Kaylin Joy arrived first, weighing 3 pounds, 15 ounces. Valerie Marie soon followed, weighing just one pound more.
Reinfelder has a condition known as uterus didelphys, and doctors say such twin births are rare. The uteri are different sizes, with the larger twin born from the larger uterus.
Neonatologist Julie Frei says the twins do not have fully developed lung function and are expected to be hospitalized for three or four weeks.
The chances of a double womb pregnancy is about one in 5 million, according to Reinfelder's physician Dr. Connie Hedmark, an obstetrician and gynecologist.
"We were concerned that the quality of each uterus was not good enough to carry the pregnancy," Hedmark told ABC News.
Doctors said they were prepared for a miscarriage in at least one uterus, if not both.
Reinfelder discovered she had two uteri after she miscarried her first pregnancy in March 2007.
She became pregnant again and carried a boy to term, giving birth about 10 months ago.
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