The Independent newspaper said Clara Rojas reportedly had the child two years ago, but remains a captive of the Revolutionary Armed Forces of Colombia, or FARC.
The relationship between the aspiring leader and armed rebel was consensual, reported the Independent, which quoted the recently published book: "Latest News From the War" by the Colombian journalist Jorge Enrique Botero.
The FARC has been fighting with government forces for more than four decades. The war claims some 3,500 lives a year. An estimated 1,500 hostages have been taken, including Rojas' former running mate, Ingrid Betancourt.