An Iowa woman who walked into a clinic with stomach pains was shocked to find out she was six months pregnant with a set of rare twins.

One week later, she was a proud new mother.

“I had really sharp pain in my left side and went into the doctor, and they said we think you might be pregnant. They told me I was six months and told me to get down to the ER. They did an ultrasound and told me it was twins,” Shelby Magnani told KTLA sister-station WHOTV.

The babies were monoamniotic twins, which means they shared one placenta and one amniotic sac, Dr. Jennifer Krupp with Perinatal Center of Iowa told the television station.

Roughly 1 to 2 percent of all pregnancies are twins, and less than 1 percent of all twins are monoamniotic, according to Krupp.

Monoamniotic twins can present serious challenges during delivery, Krupp said, adding their survival rate is about 50 percent.

“The entanglement of the cords is what we worry about. We bring the patients into the hospital at 24 to 26 weeks, so we can monitor the babies several times a day, because we know the risk of one or both of those babies dying is fairly significant,” Krupp told the station.

Magnani welcomed the twin girls – named Ava and Anna – last Thursday, WHOTV reported. The babies were delivered by cesarean section and weighing 3 and 4 pounds respectively.

“I’m still trying to process. It’s crazy how high risk mono-mono twins can be, and how good they’re doing now, it’s really a blessing,” Magnani told the station.

Ava and Ana were expected to be OK. They will spend several weeks in the hospital before being allowed to go home.

“They’re both just little miracles, it could have been so many things that went wrong that didn’t,” Magnani said.