Have you ever heard of uterus didelphys? Chances are, probably not. This is a pretty rare affliction where a baby girl is born with two uteruses. According to the Cleveland Clinic, uterus didelphys "only affects about 0.3% of the population," making it "one of the least common uterine abnormalities." 

Often just called double uterus, the condition is caused when a female fetus experiences trouble with the Müllerian ducts. Named after the German psychologist Johannes Peter Müller, the Müllerian ducts are tubes that eventually fuse together to form female reproductive organs, including the cervix, uterus, fallopian tubes, and upper vagina. However, sometimes these biological instructions get misread during the developmental period, and the tubes meant to form the baby girl's uterus are unable to properly fuse together. The result is uterus didelphys, where the tubes form two separate uteruses. 

Baby girls who are born with this condition are not always diagnosed right away; it's often only later in life when they report feeling intense pre- or post-menstrual cramps and a series of exams reveals the diagnosis. According to Nationwide Children's Hospital, these exams might include an ultrasound, MRI, sonohysterogram, or hysterosalpingography.