Leanne had felt a bit bloated just before Christmas, but was still having her periods and did not have any other pregnancy symptoms.
She woke up at 2.30am on Christmas Day with what she thought were severe period cramps.
Her mum Melanie took her to the Queen's Medical Centre in Nottingham, where staff realised what was happening and rushed Leanne into the delivery room.
At 8am she gave birth to her son Joseph, who weighed just 4lb 12oz.
From her hospital bed, first-time mother Leanne said: "It was a big surprise – I didn't know anything about it. I just became bloated in the last four weeks.
I always watched TV shows and thought 'how could you not know?' It's still sinking in really.
"I was shocked, then scared and worried and I didn't know if he was OK, but then I was happy.
"I didn't intend on having children for a long while yet but I'm happy and I'm dead excited."
The 21-year-old had recently started working at a shop in her hometown Ilkeston, Derbyshire and said she had carried on normal life throughout her pregnancy.
She said: "I did a lot of heavy lifting – he's had it tough, but he's fine. I just carried on life as normal, did my day to day stuff."
Leanne lives with her mum and grandparents but will be looking for a house with her boyfriend, Scott Curwood.
She said: "They're all very happy and dead excited. I told Scott straight away, he was shocked, but he's more than happy."
Doctors think Joseph was born around two weeks early and has suffered no lasting effects from his surprise entry into the world.
She said: "Joseph is definitely the most surprising, but the best, Christmas present ever.
Alison Atkinson, head of maternity at the Nottingham University Hospitals Trust, said: "I'm delighted that Leanne and her baby are both well and happy.
"It's rare for a woman to give birth without knowing she's pregnant – but I'm pleased that it was a happy surprise.
"It was also a lovely surprise for our team in the Emergency Department on Christmas morning.
"They were thrilled to be a part of this wonderful Christmas story – and have even clubbed together to buy a gift for the baby. We wish Leanne and her baby all the best."