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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.

The story of how a woman can carry three babies in two separate uteruses begins when the mother is inside her own mother’s uterus. In little female embryos, the reproductive tract begins as two thin tubes called müllerian ducts, the tops of which eventually become the fallopian tubes. Over time, the bottom of each tube grows wider, like an inflating balloon animal. Once they swell to meet in the middle, the puffed-up sections of the ducts fuse to become the top of the vagina, the cervix, and the uterus, leaving the still-narrow fallopian tubes dangling off each side.

In most women, this fusion occurs without a hitch. But in somewhere between 0.5 and 4 percent of the population, the walls of the müllerian ducts never really melt away into the surrounding tissue. A failed or incomplete fusion results in one of a few different anomalies. Some can be so minor that they’re never diagnosed: Plenty of women give birth from a uterus that has a small barrier protruding in from the top or a uterus that forks into two. But one of the less common müllerian abnormalities comes up again and again as a viral medical oddity. Women with a condition known as uterine didelphys don’t just have a misshapen uterus: They have two completely separate uteruses, cervixes, and sometimes even two parallel vaginas, with one ovary on each side.

Some cases of didelphys can cause issues such as menstrual blockage during adolescence, but the condition often remains a secret until a woman’s first pregnancy. Sultana reportedly wasn’t told about her didelphys until her second delivery. The doctor who delivered the young mother’s twins told the BBC that she was shocked to see the babies on Sultana’s ultrasound, and chose to deliver them via C-section for safety. In the United States, ob-gyns rely heavily on tools such as ultrasound technology to monitor pregnancies, keeping an eye out for risk factors such as didelphys so that they can plan ahead for any unique challenges expected during pregnancy and delivery. In Bangladesh, rural areas often lack access to diagnostic technologies, and the neonatal mortality rate is more than four times that of the United States. Pregnancy complications that are easily manageable elsewhere can be much more serious there.

Sultana’s story is remarkable because she faced the hazards of her rare condition without serious complication. For one, there’s the risk of miscarriage, says Beth Rackow, an assistant professor of obstetrics and gynecology at Columbia University’s Irving Medical Center. “Each uterus is more like half a uterus, not full-size. So we worry about space constraints, we worry about blood supply, and we worry about the placentas working as well.” Even if a pregnant woman with didelphys is carrying only one child, the resources her body provides to that smaller uterus can fall short of what a growing fetus needs.

Carrying more than one child at a time comes with its own set of risks. Deborah Levine, a radiology professor at Harvard Medical School, says that Sultana is “incredibly lucky” to have given birth to three healthy triplets, with only one preterm birth. Twin pregnancies in didelphic uteruses aren’t unheard of—journals seem to publish one or two case studies a year similar to Sultana’s, often from less developed countries. But safely carrying triplets in two uteruses is that much more unlikely. “Usually, twins deliver early and triplets deliver even earlier,” Levine says, which poses a risk to the infants that hospitals like to be prepared for ahead of time. “Somebody once said that if ultrasounds did nothing besides diagnose twins, it would be worth it, because of the extra care they need.”

Both Rackow and Levine were astonished that Sultana was able to deliver her first child and walk out of the hospital without anyone realizing that she was still pregnant. Though didelphys isn’t always noticeable during delivery or pelvic exams—barring menstrual complications, it can go undetected well past adolescence—it’s difficult to imagine why hospital staff might not be able to feel or see that someone was carrying around two nearly full-term babies, no matter where she was keeping them. (Members of the ob-gyn department at Khulna Medical College, where Sultana delivered her first baby, did not respond to requests for comment.)

Even with all of the risks Sultana and her triplets were fortunate enough to elude, there’s one key mystery in this case that, above all else, defies explanation, Rackow says—and it’s all about hormones. When a woman goes into labor, the brain’s hormone mission-control center sends a deluge of important chemicals coursing throughout the body to help move along the physically intensive process. The uterus in particular gets hit with wave after wave of oxytocin, which tells the muscles to push and push for all they’re worth. When that message comes in, there’s really no good way around it: Babies come when they come.

In the case of didelphys, says Rackow, there’s no reason the oxytocin produced during and after labor should be able to signal to one uterus but not the other. “The brain doesn’t know whether there’s one uterus or two,” she says. “And the uterine tissue just knows to react to that.” Labor in one uterus should absolutely trigger simultaneous labor in the other, she says. “It totally doesn’t make sense physiologically. In my wildest dreams I really can’t imagine what the explanation could be.”

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