Ye canny make this stuff up I tells ya. Helen Smitham, 25, of Distington in northwest England admitted herself to hospital on December 24th, complaining of stomach pain. She reckoned that the difficulty was a dodgy kebab that she had snaffled the previous evening. What she didn't know was that she was actually giving birth.
The baby was born on xmas eve, weighing in at a rather small four pounds and 11 ounces (2.1 kilogrammes). The Yahoo news report (via the Sun, of course) also tells us that 'Smitham's boyfriend Mark Askew, 41, was also flabbergastered.' Hehehehe. I'd say he was alright...
I've heard anecdotal stories of this over the years, but have never encountered a case of 'unknown pregnancy' myself. According to Frankly Pregnant (a site containing advice for expectant mums and the pure curious): "Unknown pregnancies seem to be most prevalent in teenage girls. It's possible that some teenagers may not be as familiar with the sexual development of their bodies and/or may be in denial about a pregnancy."