Developing in the same uterus, twins learn to share space from day one – most twins, that is. Michigan mom Sarah Reinfelder's twin girls, on the other hand, didn't have to share at all. Each enjoyed her own uterus in which to grow, kick, and roll.
How could this happen? Reinfelder has two uteruses, scientifically known as uterus didelphys. If that sounds completely strange to you, consider the fact that if you're a woman, you once had two, yourself.
"All female embryos start off with two uteruses, two vaginas, and two cervixes," says Connie Hedmark, Reinfelder's obstetrician.
These duplicate organs eventually grow together, with a temporary wall down the middle. The wall between the two vaginas is the first to disappear. Next, the cervix loses its dividing wall. And finally, the uterus becomes more circular and loses its dividing wall as well.
This whole process is complete before the baby in utero is 8 weeks old. But in about 3 percent of female babies, the fusing gets disrupted at some point.
The disruption usually happens at the final stage, and the baby is born with a uterus with a wall down the middle, called a septate uterus. Less frequently, a baby is born with a bicornuate uterus, which is almost – but not quite – two separate uteruses.
"A bicornuate uterus resembles a valentine's heart whose midpoint stretches all the way down to its bottom," says Hedmark.
And, in very rare cases, such as Reinfelder's, a girl is born with two completely separate uteruses, each with its own cervix. This condition is called uterus didelphys bicollis.
"Some women with uterus didelphys even have two vaginas," says Hedmark. "I've had patients who, when trying to conceive, had to choose which side to have intercourse on."
One uterus is usually much more viable than the other, being larger and composed of better tissue to support implantation. For this reason, most women with this condition can carry babies in one uterus only – and even then, they're at higher risk for miscarriage or premature birth. Reinfelder's situation, then, was exceedingly rare, enough so to be nationally newsworthy.
"When we first got the referral, I almost didn't believe it," says Hedmark. "We did a little statistical analysis, and we estimate the chances of a woman with uterine didelphys successfully carrying a twin in each uterus are about 1 in 5 million."
The twin girls were delivered via c-section with two separate uterine incisions by Hedmark and her medical partner, ob-gyn Breanna Pond, in February 2009.