This week a judge chose not to jail a woman who killed her two young children while suffering from an extreme form of postnatal depression. Such tragedies are rare, but evidence suggests increasing numbers of new mothers are struggling. Why?

There are few things more shocking than a mother killing her own children. But it is, mercifully, very rare indeed, and almost always a last act of desperation or a sign of temporary insanity. Which is why the judgment of Mr Justice Fulford this week, in the tragic case of Felicia Boots, who suffocated her 10-week-old baby and her one-year-old daughter in May this year, has to be applauded as a sea change in our attitudes to postnatal depression.

Judge Fulford did not send Felicia Boots to prison. He accepted a new plea of guilty to manslaughter on the grounds of diminished responsibility, and placed her under a hospital psychiatric order with the words: "A prison sentence would be wholly inappropriate … this is an almost indescribably sad case … I unreservedly accept that what the defendant did to the two children she and her husband so loved and nurtured was solely the result of psychiatric and bio-physiological factors truly beyond her control." What's more, he placed no time limit on the psychiatric order, but left that to the doctors.

All of the signs were there. After the birth of her first child, Felicia Boots had been diagnosed with postnatal depression, prescribed antidepressants and had seen a psychiatrist. There was a history of depression in the family – her brother had committed suicide four years earlier, and she was said to have been "mentally crushed" after the breakdown of her first marriage. Felicia and her husband had just moved into a new house, and she seemed unable to cope or complete simple tasks. And then, on this dreadful day, she had been surfing the internet looking for the effects of antidepressants on breastfed babies. She had stopped taking her medication two weeks earlier. Felicia then apparently had a paranoid delusion that her children would be taken into care, and killed them before attempting, and failing, to take her own life.

Puerperal psychosis affects roughly one in 1,000 women. Symptoms include hallucinations, delusions, impulses to hurt the baby or the belief that there is something wrong with it. Postnatal depression is a spectrum, with this type of psychosis at one end, and mild "baby blues" – weeping for several days after childbirth – at the other. But in the middle of these two extremes, many – if not most – new mothers experience profound lows as they struggle to adjust or cope with life as it now is.

The statistics suggest that roughly a quarter of women experience depression in the first year after childbirth. I believe, however, that the true incidence is probably far higher, for postnatal depression is often missed or misdiagnosed. The symptoms of feeling low and despondent, tired and lethargic, inadequate, irritable, tearful and unable to cope, as well as loss of appetite, insomnia and physical symptoms such as headaches and stomach pains are easily explained away by common postpartum experiences such as broken sleep, changes in marital relations and impaired health as a result of the physiological stresses of pregnancy and childbirth. Women are often reluctant or simply too tired to consult doctors, particularly if they expect having a baby to be nothing but a source of joy. Sometimes they don't even know they are depressed until they emerge from the fog and look back on those grey days.

New mothers have good reason to be depressed. They are often coping with radical change in every aspect of their lives at the same time. The experience of childbirth can be physically and psychologically traumatic. Given the lamentable state of some of our maternity wards and the paucity of midwives, countless new mothers experience much more stressful labours than they perhaps need to.

Once home, everything feels chaotic, with an endless cycle of chores and broken sleep, an overwhelming sense of responsibility and potential for a sense of inadequacy. How can one possibly know what a tiny baby might need, and am I really up to the task of looking after it, all the time? If the baby has colic, there is also the frustration of never being able to soothe one's own child. And if, as with Felicia Boots, a mother has two children under two years old, there is more than twice the work, and feelings of inadequacy can increase tenfold.

New mothers are often trapped at home alone, hundreds of miles away from their own families, as Felicia Boots was. Staggering numbers of couples move, like the Boots did, into new homes with small children or while pregnant, without the social support of local friends and neighbours who know them well. Research shows that low social support is one of the strongest links to postnatal depression. A recent NSPCC report found that over half of new mothers struggle to cope, 57% felt isolated and two in five admitted to getting angry. Other research suggests it is often as women and men (they can suffer, too) become more confident about their ability to soothe and care for their child that postnatal depression begins to ease.

A new mother has to come to terms with loss as well as gain – loss of her sense of self and identity, with no choice now but to put the needs of her child before her own; loss of freedom and a great deal of her former life; loss of her more youthful, childless body; loss of control, income or the ability to earn for herself; and perhaps even the loss of friends who are childless and consequently find it hard to understand where her priorities now lie.

Depression is a form of stasis as we adapt to radical change and accept loss. "Depression is about anger, it is about anxiety, it is about character and heredity," writes Tim Lott in The Scent of Dried Roses. "It is the illness of identity, it is the illness of those who do not know where they fit, who lose faith in the myths they have painstakingly created for themselves."

There are many ways in which accepting that sense of loss and adapting to motherhood has been made harder for women by cultural and social change. Childbirth may be natural, but that doesn't usually mean it is easy. In fact, bats probably have an easier time giving birth hanging upside down and then catching their young with their tails to stop them from splattering on to the ground below. When a new mother fails to have the intervention-free labour she planned for, or needs a caesarean section, too many feel as if they have failed before they have even begun on the remarkable journey of motherhood. Countless women, not so very long ago – just like Sybil in Downton Abbey – would have welcomed a c-section as a safer option if it would save their life and that of their child. So, too, would women throughout the third world, where childbirth is still a leading cause of death.

Just 100 years ago, new mothers weren't as isolated within the western nuclear family as they are today. Most upper-middle-class households had nursery maids and nannies. Poorer women were more likely to be dependent on a network of extended family and neighbours close by for support, and someone to just hold the baby.

Less was expected of a "good" mother, too. Children passed through people's lives, given and taken away by god. Now that we "choose" to have children, and know so much more about what the young need in terms of love, developmental stimulation and constant care, "good" mothers are expected to do and give so much more, which inevitably contributes to feelings of failure and inadequacy. Take the pressure to breastfeed – because breast is best, of course. Felicia Boots stopped her medication because she was worried it might damage her baby. So why did she not bottle-feed? Perhaps because that would have meant she was somehow failing her child. I will never forget the words of one new mother I interviewed for my book, Life After Birth, who said that she could never be friends with another mother who bottle-fed her baby. But what if that woman couldn't breastfeed for some reason? We forget how many babies died in the past because of an absence of nourishing and hygienic alternatives.

Guilt over working as a mother has mushroomed in recent decades, too, as women struggle to do both jobs well. Women have education, ambitions and a great deal to offer, and yet all too often it feels as if their own sense of sanity or the apparent welfare of their child has to suffer because of our appalling record as a nation when it comes to supporting working parents with a better work-life balance, flexible working and affordable, quality childcare. Recent research by Mintel found that six out of 10 stay-at-home mothers do not return to work because of the childcare costs. For rising numbers of low- to middle-income families (earning between £17,000 and £42,000), there is now little to be gained financially by having a "second earner" – according to a major study, Counting the Costs of Childcare, published by the Resolution Foundation last week – because of high childcare costs combined with the withdrawal of state support in tax credits. What does that do for the sanity of the mother who wants to work?

The pressure to be perfect, and to be seen to be perfect, is monumental. We have to be in control of our lives. Individualism matters more now, too, which means there is a sharper contrast between the more self-interested life of a woman pre-childbirth and the inevitable sacrifices of "good" motherhood. We have to be capable and achieve. But so many aspects of motherhood require us to give in to the reality that much is beyond our control, and that achievements come in tiny, incremental steps which are hard to measure.

We still do not know what causes postnatal depression. Some women simply struggle on, feeling under par. Others find it completely debilitating. When Lady Tebbit appeared on Desert Island Discs, she said her experience of postnatal depression was far harder to live with than the injuries she sustained when the IRA blew up the hotel she was staying in in Brighton during the 1984 Conservative party conference.

Judge Fulford's sympathy for Felicia Boots and the tragedy that unfolded will I hope contribute to greater understanding of her condition. But her dreadful story is just the tip of an iceberg. Postnatal depression is not an illness that afflicts only a small group of unfortunate women at the very end of the spectrum, who already have mental health problems and cannot cope: every woman is vulnerable, whether this is their first, second or fifth baby. And children are then vulnerable, too, for a depressed mother is less likely to be able to respond quickly and effectively to her child's needs.

With greater honesty about the difficulties countless new mothers face in adapting, and greater support for women during that crucial first year, the experience could be happier and more joyful. There is nothing shameful about saying that new motherhood is hard. Being a "good enough" parent is one of the hardest things we ever have to do.

Kate Figes is the author of Life After Birth, published by Virago at £9.99.