A British servicewoman who gave birth to a baby boy in Afghanistan on Tuesday is understood to have been "astonished" to discover she was pregnant.
The woman, who had been stationed at Camp Bastion in Helmand province since March, only learned she was going to give birth when she complained of stomach pains to military doctors.
Most women will find it incredible that a pregnancy would go unnoticed for several months or more, since there are numerous signs of pregnancy.
A woman's periods stop, morning sickness often occurs in the early stages and women feel their bodies changing and their tummy expanding as the foetus grows inside.
At around 20 weeks of pregnancy, most women start to feel their baby moving too.
But, while unusual, it is possible for a pregnancy to go undetected.
Patrick O'Brien, a consultant obstetrician at University College London Hospitals and a spokesman for the Royal College of Obstetricians and Gynaecologists (RCOG), says an obstetrician would see such a case about "once a year" - but there are no accurate figures.
He added: "It can happen. There are women who have infrequent periods anyway and also have no sickness, so these signs can be missed.
"Also if women are overweight they may not notice the bump getting bigger and they can also miss the baby moving.
"Sometimes, if all these things coincide, they can be unaware."
Women who are very fit may have no periods at all, which could make detection of pregnancy harder, and if the baby is small too, then there may be no real noticeable pregnancy bump.
More common are cases of teenage girls and young women who are in denial about their pregnancy, says Mr O'Brien.
"They're in a situation and the implications for their life are so great that they don't want to admit it to themselves or their parents, so they're just in denial that the possibility even exists."
Overall though, there are very few women who make it through to late gestation without knowing they are pregnant, he says.
Sue Jacob, a midwife teacher at the Royal College of Midwives, says every midwife has come across one or two women who did not know they were going to give birth and end up in A&E or calling an ambulance out when labour starts.
"It depends how aware women are of their body. Some just don't have an awareness of what's going on."
She says the numbers involved are small but they cross all social classes and age groups.
"There are menopausal women, teenagers and very affluent, highly-educated women. No one is immune," she says.
There are consequences for a woman and her baby of not realising or admitting that she is pregnant.
During an undetected pregnancy, women will miss out on the regular antenatal checks on high blood pressure and anaemia carried out by community midwives and miss out on scans to check the baby's growth too.
If pregnancies proceed very smoothly then babies can arrive in perfect health, but for some, complications can be missed without the benefits of regular antenatal appointments.