault but she wasn't too hard on the fast food chain for the mistake.
"It was the lunch rush and I used to work at Macca's during my uni days so I do sympathise with how busy they were," she told news.com.au.
"But being nine months pregnant makes you hangry."
The soon to be mother of two said she compromised by eating half of her husband's chips.
"The manager promised me a free one next time I go in so all is not lost," she said.
"And yes of course I still ate it - would have been a criminal waste of special sauce not to."
Mrs Ryan's friends were quick to react to her post on social media, saying the error was "cruel".
"Maybe Emily they didn't know you were pregnant and thought 'that lady does NOT need the bun'," joked one.
The Ryan family is still waiting for their own bun in the oven to arrive.
Big Macs often cops the brunt of complaints from Macca's lovers, with most usually outraged that the size doesn't fit the 'big' title.
Burger King recently came out swinging after its competitor lost its Big Mac trademark in the European Union.
Burger King leapt at the chance to troll the company, and in a viral ad created an entire menu mocking Maccas, and Big Macs specifically.
The "Not Big Mac's" menu includes meal options such as "like a Big Mac but actually big", the "burger Big Mac wished it was" and "kind of like a Big Mac, but juicier and tastier".
Other available options are "the anything but a Big Mac" and "the Big Mac-ish but flame grilled of course".
News.com.au contacted McDonald's for comment.
