Karen Humphries went the usual way about having her baby. She endured the joys and ordeals of pregnancy, grew gradually heavier for nine months, and went into labour at the expected time. But for Julie Kelley, the woman who has been charged with the abduction of Karen's baby, Abbie, it may have been very different.

We cannot yet be certain, but Julie Kelley may have been through a 'phantom' pregnancy - a rare phenomenon but well documented in psychological literature. It is a fascinating condition, especially to those who know what it feels like to be genuinely pregnant.

'I am as intrigued about this as everyone else,' confesses Dr Lorraine Sherr, a senior lecturer at the Royal Free Hospital in London. Dr Sherr, who has has four children of her own, specialises in the psychology of obstetrics.

'It ought to be very hard to fool people that you are pregnant, and it would take a midwife two seconds to discover that you are not,' she says.

'But there's a whole new move towards believing the woman and handing over control. If you go to your doctor these days and say you've done a home pregnancy test and missed a period, it's usually taken on trust that you are expecting a baby.'

Dr Sherr identifies two main categories of phantom pregnancy. One is the simple clinical misdiagnosis of a growth in the uterus: the doctor fails to read the signs, and the woman, who is growing bigger and missing periods, continues to believe that she is pregnant.

But the classic 'dream' pregnancy begins in the mind. Dr Sherr says that the most usual basis for such massive delusion is a desire to be pregnant - for instance, after the loss of a baby, sterilisation or diagnosis of infertility. Occasionally, however, it will stem from the opposite impulse: a deep fear of being pregnant.

A key question is how much the pretender believes her own fantasy. It is possible, of course, for a woman deliberately to mislead those around her and play an elaborate game of pretence. But as time goes on, it becomes increasingly likely that she will be drawn into the lie, inwardly convinced that she is with child.

'There are often a lot of social rewards and benefits from being pregnant,' says Dr Sherr. 'Someone who has negotiated themselves into that position may find it very difficult to negotiate their way out. The fantasy can start to take over, and you are sucked in.'

Pregnancy is rarely as blooming marvellous as the fairy-tales suggest. It may be accompanied by symptoms such as tiredness, sickness and a frequent need to urinate. But it also elicits the attention of those around the mother-to-be - they offer to carry her bags, force her to sit down, check her progress. When she gives birth, the inquiries after her health suddenly ebb away, and the baby becomes the focus of attention. So pregnancy can be pleasantly addictive.

'A woman may start to believe her own story,' says Dr Sherr. 'Especially if the pregnancy was created in order to cement a relationship or entice someone. It needs to continue in order to keep the partner, so it becomes a vicious circle. The psyche is a very powerful thing. During phantom pregnancy, periods can cease and the woman may grow larger. Evidence from America suggests that such women are often overweight anyway, so they can carry it off.'

It may not be a case of shoving a pillow up your jumper, which can be difficult to position realistically (soap-opera effects people, please note). Dr Sherr says nuances such as adopting a certain pose or rubbing your back may be more effective. A woman has only to wear an inelegant, smocked, maternity frock, lean back on the palm of her hand and refuse a glass of wine to have everyone guessing. And who would dare challenge her claim that she was about to give birth?

The response of the body to the mind's deepest wishes is perhaps the most astonishing delusion of all. Consider, for instance, a teenage pregnancy that the teenager desperately wants to deny. It is not unknown for a girl to carry her baby to full term without either feeling or looking particularly pregnant. The dream pregnancy employs the same powers in reverse. A woman may recreate six or seven months of virtual expectancy without having to employ any overt stratagem at all.

But the teenager and the pretender alike have to face their day of reckoning. 'The woman has created a whole scenario around the arrival of a baby, and the only way to resolve it is to get rid of the baby or to have it,' says Dr Sherr.

'Apparently, the woman in the baby Abbie case told people that the baby had Down's Syndrome or other problems. If this is true, she was clearly looking for a pathway towards 'losing' the baby.

'In these situations, the deceiver can display an uncanny intellect. And it is unclear to what extent people have control of what they say or do by this stage.'

What is clear is that phantom pregnancy, like its real counterpart, is a condition of increasingly complex implications. It may be sorted out through sensitive counselling, or through shattering events. Though numbers are too few to show a causal relationship with baby- snatching, Dr Sherr says that any woman deluded into believing she is pregnant is at high risk of doing something dramatic, quite possibly self-destructive.

'Phantom pregnancies need lots of understanding and expert counselling,' she says. 'There's very little that is systematic about these cases - the details will depend on each person's situation. But there is a theme - of loss and the burning desire for a child.'

Julie Kelley's desires may have brought her catastrophically into a drama that featured, albeit briefly, her own baby. For as long as she may have been 'pregnant', and then in charge of Abbie, she would have been enacting the experiences of a real-life mother. But, whatever her story - including unconfirmed reports that she is in fact four months pregnant - Julie Kellie now knows what it is like when your baby is taken away.