Five weeks ago, a 20-year-old named Arifa Sultana delivered a baby in a hospital in southwestern Bangladesh. As the BBC has reported, Sultana had received limited prenatal care in her rural village, but she had no problems during delivery and returned home with her husband and infant son. Less than a month later, on March 21, Sultana was rushed to another hospital with stomach pain and another burst of amniotic fluid. When doctors examined her, they realized she was in labor—this time with twins.

Sultana, it turns out, has two uteruses. In one, she’d been carrying her first son, and in the other, a second baby boy and a baby girl, each with their own amniotic sac. It was a rare type of pregnancy that begins in a way similar to the conception of any other fraternal triplets: three embryos, conceived within the same ovulation cycle (if not at the same time), gestating simultaneously. Which uterus each baby ends up in is as random as which mature egg a sperm cell happens to go for.