Katelyn Yates went to the hospital expecting to have her tonsils removed. She found out she was pregnant -- with qudruplets.

The odds of conceiving spontaneous quadruplets are less than one in 500,000, according to the Journal of Family and Reproductive Health.

Yates, a 20-year-old nursing assistant in Illinois, thought her doctor was playing a prank when he announced on April Fool's Day that she was pregnant with four babies.

"I was completely in shock," Yates tells TODAY.

After all, she had gone to the emergency room with a sore throat.

After it was determined that Yates needed an X-ray to figure out what was causing the inflammation in her throat, she was given a pregnancy test. Unborn babies are more sensitive to radiation than adults.

Not only did the test come back positive, Yates says her human chorionic gonadotropin (hCG) levels were "off the charts." HCG is a hormone produced by the placenta during pregnancy.

At the time, Yates and her now-fiancé, Julian Bueker, 21, had been dating for only six months.

"He was so excited," Yates says, noting that his reaction made her feel more calm.

Yates's pregnancy wasn't easy. She was diagnosed with preeclampsia, a serious blood pressure condition, and during her third trimester, Yates's liver and kidneys started to fail.

"I couldn't breathe on my own," she says.

When Yates reached 28 weeks and 4 days, doctors delivered the babies by C-section.

Elizabeth Taylor, Max Ashton, Elliot Ryker, and Zya Grace were born on Oct. 17, at Carle Foundation Hospital in Champaign, Illinois. Max and Elliot are identical twins.

The smallest baby was Elizabeth, who weighed 1 pound 2 ounces; while Max, the largest of the bunch, weighed 2 pounds, 6 ounces.

"They're doing phenomenal. They are gaining weight and growing," Yates says. "Elizabeth is about six grams away from reaching 2 pounds."

"It's amazing to me how well they are doing," Yates says. "Just a few weeks ago, they were the size of my hand."

Yates is trying to raise money through donations to help support her family; on Venmo she is "katelyn yates 40."

Dr. Meghana Limaye, MD, a maternal-fetal medicine specialist in the Department of Obstetrics & Gynecology at NYU Langone Health, calls spontaneous quadruplets an "incredibly rare situation."

"There were only 148 deliveries of quadruplets or higher order multiples in the US in 2021," Limaye, who did not treat Yates, previously told TODAY.