March 20, 2015 7:34 AM EDT

Bentley Thomas Williams of Belleville, Michigan, isn’t even a month old, and he has already given his parents the surprise of a lifetime.

Back in October, a 20-weeks-pregnant Danielle Williams and her husband Kyle visited the doctor’s office for a sonogram to determine the sex of their baby, reports ABC News. The technician who performed the procedure informed them they would be welcoming a second daughter to their family in March.

So imagine the look on the couple’s faces when the doctor delivering Danielle’s baby on March 3 lifted up their newborn and announced, “It’s a boy!”

“I had been up for 24 hours and I thought my eyes were playing tricks on me,” Kyle said of the reveal.

While inaccurately identifying the sex of a fetus at 20 weeks is extremely rare given today’s technology, professionals say parents should use more than an ultrasound to learn if their child is a boy or girl.

“The point of the sonogram at about 20 weeks is to check for the important stuff like brain development and the chambers of the heart,” Ob-Gyn Dr. Jennifer Ashton told ABC News. “Sex is impossible to confirm without a test like an amniocentesis that looks at chromosomes.”

Once the Williamses determined their new baby boy Bentley was healthy and not hiding any other surprises, the parents decided to have fun with the gender switch. To reveal the real sex of their baby to the boy’s grandmother, mom and dad asked grandma to change the infant’s diaper and filmed her reaction.

Baby Bentley also met his older sister Peyton, who is excited to have a brother in her life. The 2-year-old took the name planned for a (formerly) female sibling, Charlee, and gave it to her new doll instead.

Just by being Peyton’s brother, Bentley is automatically a member of Peyton Pals, the nonprofit started by the Williams family to raise awareness and funds for those with Diamond-Blackfan Anemia (DBA), a rare blood disorder.

According to Fox 2, Peyton is one of 800 people in North America with DBA and requires routine blood transfusions to stay healthy.

The entire Williams family is now back home and working on getting Bentley settled, including shopping for baby clothes that aren’t pink.

This article originally appeared on People.com

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