Angelly Arnache weighed in at 6kg (13lb) when delivered by Caesarean section at a Colombian clinic - the average size of a four-month-old.
Mercedes Pacheco, paediatrician in the northern town of Baranquilla, said: "She is the largest baby we have ever had in the clinic's history."
The girl's 25-year-old mother is not a diabetic - a frequent reason for extra large birth weight - and doctors are looking at other possible reasons for an unusually large infant.
Angelly is receiving oxygen, due to breathing problems, and an intravenous glucose solution to maintain a safe blood sugar level, but medical staff said she was otherwise in good health.
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