When Klara Dollan, then 22, woke up at 4am on the day she was due to start her new job, she thought her agonising stomach cramps signalled her period being “back with a vengeance”. She had been taking the pill with no break for more than six months, but had stopped about two weeks before. The waves of pain left her pale and shaking, but she didn’t feel she could call in sick on her first day – so she took some paracetamol on her mother’s advice, and caught the bus then the tube from the home they shared in Cricklewood in north-west London into the city.

Hours later, Dollan was in Hampstead’s Royal Free hospital, cradling a newborn baby girl: completely healthy and carried to term. Dollan had given birth by herself in the bathroom of her flat, after being sent home sick from work; a neighbour had heard her screams of labour and called an ambulance. When Dollan rang her mother and told her to come to the maternity ward, the reply was: “But you weren’t pregnant this morning!”

Amelia, now three, was a “complete surprise”, says Dollan, which many struggle to believe. How could she not have known she was pregnant? But the more pertinent question may be: why would she have thought she was?

Dollan had broken up with her boyfriend (Amelia’s father) five months before her daughter was born, and she was used to not getting periods. She had gained a little weight, but chalked that up to the breakup. A mirror selfie she took betrays no trace of her being seven and a half months pregnant. “There was nothing showing. I wasn’t feeling it. I had no symptoms, no cravings, no nausea – nothing. I was out of the loop of my pregnancy.”

In fact, the first time the thought she might be pregnant crossed her mind was as she was giving birth. By this point, it was clear this was no period. “My body was just telling me to push the pain away. Then I saw a head coming out.” What was she thinking? “I couldn’t tell you, honestly. I was in absolute shock.”

Last week, there were reports around the world of an extreme case of a woman being surprised by her own full-term pregnancy: a Bangladeshi woman gave birth to a healthy and expected baby boy, only to learn nearly a month later that she was carrying twins in a second uterus (they were also born healthy, 26 days after her first child). The physical circumstances in that case, and the fact that the woman knew she was pregnant with one child – but not three – clearly make it highly unusual. But the phenomenon of a woman carrying a baby to term without knowing she is pregnant is more common than one might think; as Dollan found out after giving birth to Amelia, this is known as “cryptic pregnancy”. A 2002 paper published in the British Medical Journal estimated that it occurs in about one in every 2,500 pregnancies, suggesting about 320 cases in the UK every year.

“This is not a particularly unusual phenomenon,” says Helen Cheyne, a professor of midwifery at the University of Stirling’s Nursing, Midwifery and Allied Health Professions Research Unit in Glasgow. “It’s rare – but it’s not that rare.” In midwifery and obstetrics and gynaecology circles, she says, if you haven’t come across a cryptic pregnancy yourself, it is not unusual to know someone – or know someone who knows someone – who has.

Early in Cheyne’s career as a clinical midwife, in 1982 or 1983, she remembers caring for a woman in the postnatal ward of the Princess Royal maternity hospital in Glasgow who had not known she was pregnant until she went into labour. She had given birth before – by then her children were teenagers – and she had chalked up her irregular periods and weight gain to age. Cheyne remembers her and her husband being in total shock. “I’ve never forgotten that. She was completely credible.”

And yet, she adds, it is “very, very hard to get your head around”. “The feeling of a baby moving inside you – if you’ve had children, it’s very hard to imagine how you might not recognise that for what it is. Having an 8lb baby inside you …” She laughs. She also adds that it is not only possible for significantly overweight women, as is commonly assumed.

Although the research is sparse – as one might expect, given the fundamental element of surprise – Cheyne says cryptic pregnancies have been recorded around the world, dating back centuries. In fact, it was more understandable when pregnancy diagnoses were dependent on indicators such as the loss of periods and nausea. With highly accurate modern tests, says Cheyne: “It’s very easy to diagnose pregnancy – if you expect to be pregnant.”

But the phenomenon cannot be explained away as women simply not feeling or noticing the signs of pregnancy, variable though they are. “Many people who are not expecting to get pregnant do get pregnant, and recognise that they are,” says Cheyne, adding that that is true even of women in war zones, refugee camps and other challenging situations where there may not be access to tests or healthcare. “If pregnancy symptoms were generally nebulous and not easily detected, [cryptic pregnancies] would happen all the time – so I think it must be something more particular to the symptoms experienced by these particular women.”

Cryptic pregnancy has been reported as a “psychological phenomenon”, says Cheyne, but she does not believe that applies to all cases. “Pregnancy is obviously a physical thing, but becoming a mother is social and psychological as well – maybe pregnancy is also.”

Understandably, when cases make headlines (a representative example: “Woman had no idea she was pregnant – until she gave birth in the toilet”), they tend to be received with incredulity, scepticism and lurid interest, as the stuff of soap operas and low-rent documentary series. Fifteen-year-old Sonia’s “surprise baby” on EastEnders in 2000 made a vivid impression on a generation of young women, while the US television series I Didn’t Know I Was Pregnant ran for four seasons. (In 2015, it was reprised for special episodes about women who had not one but two cryptic pregnancies, titled I Still Didn’t Know I Was Pregnant.)

That a woman could undergo so transformative a physiological experience as pregnancy without having any awareness of it seems to trigger deep-seated disbelief, especially among those who have experienced pregnancy. Dollan says people have questioned her common sense, her connection to her own body, and even the truthfulness of her story. She has found some mothers to be especially judgmental.

“When I tell them I didn’t have any cravings or morning sickness, that I didn’t have too bad a labour – that I just walked through pregnancy, if you will – they are like: ‘How could you not know?’ And almost: ‘How could you live with yourself not knowing?’” she says. “There’s a huge stigma, not only being a young woman who’s pregnant, but a young woman not knowing she’s pregnant.”

What about the reaction from men? “I don’t think they grasp it at all. Any man I’ve told has been like, ‘yeah, cool’, and seemed to have forgotten instantly.”

After she went public about her story on This Morning four and a half months after giving birth, Dollan says she was contacted by many women who had not spoken out about their own cryptic pregnancies out of embarrassment. For her, the proof of her cryptic pregnancy is self-evident. “All I can say to anyone who thinks I was hiding it is: why would I? Not only would I be putting my health at risk, I would be putting my child’s health at risk.”

That Amelia was carried to term and born healthy, without assistance, was a “miracle”, says Dollan, given that she had been working 12-hour days, 60-hour weeks in her hospitality job for her entire pregnancy. “I’d not lived the life of a pregnant woman for the past eight months. I was a bar manager, for Christ’s sake. I was carrying crates of alcohol up flights of stairs until I was eight months pregnant.”

Risk is inherent to cryptic pregnancy, in the gestation period but most acutely in the act of childbirth. Women can go into labour without medical assistance, sometimes in dangerous situations or entirely alone. Tragic cases where the child has been born dead or has died shortly after birth have led to the mother’s prosecution, says Cheyne, especially historically. “In a less understanding society, a woman could be charged with infanticide. People would say: ‘You must have known you were pregnant – otherwise how else would this happen?’”

Even a relatively straightforward birth of a healthy baby can be highly traumatic. “Most parents have nine months to prepare,” says Dollan. “I had two seconds – maybe a minute. Instantly, my life changed for ever.”

Unlike in Dollan and the Bangladeshi mother’s cases, past trauma can be an influential factor in pregnancies going unacknowledged, says Dr Sylvia Murphy Tighe, a midwifery lecturer and the course director at the Department of Nursing and Midwifery at the University of Limerick, Ireland. For her doctorate, Tighe studied concealed pregnancy: where women hide their babies from others and often, on some level, themselves. Given the link, she eschews the term “cryptic pregnancy” in favour of the broader catch-all “denied pregnancy”, which takes in the possibility of both conscious and subconscious rejection (although she considers the former far more common).

The 30 women she interviewed revealed “fluctuating levels of awareness” of their pregnancies, says Tighe. Some told her, years after the fact, that “they absolutely knew” even though they had said at the time that they hadn’t. Others had confided in one person – often a partner, a family member or a health professional – before denying it to everyone else, sometimes in response to that reaction.

The principal motivator, she found, was fear: these women were terrified, often for their own survival. There was also a close association between concealed pregnancy and trauma such as child sexual abuse, sexual assault and domestic violence, applicable to 11 of her 30 interviewees.

The remainder reported feeling more silenced by the social stigma of an unplanned pregnancy, fearing retribution or loss of control of their lives. (Although not all her case studies were Irish, Tighe said the country’s cultural resistance to unplanned pregnancies was a factor.) As such concealed pregnancy could be “externally and internally mediated”, says Tighe, one response was to cope by avoidance. “They might get this awareness of ‘Could I be pregnant?’, but they shut it down because a pregnancy, in their current life circumstances, is a really major crisis.”

Often the impact of this was only fully revealed with time, and in many cases therapy. Her interviewees had been reflecting, says Tighe: “Whether it was six years or 30 years after the event, they were looking back and they were ready to talk … It’s like a process of coming to terms.” At the time, however, they might feel only terror. One case study maintained that she had not known that she was pregnant until her third interview.

“We can avoid thoughts – we can push them from our minds,” says Tighe, especially if there are factors such as contraception or other medical explanations that can bolster that denial. One case study, a nurse from rural Ireland, recalled “blocking the thought”. “She said: ‘If I thought I felt a movement, I told myself maybe I had an ovarian cyst.’ She did not want to go there in terms of acknowledging that she was pregnant.”

These women’s desperate measures, says Tighe, are indicative of the need for an empathetic response to concealed pregnancy from healthcare professionals in particular – one that takes into account the lasting impacts of trauma on individuals’ approaches to motherhood. Sensational media reporting, too, did not help women to feel they could come forward.

For those women who had not experienced significant trauma but concealed their pregnancies, Tighe says, having a child was just not part of their “life plan”.

Dollan says that having a baby with her ex-boyfriend, aged 22, was not part of her plan. But she is also unequivocal: she did not know she was pregnant until she was in labour. “I would have had no qualms about telling my family if I did. Obviously, I would have been nervous to tell them – but there would have been a party, you know?”

She is also glowing about the joy that Amelia has brought into her and her mother’s lives. “It’s funny she’s so lively,” she says, “considering I didn’t feel her moving around.”

• In the UK, Samaritans can be contacted on 116 123. In Australia, the crisis support service Lifeline is on 13 11 14. In the US, the suicide prevention lifeline is 1-800-273-8255 and the domestic Violence hotline is 1-800-799-SAFE (7233). Other international helplines can be found at www.befrienders.org