Many new mothers have wept through an attack of “baby blues”, and most know that some mums are plunged into a worse state of despondency after their baby is born. Slowly, as awareness has improved, the stigma has been lifted from postnatal depression.

But for some women the black cloud descends during pregnancy rather than after it. A surprising number of women feel unable to cope at a time when they are supposed to be feeling euphoric.

Pregnancy manuals make no reference to depression before the birth. It is not mentioned at antenatal classes and is so taboo that sufferers themselves usually stay silent on the subject. Yet if they spoke out, they would realise that they are not alone.

Some research suggests that, far from antenatal depression being unusual, women are more likely to suffer an attack of the blues before the birth than after it. In a recent study published in the journal Obstetrics and Gynaecology, between 12 and 17 per cent of pregnant women were found to have significant levels of anxiety and depression. This comes as no surprise to Dr Jonathon Evans, a psychiatry lecturer at the University of Bristol, whose study of 9,000 pregnant women in 2001 indicated that 13.5 per cent were depressed during the third trimester of their pregnancy (compared with one new mother in ten who is thought to suffer from postnatal depression).

“There is this assumption that women just bloom during pregnancy, and it’s important to recognise that this may not be the case,” says Evans, who has since conducted smaller studies that produced similar results. “It should not be surprising if a pregnant woman is depressed.”

Women undergo a barrage of tests during pregnancy to check their weight and blood pressure and assess the development of the foetus, but are rarely asked how they feel – whether they are coping emotionally with the prospect of a new life turning their own upside down.

Annette Brierley, a midwife at St Thomas’ Hospital in London who also works for the children’s charity Tommy’s, says that antenatal depression can cause symptoms such as lack of interest in food or in planning for the future, compulsive behaviour such as hand-washing, days or even weeks of disturbed sleep, and feelings of isolation or possessiveness. “To many people, pregnancy is a happy time and pregnant women have no right to feel depressed,” she says. “But it is a problem that needs to be aired.”

Precisely what causes depression in pregnancy is not known, but according to Evans “both antenatal and postnatal depression are about as common as general depression and share its risk factors”, which means that “money worries, stress, a family history of depression and troubled relationships can all contribute”. Some experts think that the root cause is hormonal, with fluctuating hormone levels in pregnancy triggering mood swings in women who are sensitive to them. Others suggest that the pregnancy acts as a catalyst for underlying psychological problems.

Fear of childbirth itself may be another factor, and today a report into maternal mortality rates criticised the standard of care in some maternity units.

Moments of dark fear can strike any pregnant woman, says Liz Wise, an expert in postnatal depression for the National Childbirth Trust. Will she manage a painful labour? How will she cope with a child? And a career? Will she ever lose the weight she has gained, and will her relationship with her partner ever be the same?For most women these anxieties are fleeting and manageable. For women vulnerable to depression, though, the potential changes to their life-style seem overwhelming. “To them, there is this immense fear of a loss of control,” Evans says. “They are used to controlling their own lives and dread the fact that a baby will exert control over their daily existence.”

He and his team are about to investigate another theory: that depression during pregnancy is sparked by an evolutionary flaw. “We are looking at the idea that, as part of the preparation for being a mother, a woman’s perceptions change and her interpretation of the facial expressions of others is altered,” he says. “This might instil a deep fear in some women.” The results are not expected for two years, but this could help to explain why antenatal depression is so prevalent.

Whatever the reason, it is not just the mother who is affected. Stress hormones such as cortisol are thought to be transmitted through the placenta to the baby, and blood flow to the uterus is impaired, which increases the risk of either premature birth or low birthweight.

Dr Veronica O’Keane, a perinatal psychiatrist at King’s College London, measured amounts of corticotrophin-releasing hormone (CRH), which is associated with stress in pregnancy, in 25 expectant mothers in whom major depression had been diagnosed (but who were not on medication), and 35 who were not depressed. Her results suggested that the mothers with depression gave birth on average two days early, with three giving birth prematurely (before 37 weeks) compared with none in the control group.

“In my opinion depression is a major cause of preterm birth,” O’Keane says. “In about 30 per cent of cases there is no known cause, but within that group a large number suffer from severe psychological stress.”

Hope was raised for sufferers this year when the National Institute for Health and Clinical Excellence (NICE) acknowledged antenatal depression. But, says Ian Brockington, Professor Emeritus of Psychiatry at the University of Birmingham, their guidelines focus too much on drug treatments, which are not always the answer.

Antidepressants are not generally recommended for women during pregnancy – some, such as paroxetine, have been linked to birth defects – although in severe cases, and especially when an expectant mother feels suicidal, they may be prescribed. Indeed, a study published in The Journal of the American Medical Association last year suggested that women who have serious depression before they become pregnant risk a recurrence of the condition if they stop taking their medication. “Women with a history of depression should be very cautious about discontinuing their drugs,” O’Keane says. “If they do they have only a one-in-three chance of being well.”

Professor Brockington says that greater awareness of antenatal depression is paramount, and that it can usually be treated through psychological rather than pharmacological means. “All members of the maternity team should know about and be aware of mood problems,” he says. “The midwife has an opportunity to pick up on problems such as depression, substance misuse and alcohol misuse, though probably less than 5 per cent of mothers have any kind of problem.”

Many women find they recover “miraculously” as soon as their baby is born, though one in three goes on to suffer from postnatal depression. “Barriers surrounding pregnancy depression need to be broken down,” Evans says. “Women should not be reluctant to ask their GP for help when they are pregnant. What they need most is support.”

Where to go for help

Tommy’s, the baby charity, helps expectant mothers to explore and manage anxieties about childbirth or motherhood: tommys.org or 0870 777 3060. The National Childbirth Trust puts expectant mothers in touch with experts on antenatal depression and anxiety: nct.org.uk or 0870 444 8707.