Doctors have removed the skeletal remains of an unborn child from its mother 36 years after the baby's conception, according to news reports.

The operation happened last week, and the case may mark the longest time a fetus from an ectopic pregnancy has stayed in a woman's body, The Times of India reported.

The 60-year-old woman, Kantabai Gunvant Thakre, began complaining about an intense pain in her abdomen about two months ago, according to The Times of India. After her doctor found a lump on the lower right side of her abdomen, the woman underwent an ultrasound to determine whether the lump appeared cancerous.

Instead, the sonogram revealed a calcified mass. The doctors then turned to magnetic resonance imaging (MRI), which can take detailed images of organs and structures within the body.

"It was after the patient underwent an MRI that the doctors could make out that the mass was in fact a child's skeleton," Dr. Murtaza Akhtar, head of surgery at Lata Mangeshkar Hospital in central India, told The Times of India. [9 Uncommon Conditions That Pregnancy May Bring]

Doctors learned that when the woman was 24 years old, she experienced an ectopic pregnancy, meaning that the fetus did not develop in the uterus as it should have, but rather elsewhere within her body.

It's unclear from the news reports where exactly the fetus was located, but ectopic pregnancies often involve implantation in the fallopian tubes, said Dr. Jonathan Herman, an obstetric surgeon at Long Island Jewish Medical Center in New Hyde Park, New York, who was not involved in the woman's case. In rarer cases, implantation occurs in the ovaries or abdomen, he said.

Doctors used to remove ectopic pregnancies with surgery, but in the past 20 years or so, more physicians have turned to a medication called methotrexate that essentially dissolves the pregnancy tissue, Herman said. The drug can be used early in pregnancy, typically before there is a heartbeat, he said.

In the woman's case, the pregnancy occurred in 1978. She sought treatment, but became fearful of an operation that would remove the fetus from her body, so she went home without having surgery, The Times of India reported.

The pain returned 36 years later.

This may be a case of lithopedion, a rare condition that roughly translates to "stone baby," Herman told Live Science. When a fetus dies outside of the womb, there may be too much tissue for it to be reabsorbed by the mother's body, he said. Instead, in order to protect itself against complications, the mother's body may calcify the outside of the fetus.

It's unclear why the calcified sac did not irritate the woman until recently. "I don't think anybody can answer that," Herman said. But the human body changes with time — for instance, in people who have their gall bladders taken out, the bowels may get caught on the internal scars from the surgery years later, he said.

In the Indian woman's case, a team of doctors performed a complex operation to remove the sac, which had settled among the woman's uterus, intestines and bladder, the Daily Mail reported. The medical team found a complete skeleton inside.

"The amniotic fluid that protects the fetus might have been absorbed, and the soft tissues liquefied over time, with only a bag of bones with some fluid remaining," Dr. Mohammad Yunus Shah, one of the surgeons who treated the woman, told the Daily Mail.

It's unlikely that doctors would fine a lithopedion case in areas with accessible health care, Herman said. In most women who have an ectopic pregnancy, physicians locate and remove the fetus.

The condition "is going to become more rare as modern medicine spreads throughout the world," Herman said. "With current medicine, you just don't see them."

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