Sometimes I nearly forget it happened, and then I stumble on some tiny thing which brings it all back. One evening a few weeks ago, I was tidying up when I found a long-lost pot of L'Occitane honey face cream under our bed. As I unscrewed the lid, the sweet, intense scent spilled out into the room, and I felt a strong rising flutter of panic, a bird in the house.

A year ago, when it happened, I suddenly stopped using the cream. No point, no time. But having found it again, I washed my face in very hot water and patted it dry, and then I rubbed the cream around my eyes, into my cheeks, my forehead, my throat, and got into bed, and slept well.

This morning, I put on a jacket that hasn't been worn since last spring. In the pocket was a note on a torn scrap of lined paper: 'How do I know this is PND rather than me just failing to cope and needing something to pin it on other than my own inadequacies?' Oh yes, I must have written that before a GP appointment, frightened that the point of the visit would escape me at the crucial moment, or that I would be crying so hard I'd be unable to make myself understood.

When I was very pregnant with my second child, I bumped into a neighbour, Helen, on the street. We'd met at antenatal yoga classes in 2001 and delivered daughters on successive days. On her fridge, tamped down by magnetic letters, is a picture of our babies, side by side on a carpet, all four cheeks pink with teething. Now we had one of those chaotic pavement chats, trying to restrain small lunging girls while Helen jiggled the buggy containing her second daughter. She asked me how I was doing.

'Fine, OK,' I said. 'Well, dreading it, actually.'

'The birth?'

'No, that's the easy part. No, I'm dreading the bit after that. Can't believe I'm doing it again. It really was the worst year of my life.'

She looked at me for a moment. I could see how startled she was. 'Was it?' she said. 'It was the happiest year of mine.'

Helen and I had done the whole thing together. We'd introduced our babies when they were a few days old, on the way to the park on what must have been the earliest outing there. We'd shared hurried cups of tea under awnings swollen with rain, icy trudges along the high street as dusk fell, the usual moans about breastfeeding and centile charts. I'd always assumed we had everything in common. Here was evidence to the contrary.

Had Helen really been that happy? Had I been that unhappy? When I looked back, I remembered mostly a numbing grey monotony: the circuits around the block trying to use up another half hour, the sprints to the GP, the anxiety that set in at naptime in the knowledge that, when the baby woke up, I'd have to find some way to entertain her. Had I simply blocked out all the good bits?

'I'm not very good at babies,' I used to tell people who asked how it was going. 'I find them a bit boring.' Boring was a lie. I didn't find my baby daughter boring; I found her unfathomable, frightening, out of my league. I did not know why she cried, and her crying drove me frantic with worry. It took me a while to work out that mostly she cried because she was tired. Soon, with the help of Gina Ford's The Contented Little Baby Book, I had a lively baby who ran like clockwork and who hardly cried at all. She slept through the night very early; by day, she was blissed out in her cot for a safe two hours every lunchtime. Everyone commented on how 'good' she was. I didn't know if she was good. I didn't know what she was. I couldn't imagine how the other mums I knew, whose babies dozed off for 20 minutes in the buggy here and there, coped. What did they do with their babies for the rest of the day?

In my head, I called it The Year of Slip-On Shoes. I lived in my Birkenstock clogs and, if it rained, my heels got wet. There never seemed to be time for laces or zips, just as there was never time to read, hang up laundry, buy stamps, phone friends or eat properly. This sense of living flat-out began to show up in the form of colds, styes, mouth ulcers, rhinitis. Bad luck took me under siege; the simplest things defeated me. Rather than feeling irritated by just-missed buses, unannounced rainstorms, offhand GPs, snaking post office queues, sweet-potato puree on the floor, I felt beaten. I cried in shops all the time. My margins had gone. And yet that is where I had ended up: on the margins of my own existence. 'Something is pushing them/To the side of their own lives,' a line from Larkin, came back to me as if on a loop.

If you need one, there is a sort of baby manual: you can find it in the childcare books, or alongside the baby-shampoo samples in the Bounty pack you're given by the midwives after delivery. Grateful that someone seemed to know what needed to be done, I went through the literature, ticking all the boxes. I fed my child, winded her, changed her, bathed her, cut her nails, shook toys at her, sang nursery rhymes and kissed her. I also felt sorry for her. I knew she'd been shortchanged. Dull-witted, joyless, I really wasn't much of a mother.

Though I had no energy, parenthood seemed to have given me a strange electrical charge. Within a few weeks of the birth, I was reaching out to light switches, sure in the knowledge that the bulb would flare and die, and getting a small grim shock of satisfaction when it did, even though the business of replacing it seemed to require infinitely more time and effort than I could spare. I'd never bought so many bulbs.

Eventually, we called in an electrician, but he couldn't find a problem. Still, it was obvious something was wrong with the wiring. Really badly wrong.

As far as I was concerned, my one stroke of luck that year was finding wonderful childcare in the form of the warm and capable Monika. I returned to work and, bit by bit, to normal. Around the time my daughter started to copy animal noises and sing the tune of 'Hot Cross Buns', I suddenly found myself jolted by the excitement of being with her. I began memorising things she had done or said and passing them on to her father and grandparents. I started creeping into her room when she was asleep, thumb in mouth, comfort muslin in fist, to stare at her and listen to the peaceful sound of her breathing. Something was coming to life. Lamps stayed lit.

For a long time, the prospect of having another baby seemed like a very bad joke. Having had one, I simply couldn't see why you would want to go through it again. When a friend got in touch to say she was expecting her third child, I emailed back, 'Are you completely mad?' The horror of the first year became my party piece. When other mothers looked at me sidelong, I told myself they'd forgotten, or were deluding themselves. I was in good company: I'd read Helen Simpson and Rachel Cusk. And yet, as the bleakness receded, I took a deep breath and prepared for round two. As my husband pointed out, 'Everyone says it's much easier second time around.'

Exactly a year ago, we came home from the hospital with another baby. It was immediately apparent that, this time, everything was going to be fine.

Night feeds were quiet and quick. Our three-year-old was besotted.

Standing on the doorstep, my midwife laughed at the way I was holding our son, hammocked in one arm: 'You old-timer,' she teased.

When the baby was about 10 days old, I was feeding him on the sofa, watching a little muscle behind his ear moving with each swallow, when I found myself stroking the pale hair on his temple with my fingertip and was swept away by a strange, wonderful, disorienting flood of emotion so strong that, if I'd been standing up, I'd have been knocked off my feet.

So this is what it feels like, I thought, amazed. It was as if a red velvet rope had suddenly been lifted for me; as if I'd suddenly proved myself worthy of entry to some legendary but difficult-to-locate event.

One morning, a few days later, as the baby stirred in his moses basket next to the bed, I opened my eyes and, without any sort of warning, found myself thinking, 'Oh no. Not another day of this.' When I'd gone to sleep, the world was full of colour, and there seemed to be a point to everything; now, I was staring down into a black hole. I got up, fed everyone, pulled on all our coats and took the buggy to the supermarket, moving through the aisles in a daze, flinching at the interaction. It all seemed too bright, too loud, too sharp. I came home without milk and carrots, and cried as I put the butter away. This time, the bleakness was compounded by disappointment. How stupid I'd been, daring to hope it would be different this time.

My parents, who live nearby, came up in the afternoon, when my daughter was at nursery school, to take the baby out for a walk. I could hardly wait for them to leave so I could drag myself upstairs, draw the curtains, and slide into bed. In the quiet house, another line from Larkin came at me like a fist: 'Unreachable inside a room'. All I wanted was to be unreachable. I lay on my back, tears trickling into my ears, and clutched the sheet. Silence, darkness. My mind emptied. Distantly, the front door banged open, and I could hear my parents, chatting as they manoeuvred the buggy back into the hall. The hands of the alarm clock showed that an hour had passed. The baby would need a feed. Somehow, I got out of bed.

So time flew when I had it to myself. But at other times, when I needed it to speed up, it crawled with a terrible, sickly lethargy. One morning, I sat on the floor next to the sheepskin while the baby experimentally moved his arms and legs. I looked at my watch. His next nap was hours off. The morning yawned ahead of us. I had no idea how to get through it. Jerkily, with substantial effort, the baby moved his fist against the orange giraffe on his babygym. Desperate, I checked my watch again. Twenty seconds had gone by.

Stumbling through the days as though through sand dunes, painfully conscious of my failure to enjoy anything, I stopped bothering to apply face cream or pour posh stuff in my bath. My wardrobe shrank to a grey jumper and a pair of old cords. Lightbulbs started blowing again. Pop, pop, pop.

When my husband was at work and my daughter busily occupied with nursery school or Monika, my world consisted of feeds and naps, and my son was such an obliging baby that he even made these easy. Yet this surprise gift, like all my other advantages - whether fortuitous or deliberately built-in - only made me feel worse. Here I was, with a conspicuously good-natured baby, loads of affectionate childcare for my lovely daughter, understanding husband, enthusiastic family backup - and still I was unable to cope. My husband, midwife and health visitor all said it would get easier, not to worry. But worrying was all I could do.

It was on the relatively rare occasions when the baby cried that I felt myself spinning out of control. At those moments, my anxiety seemed to burst out of my head, forcing its way into every organ and muscle in my body. Sweat poured off me, my pulse raced, I struggled to get air into my lungs and my stomach filled, as efficiently as a lavatory cistern, with acid. When he fell asleep, I could not enjoy the peace for dreading the next storm, those exhausting tidal surges of adrenalin.

Two or three weeks went by. After one particularly wretched day, I went online, found the Mumsnet website and furtively looked up some old PND threads. On one of these I found the Edinburgh Test, a questionnaire designed to pick out signs of post-natal depression. Oh, I remembered the Edinburgh Test. My health visitor had run me through it when my daughter was a few months old. Not wanting to appear a miserable wuss, I'd put a positive spin on things, thinking that everyone would, and at the end, when she said, 'Oh, you're absolutely fine,' I'd felt a dull ache of disappointment.

It felt like a cheek, doing the test again: I didn't have PND. I wasn't ill or anything; I wasn't contemplating harming myself or the baby. I was just stuck in the baby blues. But, embarrassedly, I did the test anyway. My score came up: 19. At the bottom of the page, it said, 'A score of 12+ indicates the likelihood of depression, but not its severity.' The next day, a Friday, I rang my health visitor and said, 'Mary, I don't feel like myself.'

With Mary's immediate acceptance and confirmation from my GP, the shadows began to shrink back a little. It seemed that the reason why I felt so lost in motherhood might have less to do with my failings as a mother and more to do with something being wrong with my head. Until this point, having never encountered it, I'd always been rather brisk and dismissive about depression, but now I found myself grabbing on to the diagnosis, trusting it to lift me out of the blackness. Hope, an emotion I'd given up on absolutely, began to sneak back in, as people I trusted told me that it did not have to be like this.

Finally, I had the glossary for my misery. My husband told his employer and started arriving home earlier than usual. My parents, who had both been so generous with their time anyway, now cleared their diaries and announced that one or other of them would come up every day, to take the baby out for an afternoon walk, play with my daughter, buy groceries, or just keep me company. I'm ashamed to say I have spent my whole life taking my parents for granted. Well, I won't make that mistake again.

My GP prescribed me Lustral (sertraline), one of the newer SSRI antidepressants compatible with breastfeeding, but I was almost as scared of this sort of medication as of the illness, and left the green slip in my pocket. Perhaps I wouldn't need it. After all, things were getting better.

Even if I could not experience happiness quite yet, I had begun to believe in its existence again, and that felt like progress.

Searching for a pattern, I had begun to make marks in my desk diary on my bad days, and the four days after that call to Mary are unmarked.

Then, in the middle of Wednesday, there's a huge 'X'. It was a bright day, the lemony winter sunshine pouring on to the floorboards and the dark pink rug, the litter of the dolls' teaset and scattered plastic rattles. The baby, overtired, cried hard when I put him down for a nap. As the noise twined down the stairs, I felt the sweat springing in response from my palms and forehead. The room seemed to be compressing itself around me, crushing the air out of my lungs. Acid jetted into my stomach; I could feel the level rising by the second. The baby stopped crying after a moment or two, but I did not. Light and air seemed to have been sucked out of the afternoon. My father took the prescription and went to the chemist.

I told my mother I thought I might be going mad. 'You're not mad. You're sad,' she said. I took the pill at bedtime.

The next day I was slightly groggy but coherent: the diary page remained blank. My health visitor had arranged for me to see a counsellor in the afternoon and guilt mounted as I told this pleasant person about my easy baby and my attentive family. Seeing her actually made me feel worse, a fraud, a time-waster. I told her apologetically - and aware that this itself sounded like an evasion - that I didn't think my PND was about 'issues', but some sort of chemical imbalance. One day I'd felt fine: the next, I was someone else. She asked me if I wanted to see her again; it felt impolite to say 'no', but I said it.

The following morning, my appetite seemed to have dwindled away to nothing: a side-effect of the Lustral, apparently, like the dry mouth, upset stomach and light, lurching sleep. As prearranged, my husband and I went to look around a local primary school. Sitting on a moulded plastic chair in the gym, surrounded by parents who looked smart, organised and in control, I felt that tight bright panic flowering and spilling over yet again, forcing every other thought from my head. My hands left damp stricken prints on my thighs. On the tour, when faced with a small table of expectant-looking nine-year-olds making pipecleaner portraits of the Tudors, I backed behind my husband. I no longer had the confidence to even smile at them.

This new jitteriness was a blow, but the information slip in the Lustral pack warned that increased anxiety was to be expected, so I tried to be philosophical. Another GP who spoke to me on the phone that afternoon, when my nervousness showed no signs of abating, told me that because it was now late on Friday I was in 'the NHS black hole': there was no real help available. Her only suggestion was to go to A&E if I felt worse, and ask for the duty psychiatrist. Having ascertained that I wasn't on my own, she pointed out that I was lucky to have my mum in the house with me: 'Lots of people aren't so fortunate, you know.' It was like talking to someone from Hawaii, or the moon, someone who had no idea what I was experiencing at all.

The next day, I was limp with tiredness. The tablets got in the way of sleep: I'd lain awake, thoughts tangling in my head, until 4.30am.

Mistaking my exhaustion for calmness, my husband said he thought it would do us all good to go on an outing. I didn't realise what a mistake this was until we were all in the car, when I felt the onset of another panic attack. But we were already on the way and our daughter was excited, so I kept my head down and gripped hold of the underside of my seat and tried not to scream, and it passed, and we got there and parked and found change for the meter and unloaded the buggy and unclipped the children from their car-seats and crossed the road and went in to see the dinosaurs.

At one point, standing by some fossils, I remember asking my husband how happy he was, at this moment, out of ten.

'Seven?' he said. He had his daughter's hand in his, his new son asleep in the buggy, and I was getting the help I needed. Things really weren't that bad, were they?

How could we be in the same place but having such different experiences?

Queuing at the cafeteria with a tray of sandwiches and tomato soup, I felt the terror surging up again, more demanding, more insistent than before. How on earth was I going to survive this? For the first time, I began to suspect that I couldn't. Perhaps I would get stuck in it.

'Mummy's crying again,' my daughter said to her father, making a worried face. I put the tray on the formica tabletop and, as quietly as I could, said I thought we'd better go home. I can't remember much about the next 24 hours, just a sense that family life was going on somewhere very far away from me, through a very thick insulated glass wall; and, nightmarishly, the theme music from CBeebies's Bobinogs

My parents and husband talked on the phone that afternoon. Someone said they thought I should stop taking the Lustral, but I refused and swallowed the next tablet: it was obvious, I had to get through the side-effects in order for the medication to work. The next day, my husband drove us all over to my parents'. I wasn't speaking much by then and I think I'd stopped crying, too. My world had shrunk to the darkness within my head.

There I was, trapped inside, stunned, while my family did and said things which I did not understand and which did not interest me. I remember climbing up the stairs to the flat where I had grown up, dully placing one foot in in front of another, and my dad opening the door and holding out his arms to me, and the feeling of his old jumper under my chin.

My mother helped me into bed, tucking me up under an old Rupert the Bear quilt and kissing my forehead. For the first time in days, I slept. When I woke up a few hours later, my parents' out-of-hours GP service was on the phone. A kind voice told me that if the Lustral was making me feel like this, I shouldn't feel worried about stopping. But I was worried, very worried. I was taking the pills to make me feel better, but that would only happen once they'd stopped making me feel worse. No, she told me, it was probably wisest to stop. I took her advice.

On the Monday ('X'), my husband and I went back to see the sympathetic GP.

'You were on the lowest possible dose - you've been extremely unlucky,' she told me. Apparently, of every 100 people prescribed sertraline, one will have a bad reaction. One in 100 sounded like very bad odds to me, especially since the tablets are presumably given to people who are already at rock-bottom. I took away another prescription, this time for an old-fashioned tricyclic antidepressant, which is also fine if breastfeeding, but put the packet unopened in a drawer. Tuesday, another 'X'. That night, feeling as if this was the bravest thing I had ever done, I popped the first pill from the blister pack, knowing that I wouldn't feel the beneficial effects for a month or so.

Wednesday, 'OK/good'.

Thursday, 'OK/good.'

I believe the sertraline was in some way a catalyst for the illness, plummeting me into a distillation of the depression; as if, in those poisoned four days, I'd gone through what might have otherwise taken four weeks, maybe four months. As the Lustral worked its way out of my system, I started to edge towards feeling normal again. A little bit shaky at first, for sure, but once again, distinctly myself. And normality began to feel like the most astonishing treat. It still does.

Yesterday, on our son's first birthday, we had a tiny party: champagne and fairycakes in the kitchen. My parents came, and my sister; my husband got back early from work. Our daughter blew out the single candle and toasted her brother with a teaspoon of Moet in an egg cup, and then the birthday boy got his first taste of refined sugar, mashing icing into his mouth and crowing with pleasure, flashing his four teeth. For all of us, it has been quite a year.

I'm nearly off the ADs now. As the dosage reduces, the happiness has remained constant, a sort of warm background hum as we all get on with the everyday business of work and nursery school and laundry and runny noses and tax returns and bolshy bus drivers and finding long-lost pots of face cream under the bed. But each flash of specifically baby-related joy (that whoop of uncontrollable laughter when you hot-potato his tummy, his delight when a stranger smiles at him in the street, the satisfaction he shows when he manages to say 'duck', or something rather like it) leaves me with an underlying sorrow: to some extent, my son has been my first baby.

So much for being 'an old-timer'. In mourning my daughter's lost first year, I torment myself, wondering how much she has missed out on. More than anything, I wish it could have been different for her, just as I wish it could be different for the children of the other mothers who suffer without knowing that they are ill, and that it doesn't have to be like this. Because it really doesn't.

Blimey, babies are amazing. I never knew.

· For more information, go to www.pnisha.org.uk.