What's less obvious, though, is that it also affects new fathers. The research is new, and since the earliest stuff in the Eighties there's been a trickle rather than a flood.
So far, statistics about incidence and timing are unreliable, with few replications of results or methods. However, the most robust UK study so far found 9 per cent of men depressed at six weeks post-partum, with 5.4 per cent depressed at six months - though one study went as high as 20 per cent at six months.
New babies bring stresses, pressures, expectations, the need for more cash, broken nights - all of them felt by men as well as women. "There are similarities in the postnatal depression suffered by men and women," says Dr Malcolm George, from the men's studies research group at Queen Mary and Westfield College, University of London. "Many of them arise from the changed circumstances, and the problems both partners may have finding the emotional closeness to get through the difficulties .There's still an idea that good personal relationships are important only to women, and that's just rubbish. Postnatal depression is an exchange phenomenon. Each partner can make the other feel worse."
Classically, when men feel depressed, their strategy is to look for distraction - "I've had enough of this. I'm off to the pub" - leaving the mother feeling even more isolated. "Once you're depressed, your ability to solve problems is reduced, and things get worse," says Dr George.
Psychiatrist Clive Ballard said there's been little progress in research since his team published their study of incidence in the mid Nineties. "It's partly practicalities. If a a group of subjects is difficult to get hold of - such as fathers who don't know, or who don't admit, they're depressed - you tend to choose something easier to study."
Dr Ballard would like to see more active "seeking out" of fathers with depression. "In one or two areas now they routinely screen the partners of women with PND, but that reaches only some of the affected men. Why can't health visitors at least ask the mothers they're seeing, about their men?"
There exists, say some, what one psychotherapist calls a "benign conspiracy" to exclude fathers from active parenting. New parenthood produces female- only dialogues between grandmothers, sisters, midwives, health visitors and new mothers themselves. Adrienne Burgess, in Fatherhood Reclaimed (Vermilion, 1998), says the most usual interpretation of paternal depression is that the father is feeling displaced - sulking, in other words, like a "greedy child". She thinks it more likely that it springs from the fact that he is a "potentially active parent who has been disabled".
For all the talk about New Man, and the way even car ads are designed to appeal to men by showing their products as kid-carriers instead of babe-magnets, it seems we've a long way to go before fathers stop feeling marginalised.
Duncan Fisher, dad to Miriam, 21 months, aims to convince the UK's major parents' organisation, the National Childbirth Trust, of exactly this. Speaking at NCT's national conference recently, he challenged the overwhelmingly female Trust to let go, and to examine ways in which men are excluded from NCT and, more generally, from being active fathers.
"Everything to do with family life focuses on the mother. I remember, when Miriam was small, I felt miserable at the way the pressure was working against the egalitarian parenting my partner Clare and I'd both wanted. I felt a failure. None of the health professionals involved me; in the hospital, the teaching session demonstrating how to bath the baby was in the morning when most dads would've found it hard to be around, even if they'd known it was on. It's not that we want to be whinging victims - we want to play a real part in the lives of our children."
Duncan Fisher admits to intense irritation at cartoons about silly old Daddy keeling over at the idea of changing a nappy, and "jokey" pieces about disastrous afternoons looking after baby. "Who are these fathers? All the ones I know change nappies without fainting, and manage childcare perfectly well."
They don't feel resentful of their children "stealing" their partners, either - he rails at the idea that there are hordes of men who can't cope with their partner breast-feeding, for example, and feel jealous at the baby's use of the breasts.
"Is this really true? How many men feel like that? I thought it was a lovely sight."
However, Fisher, 36, who is from Crickhowell, says that professionally run groups are unlikely to work as well for men as for women. He points out that men who are depressed or miserable, or who seek reassurance in their new role, are probably unwilling to join groups. "Privacy's critically important. Maybe they'd join a fathers' group later, but not at first. But I'd like to see couples' antenatal classes extended, to six months after the birth."
He proposes imaginative use of the Internet, where men could seek mutual support or even professional advice, at any time of day or night. "There could be leaflets given to all new fathers, including information on depression, and an acknowledgment of the pressures they might be under." He would like to see more men brave enough to visit toddler groups: "I go along to a local one, mainly because Miriam adores it, but I feel uncomfortable - about as comfortable as a woman in a rugby club bar."
Ongoing studies show that the children of postnatally depressed mothers are affected behaviourally and developmentally; Malcolm George is certain that research would show similar effects on children of a father's depression. "We already know depression of any kind increases marital breakdown; it's important to get the message across it's not just mothers who get PND."
Duncan Fisher wants parents, and professionals, to accept that mothers and fathers are in it together, and that mothers and their babies, can only benefit from fathers' active involvement and support. "When you really meet the needs of mothers, hey presto, you've met the needs of fathers as well."
Heather Welford is the author of `Postnatal Depression' (Thorsons, pounds 5.99).
Paul Chapman, 31, pictured above with son Jacob, 4, and daugher Daisy, 6, didn't want help for his depression after Daisy was born
I didn't want people to think, `oh, poor sod, he can't cope with it all."
Paul, a civil servant in Birmingham, is father to Daisy, and Jacob. After Daisy's birth, Paul's partner Jane suffered severe postnatal depression, which took some two and half years to resolve with professional help. But Paul was suffering, too, from feelings of inadequacy.
Working long hours, and also following a part-time degree course, he got into the habit of drinking after work rather than coming straight home, sinking 10 bottles of strong lager two or three times a week. "I dreaded going home. The drink was a way of getting Dutch courage, stifling the worst of what I felt."
Even when he realised he'd have to seek some sort of support, he couldn't see a way of getting it. "Despite all the professional help Jane was getting, I felt I was expected to stay in the background. I was also getting the odd snide comment along the lines of `get your act together'. The only thing that kept me from walking out was the kids - I just couldn't leave them."
In time, Paul saw a stress counsellor at work about his alcohol problem, and as the pressure eased, he shared confidences with a (male) friend.
Looking back, Paul feels fathers are "pushed out of parenting. When problems arise, we're just expected to get on with it - but the truth is, we don't always manage to.
Paul Chapman is willing to offer support for new fathers. Write to him c/o NCT, Alexandra House, Oldham Terrace, London W3 6NH