They told their friends and family they were having a daughter and bought everything pink they could lay their hands on.
And the couple, from Belleville, Michigan, US, even picked out the name Charlee for their second daughter.
But when their baby was delivered by Caesarean section they got a HUGE surprise when they saw there was something 'there' where there shouldn't have been!
After adjusting themselves to having a son, instead, the couple then filmed themselves breaking the news to family during hospital visits.
They've now uploaded footage of the mix-up – and it's hilarious.
Story continues after video...
In one scene, one of the baby's grandmothers walks in to do a nappy change, when she's suddenly left gobsmacked.
As she opens up the nappy, she exclaims: "Well, child welcome to the world... I can't believe it!"
In the next scene, the baby's other grandmother finds out that he is boy and not a girl, saying: "It's a boy? You guys are not joking?"
Later the new mum tells her sister about the unexpected sex of her baby via Skype.
The new aunt says: "Are you kidding! Oh my God! Oh my goodness! Yay!"
The Williams' swiftly changed the name of their 7lbs 5oz baby to Bentley Thomas.
The parents later told their daughter Peyton: "This is Bentley, your baby brother."
"Why?" the baffled two-year-old replied!
Dr Lisa Masterson, an obstetrician from Los Angeles, California, told Inside Edition that getting the baby's sex wrong is more common than people think when sonograms are involved.
She said: "There are multiple things that factor into the accuracy of the ultrasound.
"The fluid that's around the baby, because there's less fluid then everything is kind of squished up and you can't delineate all the borders.
"The position of the baby. If the legs are crossed and you can't really see if there's a penis or not."
More on Parentdish: How to tell your baby's sex at the 12 week scan