"That's not her heartbeat." Megan learnt she was pregnant less than 12 hours before giving birth.

“One doctor looked at another and said, that’s not her heartbeat. At that point my stomach dropped. They said to me: you are pregnant and you are going to have a baby today.”

Megan Paolini was 21. She was a Melbourne business studies student, and had only been with her boyfriend Matt for less than a year. He’d rushed her to hospital thinking she had appendicitis.

Megan vomited in shock as she digested what she was being told. She was about to become someone’s mother.

Listen to the full interview on The Quicky. Post continues after podcast.

Megan’s son Theo made his entrance into the world 11 hours later, at a healthy weight of 3.49 kilograms.

“He was born in early Feb, and I’d only had my 21st in the November. I’d had a normal party, I was working part time, and I had been with Matt 10 months. He was my first boyfriend. We definitely weren’t in a serious relationship, we were cruising along in a very, very carefree unattached life,” Megan told The Quicky.

When Megan gave birth in 2015, her story hit the headlines. How could she have given birth, and not know she was pregnant? The backlash was brutal.

“What a joke how can she expect us to believe this.”

“Absolutely pathetic obviously she’s making it up for five minutes of fame.”

But as Dr Tamara Hunter, a specialist in obstetrics and gynaecology told The Quicky, while it isn’t common, it can certainly happen.

“It seems too hard to believe. But if we look at the early symptoms; nausea and vomiting in the first trimester could easily be confused with a medical illness like the flu, or anxiety or even indigestion,” she told host Claire Murphy.

Megan had very small bouts of morning sickness that lasted about two weeks. That was it. So she didn’t think anything of it.

“Not having a period can actually occur quite commonly. One of the most common instances where woman can have periods six to 12 months apart is polycystic ovary syndrome,” said Dr Hunter.

Female athletes, and those recovering from eating disorders can also not have their periods. As can those who are just stressed, or have poor diets in general.

In Megan’s case, she had been on the pill, and was purposely skipping her period.

When it comes to weight gain, Dr Hunter says that's the easiest to explain away.

"Gaining less than 6-12 kg can be quite normal in pregnancy, but also normal in society. Anything can be confused, poor diet, not exercising," she said.

During Megan's pregnancy she only gained 5kg.

"I'd always carried a little bit of extra weight around my middle, and I'd always had big boobs," said Megan.

As for the baby's movements, she didn't feel a thing. It was explained that her placenta was sitting between her and Theo in the womb, so any movement he made was completely padded.

"When Theo was born his legs were really tucked up tight, so after he was born we had to work on turning them out and getting them sitting properly. His natural reaction was to curl into a ball," she added.

To explain the absence of a baby bump, Dr Hunter says while some women carry in front, others carry behind.

"If a woman is overweight often it's difficult to determine. If the baby is quite small, some women don't get much of a bump at all," she explained.

She also suggests that psychology has something to do with it. So if you don't think you are pregnant, don't think you can get pregnant, or it's just not something that's even on your radar, you might not realise the signs your body is giving you.

Erica Dillworth was told she'd never have a baby. But when she went to get blood tests to start IVF, she found out she was 34 weeks along.

"As I discovered... I carry a pregnancy quite well. As soon as I found out though, my belly popped and I had to buy maternity clothes," she told The Quicky.

But Dr Tamara has a different theory.

"I think psychology has a huge involvement in reproductive medicine. The very fact that this woman had been told numerous times she was unlikely to be a mother, and it wasn't until the moment of realisation she saw the things she couldn't see before.

"Could her belly have just popped out? Probably not. Could she have seen things differently? Absolutely."

In Megan's case, she had initially been terrified about being a young mother. While she was in labour, she had conversations with a social worker organising adoption agencies they could team up with.

"I didn’t even want to entertain the idea of becoming a mother, a young mother at that. It was a stereotype I was so afraid of," she told Mamamia.

When Megan was discharged from hospital, baby Theo was put in a temporary foster home while she and Matt worked out what they were going to do.

A week later, they decided to bring him home.

"It would have been nice to have some more warning for what we were about to be thrown into, but I wouldn’t change one thing about Theodore. He is my son and the most wonderful thing that happened to me," she said.