Woman with 2 uteruses carrying a baby in each

A woman who has been pregnant for more than 20 weeks is due to give birth to two babies. But they're not twins, because they’re growing inside separate uteruses and one is four days older.

Angie Cromar, a Murray, Utah, woman, was born with two uteruses, according to CNN affiliate KSL.  The condition is known as uterus didelphys.

Read and watch her video here.

CNN.com spoke with Dr. Peter Greenspan, an associate chairman in the department of obstetrics and gynecology at the University of Missouri-Kansas City School of Medicine, about the condition. He is not involved with Cromar's care.

Usually women aren’t even aware they have double uteruses, he said.

This condition occurs during fetal development, when the two small tubes that would normally fuse together to create the uterus fail to join. This creates two separate uteruses.

Greenspan said he has seen several pregnancies in women with this condition and most of them conceive in one of the organs.

“I’ve a patient who had a baby on her right uterus, then come back pregnant years later in the left side and have a baby out of that side. It’s kind of interesting,” he said.

While it's rare to conceive in both at the same time, it has happened, he said. Doctors usually don’t remove an extra uterus unless it creates complications such as repeated miscarriages.

“The issue is preterm labor, because it is a smaller cavity. It’s half the uterus,” said Greenspan. “There are premature contractions. The other issue is they have trouble and can’t remain pregnant when they get to a certain point because the uterus is too small or too tight.”

Cromar, a labor and delivery nurse told KSL that she knew that risk of miscarriage or premature birth is slightly higher in double uterine pregnancies.

"I'm a little nervous, just because I know what can happen, but I'm really excited," she said.