The boy was three months premature and was taken to hospital with his mother, where he remains in intensive care.
The head trainer at the gym, Horacio Reis, said the birth had been "a surprise".
"A week ago I acted as a judge in a competition in Chile and this girl was competing. All went well and she won the competition," the AFP news agency quoted him as saying.
"I could see she was a big girl, round and strong. That's all I noticed."
The local press reported that Poblete was ultimately kicked out of the hospital because she couldn't pay the medical bills—suggesting that poverty and/or lack of health care may have been factors in Poblete's womb's sneak attack—though her hospital disputes the claim. [BBC]