Pregnancy is supposed to be a joyous time. Pregnant women, so the cliche goes, bloom and glow. For many women, though, this is is not the case. "As the days passed, nothing could distract me from the darkness rolling in," says Jodie Santos, a documentary film-maker who suffered a severe depression while pregnant with her first baby. "I started having panic attacks, usually when I couldn't sleep, and I knew I was in trouble."

Researchers estimate that around one in 10 pregnant women develop some degree of depression. Much has been done to raise awareness of postnatal depression, but studies show that a woman is more likely to be depressed while 32 weeks pregnant than she is eight weeks after the baby is born. But it is only now that prenatal depression is catching the attention of GPs, midwives and health visitors.

Being depressed in pregnancy means experiencing anything from anxiety, irritability and sadness, to obsessive compulsive disorder (OCD), agoraphobia and suicidal thoughts. Not much is known about why this happens. It could be down to hormonal shifts or any number of psychological or social factors: a history of depression or anxiety, an unplanned pregnancy, relationship stress, a lack of support from family or friends, a previous miscarriage or stillbirth, a past trauma or concern about the future. What seems undisputed is that pregnancy depression can happen to any woman, regardless of her personal circumstances or psychiatric history.

Delphi Ellis, a mother of two from Bedfordshire, "sailed through" her first pregnancy. A few years later, she suffered a miscarriage. She conceived again, but when 18 weeks pregnant, experienced some bleeding. "I went into a spin of panic," she says. "Though the baby was fine, I started to go downhill mentally."

She began going to the loo every 15 minutes in order to check that she was not bleeding. Then she started compulsively washing her hands. "I was afraid that I would somehow infect the baby and then he would die." This worry became overwhelming, "I would lie in bed and cry for three or four hours at a time. I felt completely out of control. I knew that if mother nature decided on a stillbirth or a premature baby there was nothing I could do. I became so obsessed with hygiene that once, when a woman brushed against me in town, I had to go home and wash. I didn't use makeup, or creams, or wear perfume in case it harmed the baby. I wouldn't even eat takeaways. It was an appalling, helpless feeling of huge anxiety."

As the weeks ticked by the depression deepened. "It got so I couldn't sleep for more than a couple of hours at a time. I felt isolated. I started comfort eating - I put on three stone in the last eight to nine weeks of the pregnancy. I washed my hands so much they cracked and bled. At one point, late in pregnancy, I stood at the top of the stairs and thought, "If I throw myself down, they'll have to take me in and deliver the baby."

Ellis's partner put the tears down to stress and hormones. "He was wonderful, but I was able to hide the true extent from him," she says. "It was like being two people: I knew I was being irrational, but the behaviour also seemed logical to me. The fears were layered on to one another - the fear of harming the baby, the fear of the baby dying, the fear that I was going mad."

For Santos, her profound sense of connection to her unborn child both inflated and soothed her depression: "I have read that being pregnant is the closest you'll ever get to the other side," says Santos, "with the soul inside of you straddling the worlds of darkness and light. Even in my most difficult moments, I have always felt this joy radiating from the life within me."

Sharing such complex feelings with others can feel impossible. Many women say that the pressure to sound upbeat while pregnant is immense. Nicky Stanley, professor of social work at the University of Central Lancashire, has studied how women with depression in pregnancy are treated by health professionals. "It can be very hard to disclose depressed feelings during pregnancy," she says. "The reactions can vary so widely." While some midwives, health visitors and GPs are very supportive, "some women say they were treated condescendingly, or not taken seriously."

Many midwives avoid asking difficult questions, says Stanley, "because they know that if they open the Pandora's box, they simply will not have the resources to offer the support that the woman will need." This year's guidance from the National Institute of Clinical Excellence on mental health in pregnancy and afterwards attempts to tackle such problems, outlining questions for midwives to ask pregnant women at their early appointments, including whether they have felt depressed, and whether there is anything they need or want help with. But clearly, many women will still slip through the net.

Ellis has set up a website and support group to help those women. Through this network, she sees the far-reaching effects of these depressions: "One woman had been desperate for a baby, but by the time she was four months pregnant she had become practically catatonic with anxiety, believing she would never be a good enough mother. She went to the GP who told her to 'just get on with it'. She terminated the pregnancy, and regretted it deeply."

It is important, then, for family, friends, partners and health professionals to take this problem seriously - to provide support, to listen and to find help. "The worst-case scenario is suicide," says Stanley. According to government statistics on maternal death, "mental health problems are the largest identifiable cause of death in the perinatal period." There is also a danger that a depressed woman will struggle to care for her other children. She may self-medicate using alcohol, cigarettes or drugs, struggle to eat well or show up at antenatal appointments.

Santos eventually realised that for the good of her baby she needed to get help. She saw a sympathetic doctor and was put on antidepressants (some, but not all, are safe to take in pregnancy). Ellis, meanwhile, had a switched-on midwife. "It was only when she noticed, at 32 weeks, that something was wrong, that I talked honestly," she says. Bedford hospital allowed her to come in whenever she was worried, and have the baby's heartbeat monitored (she showed up a lot). Though the waiting list for counselling was too long, she saw a therapist privately and realised that she had never come to terms with her miscarriage, and this at least partly explained her extremes of anxiety, OCD and depression. The good news is that there is no firm evidence that antenatal depression and postnatal depression are linked. As soon as Ellis's baby, Harvey, was born, "It felt like a black mist lifting," she says. "I've never been depressed since."

· For more help and info go to depression-in-pregnancy.org.uk