'I Gave Birth The Next Day': How Do Women Give Birth Without Realising They Were Pregnant?

When preparing for a baby, it can feel like nine months is never enough time -- but imagine having less than 24 hours.

At the age of 21, Erin Bussenschutt decided she wanted to go on birth control.

When she went to the doctor to get a prescription, however, she was told they wouldn't be able to give her one.

"I said 'oh why?' and they said 'because you're already pregnant'," she tells The Sunday Project.

While at work the next day, Erin went into labour. She was 38 weeks pregnant and had no idea.

Erin with her son Andrew. Image: supplied

It sounds outlandish, but Erin's story of a surprise birth is far from unheard of.

In Britain and Germany, it's estimated one in 2500 births are a total surprise.

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The phenomenon is more common in women who are experiencing their first pregnancy and have strong abdominal muscles, so the baby rests closer to their spine.

Women often continue to have regular periods, grow no visible 'baby bump', and test negative when using home pregnancy tests.

Erin was almost 7 months pregnant in this photo. Image: supplied

Appearing on this weekend's The Sunday Project, Erin explains how there is still a culture of disbelief around surprise births -- people who can't accept the idea that women can carry a baby to full term without knowing.

"These pregnancies do happen," she said.

"I think the important thing is for when women say that they didn't know, we believe them. There's no reason not to believe them."

Catch the full story on The Sunday Project on Sunday 6.30pm.