The twins were born 11 weeks apart. Such a long interval between twins is rare, but not unheard of. (The world record — twins born 87 days apart — was set in 2012). But it’s not just the separate birthdays that set these twins apart — it’s the fact that each one gestated in a separate womb.

That’s because Konovalova has a condition called uterus didelphys, which essentially means that she has a double uterus.

Uterus didelphys is surprisingly common — one in 2,000 women have it. The conditions occur early in development, when the two tubes that all female fetuses have, which normally fuse into one uterus, instead develop into two separate wombs. But the odds of a pregnancy like Konovalova’s, where each baby develops in its own womb, are miniscule — just one in 50 million, Scientific American reported. (For a woman with uterus didelphys, the odds of conceiving one twin in each are 1 in 25,000).