Amy Furber, 19, was prescribed antibiotics for an apparent lump on her ovaries after she began losing weight and suffering sharp stomach pains.
But after months of tests doctors told her she was pregnant just nine days before Miss Furber gave birth to her son David.
Miss Furber said: 'I just couldn't believe it when they told me I was pregnant. I was totally surprised but I'm really happy.'
The supermarket checkout worker's ordeal began in March when she went to her doctor complaining of stomach pains and feeling tired.
Blood tests appeared to show a cyst on her ovaries and she was sent away with antibiotics - which women are warned not to take during pregnancy.
It never crossed her mind that she might be expecting and she even started to lose weight.
The teenager from East Huntspill, Somerset, continued partying with her friends. She even walked the two-and-a-half mile route of Somerset's Bridgwater Carnival last month - blissfully unaware she was heavily pregnant.
But days after the parade she returned to her doctor after getting a splitting pain in her side.
More blood tests were taken and the next day, on November 20, she phoned up for the results and a nurse told her she did not have a cyst - but was pregnant.
She said: 'I just didn't know what to do. I broke down in tears and called my mum straight away.
'I then went for a scan two days later and mum told me to expect to see a small bean-sized shape.
'But instead there was a baby smiling back. I thought it was someone else's scan - but the nurse said that was the baby inside my stomach.
'They said the pain in my side was the baby's legs kicking me. I just could not believe it.'
Amy was given a due date of December 13 and spent a frantic days buying nappies, buggies and baby clothes - many of which were donated by her local community.
But she went into labour two weeks early - just nine days after discovering she was pregnant.
Baby David was born at 7.45am on November 29 in Musgrove Park Hospital, Taunton, after a 14 hour labour, weighing a healthy 7lbs.
A delighted Amy named him David Martin, after her dad and brother.
She said: 'David is such a lovely baby and I am really pleased. Everything is just settling down now.
'I would like to say thanks to all the midwives at Musgrove Park Hospital in Taunton and to everyone who has donated bundles of baby clothes and equipment.
'Next time I'm hoping for a bit more warning!'
The baby's father Stuart Turner, 21, is said to be delighted about the birth, however Amy will raise the child as a single mother with the support of her parents Dave, 53, and Amanda, 46.
Mr Furber said: 'It was a real shock and the time but now it's marvellous - we are both really pleased for her.
'She didn't put on any weight during the pregnancy so no-one had a clue. But they did a scan and there he was - waving back at them all.
'The doctors said it can happen on occasions.'