Erin Bussenschutt was 21 years' old when a routine doctor's visit to begin birth control changed her life.

After taking a blood test before receiving the Implanon contraceptive implant, Ms Bussenschutt was told the procedure could not be carried out after all — because she was already pregnant.

She went into labour at work the very next day, delivering son Andrew 38 weeks and two days into her surprise pregnancy.

Erin Bussenschutt was five months' pregnant when this photo was taken. Photo / The Sunday Project
Erin Bussenschutt was five months' pregnant when this photo was taken. Photo / The Sunday Project

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Speaking with host Lisa Wilkinson on tonight's Sunday Project program, Bussenschutt revealed the brutal reality of welcoming a child into the world almost out of the blue.

While she was lucky enough to have the support of her family, Andrew's father has never wanted to be involved with his child.

"I contacted (the father) after he was born and told him this is what has happened and he made the decision of not wanting to be involved, that's what he stood by," Bussenschutt told Sunday Night.

She was almost seven months' pregnant in this snap. Photo / The Sunday Project
She was almost seven months' pregnant in this snap. Photo / The Sunday Project

"I felt absolutely gutted and I felt very alone … you know I had the support of my family, I had my mum and I had my dad, but it's different support from the family as opposed to, you know, the child's father."

But four years on, Bussenschutt - who is now engaged to a new partner and expecting her second child - wouldn't change a thing.

"He gave me a new purpose and a new reason for making something more of myself and he made me realise that there are more things in life that are important than what I was stressing about," she said.

Erin Bussenschutt welcomed son Andrew a day after learning she was pregnant. Photo / The Sunday Project
Erin Bussenschutt welcomed son Andrew a day after learning she was pregnant. Photo / The Sunday Project

However, Bussenschutt's surprise pregnancy is far from unique.

In fact, according to the Sunday Project, it is believed that up to one in 2500 births in Britain and Germany are a surprise, with the phenomenon more common in first pregnancies and in women with strong abdominal muscles, which means the baby is positioned closer to the spine.

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Fellow Aussie Caity Mason is another statistic, after delivering her son Flynn 10 months ago, just 24 hours after learning of her pregnancy.

Caity Mason also had a surprise pregnancy. Photo / The Sunday Project
Caity Mason also had a surprise pregnancy. Photo / The Sunday Project

Throughout the pregnancy, Mason had no morning sickness, still got a period each month, and showed no signs of an obvious baby bump.

On January 21, she visited her GP after experiencing cramps. She was sent to hospital, where she was told she was 10 centimetres dilated - and in labour.

"I was in complete and utter shock, it was just like, 'how far along am I?'

"Clearly far along, because I hadn't been with anyone for a very long time," she told the Sunday Project.

Like Bussenschutt, the father of Mason's baby has not been involved, a situation the young mum sometimes finds tough.

"I think I could do better, I worry sometimes that maybe I'm not enough for Flynn," an emotional Mason said on the program.

" … Father's Day was tough, because Flynn doesn't have a dad, and I don't want him to miss out on that, at all.

"He is the light of my life, the best thing that could have ever happened to me."

While many people struggle to believe a woman could carry a child for nine long months without realising it, Professor Euan Wallace, an obstetrician and gynaecologist from Monash University, told the program it was entirely possible.

"These pregnancies do happen … there's no reason not to believe them," he said.

"Some women do fall pregnant on the pill for a variety of reasons and for a small number of women when they have those pill-free weeks, they get a bit of light bleeding so they are assured or why would they expect they are pregnant."