A Colombian woman has given birth to a baby whose abdomen contained the tiny, half-formed — but still growing — body of her own twin sister.

This type of birth, an example of “fetus-in-fetu,” is very rare but not unprecedented.

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The condition was described in a British medical journal in 1808 and is thought to occur in about one in every 500,000 births. In recent years, similar births have occurred in India, in Indonesia and in Singapore.

The latest case was even more unusual, because doctors clearly identified the fetus-in-fetu during the pregnancy, said Dr. Miguel Parra-Saavedra, a high-risk pregnancy specialist in Baranquilla, Colombia, who oversaw the birth.