The 15-year-old boy was handed the scalpel and allowed to make the incision as part of his on-the-job training.
The new mum had had an epidural and was awake throughout the procedure, but did not see who had made the cut.
However, she did know that the student would be observing her operation.
The incident happened in a Swedish hospital, where the doctor who allowed the untrained teenager to help out expressed regret.
A spokesman for the the hospital, in Malmo in Sweden, said: "I don't know how deeply the [student] cut, if it was a mere scrape or a cut several millimetres deep."
The hospital says the woman was never placed at risk and so there is no need to report the incident to the authorities.
According to The Local newspaper, the hospital's regulations say no students under the age of 18 are allowed in an operating room.