A few weeks ago, I was tidying up when I found a long-lost pot of face cream under the bed. As I unscrewed the lid, I felt a rising flutter of panic. A year ago, when it happened, I stopped using the cream. No point; no time. But, having found it again, I washed my face and patted it dry, then rubbed the cream around my eyes, into my cheeks, down my neck, and got into bed and slept well. "I'm not very good at babies," I would tell people. "I find them a bit boring." Boring was a lie. I didn't find my baby daughter boring; I found her unfathomable, frightening. I didn't know why she cried and I was frantic when she did. With the help of a baby manual, I soon had a lively child who ran like clockwork and hardly ever cried. Everyone commented on how "good" she was. But I didn't know if she was good - I didn't know what she was.

I called it "The Year Of Slip-On Shoes".

There never seemed to be time for laces, just as there was never time to read, hang up laundry, phone friends or eat properly. Rather than feeling irritated by just-missed buses, unannounced rainstorms, off-hand GPs, long post-office queues or sweet-potato puree on the floor, I felt beaten. I told a friend that it was "the worst year of my life".

My one stroke of luck that first year was finding a wonderful childminder. I returned to work and, bit-by-bit, to normal. By the time my daughter started to copy animal noises and sing "Hot Cross Buns", I found myself jolted by the excitement of being with her. I started creeping into her room when she was asleep, thumb in mouth, to stare at her and listen to her breathing. Something was finally coming to life within me.

For a long time, the prospect of having another baby seemed like a bad joke.

Yet, as the bleakness receded, I prepared for round two. When we came home from the hospital with our second child - a boy - it was immediately apparent that this time everything would be fine. Night feeds were quiet and quick, and our three-year-old was besotted. When the baby was about 10 days old, I found myself stroking the pale hair on his temple and was swept away by a wonderful 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. But then, one morning, a few days later, 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; now, I was staring down into a black hole. I got up, fed everyone and took the pram to the supermarket, moving through the aisles in a daze. It all seemed too bright, too loud, too sharp. I came home and cried. How stupid I'd been, daring to hope it'd be different this time. After one particularly wretched day I went online and furtively looked up postnatal depression (PND) websites. On one I found the Edinburgh Scale, a questionnaire designed to pick up signs of PND. My score was 19. At the bottom of the page, it said, "A score of 12+ indicates the likelihood of depression, but not its severity." I rang my community nurse and said, "Mary, I don't feel like myself." With Mary's quick acceptance and confirmation from my GP, the shadows began to shrink back a little.

I'd always been rather dismissive about depression, but now I found myself grabbing on to the diagnosis, willing it to lift me out of the blackness. Hope began to sneak back in as people told me it didn't have to be like this.

My husband started coming home earlier and my parents, who had both been so generous with their time anyway, decided that one of them would come every day to take the baby for a walk, play with my daughter, buy groceries or just keep me company. My GP prescribed the antidepressant sertraline, but I was almost as scared of the medication as the illness, and left the script in my pocket. Perhaps I wouldn't need it, I hoped.

Searching for a pattern, I had begun to make marks in my desk diary on my bad days, and the four days after that fateful call to Mary are unmarked. Then, in the middle of Wednesday, there's a huge "X". The baby cried hard when I put him down for a nap. As the noise twined down the stairs, I felt the sweat springing 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. 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.

After a while, my appetite seemed to have dwindled away to nothing: a side effect of the medication, like the dry mouth, upset stomach and light sleep. On a family outing one day, my daughter announced to her father, "Mummy's crying again", and we immediately went 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. At the weekend, my husband drove us all over to my parents'. My mother helped me into bed and, for the first time in days, I slept. When I woke up, my parents' GP was on the phone. A kindly voice told me that if the medication was making me feel like this it was probably wise to stop. I took her advice. On the Monday ("X"), my husband and I went back to see the GP. "You were on the lowest dose - you've been extremely unlucky," she said. Apparently, of every 100 people prescribed sertraline, one will have a bad reaction. I took away a different prescription, but left the packet unopened in a drawer. Tuesday, another "X". That night, feeling as if it was the bravest thing I'd ever done, I popped the first pill from the blister pack.

As the sertraline worked its way out of my system, I started to edge towards feeling normal again. It felt like the most astonishing treat - t still does. Recently, for our son's first birthday, we had a tiny party: champagne and fairy cakes in the kitchen. It's been quite a year. I'm nearly off the medication and, 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, play - and finding long-lost pots of face cream under the bed.

But each flash of specifically baby-related joy leaves me with an underlying sorrow: to some extent, my son has been my first baby. In mourning my daughter's lost 12 months, I torment myself, wondering how much she has missed. 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 they are ill, and that it doesn't have to be like this. It might sound odd, but babies are amazing: I never knew.

Postnatal Depression Awareness Week runs from November 12 to 18. For more information on PND, visit www.panda.org.au.