Helen Thompson was asleep at her Salisbury home when she woke suddenly with stomach cramps. Her husband Dave was working a night shift, but the pain was so bad she phoned him at work, then called the doctor, who told her to go to hospital. The next day the couple returned home to introduce their two daughters, aged six and four, to their new baby sister Jessica, a bouncing 6lb 15oz and, insists Thompson, a complete surprise. "The doctors and nurses couldn't believe that I didn't know I was pregnant," she says. "I had been to work as usual on Monday, and the next day Dave had to phone in and say I wouldn't be coming in as I was having a baby."
It does indeed seem hard to credit that a woman could carry a child for nine months without experiencing morning sickness or tender breasts, or noticing a halt or change in the pattern of her periods. And, more to the point, is it possible that a woman could fail to notice an enormous bump where her belly used to be?
When EastEnders's Sonia Jackson gave birth out of the blue last year while making the tea for her boyfriend, one TV critic dismissed it as "fictional nonsense". But for once, the soap storyline seems not so very implausible. According to Peter Bowen-Simpkins, a specialist from the Royal College of Obstetricians and Gynaecologists, the phenomenon of "concealed pregnancies" is more common than one might think: "I would guess that if you ask obstetricians who have been in practice for 10 years, most would say they have come across cases where the mother didn't know she was pregnant."
In many cases, he says, the women say they have carried on having periods, or what seem to be periods, until late in the pregnancy. These "decidual bleeds" are difficult to explain, he says, but a woman who believes she is menstruating normally is less likely to notice other symptoms of pregnancy - if she has them at all.
Even the apparent lack of a swollen abdomen can be explained in unusual cases, for example if a woman is overweight, or very young. "For many of these women it is their first pregnancy, so they have very strong abdominal musculature," says Bowen-Simpkins. "I can think of three very slim women who managed to hide their pregnancies from everyone."
Claire Argo was particularly surprised to give birth to baby Liam last July since she had lost a stone and a half over the period in which she later discovered she had been pregnant. "I got a dull backache after work one day, which got stronger and stronger," she says. "Eventually I said to my boyfriend that I couldn't take it any more, and drove myself to hospital. They asked me could I be pregnant, and I said no. I had been on the pill and had been having periods all the way through. Looking back, I suppose they were slightly lighter than normal, but if you're still having them you don't think that something might be up. At 4am they did a test and told me I was in labour. Liam was born at two minutes to eight.
"He was a total shock. I didn't even have a bump. I had been dieting, so I probably put any changes down to that. But to be honest it was a relief, because I was worried about going to hospital in case they said I had a tumour in my spine or something."
The mystery, says Bowen-Simpkins, is why concealed pregnancies happen. He believes that in the majority of cases where women say they didn't know they were expecting, they may have had an inkling but didn't want to deal with the possibility and so blocked it out of their minds.
"Often women convince themselves they are not pregnant because they don't want to deal with it. They have deliberately hidden the facts from themselves. I don't believe they are lying, they genuinely do believe they are not expecting." This may also account for some of the lack of symptoms, he believes. "Lots of women who are very pleased to be pregnant, consciously or unconsciously will stick out their tummies a bit. Just the converse happens with people who are terrified of it sticking out."
"I don't believe that it's possible not to know when you are eight months pregnant with a baby kicking inside you," says Maureen Marks, a senior lecturer at the institute of psychiatry at the Maudsley hospital in London. "But I do believe that people can convince themselves otherwise. The human mind has a wonderful capacity to deny reality."
Studies have shown that a high proportion of women in this position have a "dissociative disorder", says Marks, where "they can dissociate from what's going on inside them completely", even to the extent where some are unable to experience physical pain until they see the wounds. The prospect of a baby may be frightening or inconvenient, she says, and a mother with a tendency to dissociate may manage to convince herself that it's not happening.
"We all have ways of denying reality, it's one of the mechanisms we have to defend against harmful experiences or knowledge. It's not something that you would send someone off to a psychiatrist about, but it can be dangerous in some cases if you don't notice things happening to you that can be harmful."
Argo says she was hurt, however, that some friends and colleagues didn't believe that she hadn't known. "I was not in a problematic situation. I was 27 at the time, so it's not that I had any reason to cover it up. I had been with my boyfriend for seven years, and so had a supportive partner and family." She and her partner Paul "wouldn't be without Liam now. He's wonderful."
Both Marks and Bowen-Simpkins describe Argo's case, and particularly Thompson's as "extremely unusual" in that neither woman fits the stereotype of a denied pregnancy - usually very young women on their first pregnancy. "I would never say always," says Bowen-Simpkins, "there are always exceptions. But certainly in my experience, most of these people have deliberately hidden the facts from themselves, though they are quite genuine in their beliefs. The mind is a very funny thing."