Rakiya Abubakar was one of over 200 schoolgirls abducted from the northeast Nigerian town of Chibok in April 2014, army spokesman Sani Usman said in a statement.
He said soldiers had "during investigation of arrested suspected Boko Haram terrorists discovered... Rakiya Abubakar, with her six-month-old baby."
He said she was a senior school student at the time of her abduction but did not specify when or where she was found.
Of the 276 girls initially seized, scores escaped in the hours after the kidnapping. There are currently 195 schoolgirls still missing.
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In October, 21 Chibok girls were released by Boko Haram after negotiations with the Nigerian government brokered by the International Committee of the Red Cross and the Swiss government.
The release was hailed as a breakthrough that would lead to the recovery of remaining girls in captivity.
The Chibok girls drew global attention to the Boko Haram insurgency engulfing the country when US First Lady Michelle Obama joined the #BringBackOurGirls online movement.
Despite winning back swathes of territory from the jihadists, President Muhammadu Buhari has faced intense criticism for failing to recover the young captives, who became the defining symbol of Boko Haram's brutal campaign to establish a fundamentalist Islamic state in the country.
Nigeria has recently trumpeted a major victory in its battle against Boko Haram, claiming in late December that its army has routed the jihadists from their Sambisa forest stronghold.
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