It sounds pretty unbelievable, right? Going a full nine months without realising that you’re pregnant.

No morning sickness, no sore boobs, no baby bump. But, the truth is some women only find out they’re expecting a baby when they’re months into the pregnancy or, even more surprising, during the actual birth.

According to the BMJ, ‘Cryptic pregnancies’, as they are known, are estimated to occur in around one in 2,500 cases, suggesting that for around 320 women this is their reality.

With so many typical indicators of pregnancy, it may be difficult to understand, but according to experts it does happen.

“It is very rare for a woman to not know she is pregnant but whilst it is not common we do see it from time to time,” explains Mr Narendra Pisal, consultant gynaecologist at London Gynaecology.

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So how can you have a surprise baby?

“Firstly, every woman ‘carries’ their pregnancy differently and women who have a long abdomen, may have more space for their uterus to develop upwards rather than outwards which may give the appearance of a ‘smaller bump’ which can therefore got undetected for a while,” explains consultant gynaecologist Miss Meg Wilson.

“The female pelvis and abdomen is well designed to accommodate an enlarging uterus. As the uterus gets bigger with a developing pregnancy, the loops of bowel which fill the abdomen are pushed upwards and out to the sides.”

According to Miss Wilson the size of a pregnant abdomen is also influenced by the size of the developing baby.

“If babies are very small (growth restricted) then they may not take up much space,” she explains.

Your pre-pregnancy bodyweight might also impact the concealing of a baby bump.

“Obesity is also another factor, so a developing baby bump is not always as noticeable to see,” Miss Wilson adds.

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So why the periods?

Some women assume they can’t possibly be pregnant because they continue to have what seem like periods. But it is possible to bleed while pregnant.

“With regards to bleeding in pregnancy, it is not possible to have ‘periods’ through the pregnancy as this is shedding of the uterine lining (endometrium) in a cyclical manner,” explains Meg Wilson.

“However, some women do experience bleeding which can be a like a period during the pregnancy, sometimes from the surface of the cervix or bleeding from the edge of the placenta.”

And if you had irregular periods before pregnancy, that can also throw you off the ‘I’m pregnant’ scent.

Likewise, perimenopausal women may mistake signs of pregnancy for the menopause.

Baby kicks or bowel movements?

According to Liz Halliday, Deputy Head of Midwifery at Private Midwives there are some other explanations why women might not realise they are pregnant until they go into labour.

“A quiet baby or a placenta situated at the front of the uterus might mean that a woman isn’t aware of movements,” she says.

And women who suffer from stomach conditions, like IBS may also mistake movements for gas or bowel pain.

So there you have it, according to the experts, it is totally possible to not have any idea you might be pregnant until you’re actually in the delivery suite.

Now all you have to do is give birth.