Emily Wackett, 21, had no idea she was pregnant and believed she had piled on the pounds due to eating too much over the festive period.
She did not realise what was happening when she suddenly started having severe stomach pains while working a night shift.
Ms Wackett called her mother Lea Wackett, 60, and sister Vicky, 29, who rushed to help her at the Nightingale care home in Welborne near Dereham, Norfolk.
Emily Wackett did not know she was pregnant until the very last minute
The pair dialled 999 as they were so worried and a paramedic confirmed that Ms Wackett was in labour and having contractions.
Ms Wackett who had no morning sickness or baby bump during her pregnancy was helped into the back to be taken to the Norfolk and Norwich University Hospital.
But her baby boy weighing just under 7lbs was born in the back of the ambulance on the A47 at Costessey around five miles from the hospital.
Ms Wackett, who lives with family in Dereham, had been in labour for just over an hour when her son made his sudden arrival at 3.18am on January 31.
The 21-year-old thought she piled on the extra pounds through over-eating
She insisted that she had no suspicions about being pregnant when she turned up for her first ever night shift at work.
Ms Wackett said: “I suddenly felt a lot of pain in my stomach. I didn't know what was wrong.
"I know someone who had appendix problems recently so thought maybe that was it.
“I called home. My sister answered and I told her I was in pain and needed help.
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Her mother and sister joined her in the ambulance for the 15 mile trip to hospital as her son, who has not yet been named, threatened to appear at any minute.
I suddenly felt a lot of pain in my stomach. I didn't know what was wrong
Vicky, who is 11 weeks pregnant, said: “With the ambulance rushing to the hospital, I had to hold up the paramedic while Emily was giving birth.
“Mum couldn't breathe in the panic and had to be treated in the hospital herself."
Ms Wackett's boyfriend of two years Mathew Charles, 20, a banking and finance student at Middlesex University, was in bed 120 miles away when the drama began.
Ms Wackett also though she might have had appendix problems
He got a phone call in the early hours from Vicky's boyfriend Tim Baxter, 29, saying: “I don't want to alarm you mate, but we think Emily is having contractions."
The baby was born while Matthew was on a £170 taxi journey from London to the hospital.
He said: “I was in shock. My immediate concern was for the health of Emily and the baby.
“I just wanted to get there as quickly as possible. I spoke to Emily on the phone on the way up and was just so relieved to hear that everything was okay.
The little baby was born in the back of an ambulance on the way to the hospital
“When I first saw her and my baby in the hospital I didn't know what to say. It was incredibly emotional.
“We didn't plan this but we are so happy. We thought we might have a child one day, when we're a bit older, but hadn't talked about it seriously.”
Ms Wackett added: “Everything hasn't sunk in yet, but we're really happy. Our baby is really beautiful and is so quiet and well behaved.
“I've got a lot of support around me and we're adapting really well."
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The family were still mystified today about how Ms Wackett had been pregnant for nine months without knowing.
Lea who works with Ms Wackett, said: “We've been told that the baby may have been kicking the placenta, which is why she didn't feel any kicking.
“When she came into work that day I just thought she had the hump about having to do a night shift. I think it will put her off doing night shifts from now on.”
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