Though the general biological rule is one uterus per woman, Utah resident Angie Cromar, 34, has two, a rare condition known as uterus didelphys. This allowed her to conceive in one uterus one day, and the other a few days later. She's now about 20 weeks along, and her respective fetuses — which are at slightly different stages of development — are doing well. A labor and delivery nurse herself, Cromar already has three children under the age of eight. (Watch a local report about Cromar's pregnancies.) Here, a few infrequently asked questions:

What are the chances of this happening?
Approximately one in five million. Fewer than 100 cases of uterus didelphys have been reported worldwide, Dr. Steve Terry, Cromar's OB/GYN, told local Utah TV station, KSL: "She's a member of a small, elite club." According to the Mayo Clinic, approximately 4 percent of women have some sort of uterine abnormality.

How does one end up with two uteruses?
In a female fetus, the uterus begins as two small tubes that normally join together to create a single womb. Sometimes, the tubes don't join completely and develop into separate cavities.

When did Cromar realize what had happened?
She's long known she had two uteruses, but her previous pregnancies have been straightforward single-uterus affairs. Cromar and her husband first learned she was doubly pregnant during her first ultrasound.

What are the risks?
The situation could trigger complications like pre-term labor and low birth weight. So far, the babies appear to be developing normally. "I'm a little nervous, just because I know what can happen," says Cromar, "but I'm really excited."

Best sources: KSL, KTLA, Mayo Clinic, Telegraph