Until now, doctors believed that only identical twins — which come from a single, split embryo — can share a placenta, the mass of tissue inside the uterus that delivers nourishment to the fetus via the umbilical cord.
In Thursday's New England Journal of Medicine, doctors at the University of Washington in Seattle reported that a 48-year-old woman gave birth about two years ago to male and female twins nourished by the same placenta. Each baby had his or her own umbilical cord and amniotic sac.
The babies were conceived via test tube fertilization with another woman's eggs. Experts said the procedure, in which eggs and sperm are mixed in a dish and the fertilized eggs are inserted in the woman's uterus, could have somehow caused the outcome.
"I think there are other cases out there" that were missed, said lead author Dr. Vivienne L. Souter. But "I think it's very rare."
Fraternal twins occur when two sperm fertilize two eggs at the same time; they are relatively common with test tube babies because multiple embryos are put in the uterus to increase the chances of producing a baby.
In this case, the babies were of the opposite sex, so they could not be identical twins. But the woman's doctors were puzzled over how there could be only one placenta. Initial blood tests added to the confusion, indicating a 99.9 percent likelihood the twins were identical.
Partly because of concerns the girl might have a rare genetic abnormality that can cause sterility, the case was referred to experts at the university, where Souter was then a fellow in medical genetics.
"The parents were really anxious" about the babies, Souter said. "We examined them. They looked like a completely normal boy, a completely normal girl. I initially thought, `This doesn't add up."'
Souter consulted with top specialists, including geneticists from Australia's twin registry, and they solved the mystery by doing sophisticated DNA testing of skin from each baby to prove the twins were not identical. Their blood was similar because they shared blood circulation in the womb.
"Because these things get so deeply rooted in medicine, it's very difficult to convince people that there's an exception to the rule," said Souter, who is on medical leave caring for her 3-month-old fraternal twins.
Dr. Laura Riley, chairman of the obstetrics practice committee of the American College of Obstetricians and Gynecologists, said this is the first such case she knows of to be so convincingly documented.
A 1970 journal report claimed this had happened, but was disregarded because of limited data.
Riley said whether there is one placenta or two is important because if twins share blood circulation, one can receive more of the blood, and therefore more of the nutrients and oxygen. This is dangerous to both babies.
Dr. Roger B. Newman, vice chairman of obstetrics and gynecology at Medical University of South Carolina, said this outcome is much more likely with test-tube babies.
"Somewhere in laboratory or in the process back toward the mother, (the fetuses) become disrupted and fuse together," he said.