I knew it in pieces long before I could bear to recognise the central, howlingly obvious fact. I knew that I had gone two months without a period. I knew that my last bleed had been a vague and unconvincing thing. I knew that there had been an interval when I wasn't covered by the pill, and I knew that I'd had sex during it. I knew that I'd been sick when I took the morning after pill and I should have gone back to the doctor – but then, there'd been that bleed (in retrospect, probably a sign of an embryo implanting rather than the reassuring all-clear). I knew that I felt tired, and my breasts were swollen, and my stomach was growing round and distended in a way that no quantity of sit-ups seemed able to arrest.

I knew all of these things, but I did not know I was pregnant. And so I proceeded as a not-pregnant woman of 20 would, starting her second year at university. I did my reading and went to lectures and seminars, and in the evenings I went to pubs and gigs with my boyfriend and our friends. I had a plan, and no doubt whatsoever that my life would accord with it: two more years of studying, a First at the end of it, a year of travel with my boyfriend, then jobs in journalism which would probably mean London, a decade of career success . . . then, and only then, kids.

We were so sure that this was the plan that we discussed what we would do if an unplanned pregnancy intervened. “We couldn't have a baby now,” we would say solemnly to each other, “so we would have to get an abortion”. But to get an abortion, I would have had to know I was pregnant, and I couldn't let myself do that. I found out at an appointment to reissue my pill prescription. I handed over my urine sample for a routine pregnancy test, and watched my doctor perform it. Her mouth fell open – the proper jaw-flap of a person who was not expecting this – and she told me I was pregnant. At least 12 weeks pregnant, according to the internal exam.

This news, catastrophic as it was, came to me as a confirmation rather than a revelation: the breasts, the belly, the tiredness, the absence of menses. Ah yes, I was pregnant. I was also devastated. My GP sat me down, handed me a tissue, and told me all options were open. “But it's too late!” I cried – I didn't know the abortion time limit, but that summer the issue had been debated, and the idea of a first-trimester cut-off point had lodged with me. And then when my doctor reassured me that I had many weeks to make my choice, I snuffled,“And I've been drinking so much.” (Foetal alcohol syndrome had also had a moment in the news.) "That doesn't matter," said my GP, and she told me to come back the next day and discuss things further.

Before that conversation with my GP, I had no concrete idea what it meant to be pro-choice, although that's what I called myself. But by giving me that choice, my doctor gave me my life back when I felt it had just been taken from me irrevocably. Not every doctor would have done this. Some might have scared me with talk of how urgently I needed to make a decision. Some might have felt unable to offer me a choice, if they worked in a practice where that second signature was hard to come by. And some might have looked at me through their own religious or moral scruples, seen my case as the definition the ever-maligned “social abortion”, and refused to make it easy for me. I was lucky, and with that luck, pregnancy was no longer a thing so final that I was afraid to even acknowledge it: I could still decide.

I went to tell my boyfriend: we cried and cried. I told my mum, and my mum did not cry: my mum listened carefully and then said: “Whatever you decide to do, me and your dad will support you.” There was good fortune in that statement too. For example, I was fortunate that my parents could support me materially as well as emotionally – a spare room when I needed it, extra money for better food, the petrol money to come and get me if everything felt too much. But I was also fortunate that the emotional support was profound, and totally impartial of my choice. I knew that my parents would walk me into a termination appointment if I needed them to, and I knew that they would look after me at every stage of pregnancy and beyond. I knew it, because they had always trusted and cared for me like that.

I thought about the decision that was mine to make. And surprisingly, solidly, I realised what I would do: I would have this baby. At the time, I didn't know that there is a critical difference between unplanned and unwanted. At the time, I would barely have described myself as “wanting” children. I had never felt that cooing hunger which teenage girls called "broodiness", the longing to put their arms around a baby – even when small, I preferred reading to playing with dolls. And I will never feel the ravenous grief that older women call broodiness, either, the anguish of love with no object. But I did want a child, and specifically I wanted a child with the man I was with. It was ten years premature, but this was that child.

So I told my boyfriend, again, knowing that there was a terrible unfairness in what I was telling him. “I’ve chosen this,” I said, “but I’ll understand if you want to walk away”. It felt like the only choice I could fairly give him, but later he said it didn’t really feel like a choice at all: if this baby was to be born, he knew he would stay with me and look after it. And he did, unfailingly. One day, at about six months pregnant, I rang him up in tears about an essay mark. Incoherently upset, I choked out that I was “by the hospital”. He ran round to get me, and when I saw him he had a look of terror that suggested he was expecting something much worse than a low grade. “When you said you were by the hospital, I thought you were having a miscarriage,” he said, relieved that this pregnancy we hadn’t wanted would not after all be lost.

I cried a lot, actually. I knew I wanted this baby, but I didn't know if I would be able to finish my degree. (My department stretched every deadline to make it possible. I was lucky, again.) I knew that even if I did, the Big Plan with the travelling and the graduate traineeship on a national title was over. I'm ambitious, but I'm not an idiot, and I grieved for it. And added to that, I simply found pregnancy hard. The tiredness was hard. The transformation of my body was hard. I did not have a beautiful bump. I had stretch marks and sore patches where my non-maternity Levi's rubbed. The things people said were hard: the older woman at the bus stop who openly inspected my left hand for rings and snorted when their absence confirmed my fecklessness; the rugby lads who watched me struggle with my shopping bags and shouted, "Shouldn't your boyfriend be helping you?"

Sometimes, I would resentfully hope that the baby was a girl. “Because then you might know what this is like one day,” I would think, with the bitterness of someone who had never had to think very much about the material fact of being a woman before now. I had understood sexism in the abstract, but always believed that what was true for other women didn't have to be true for me. The brute fact of pregnancy and what it was doing to my life changed that: suddenly my sex was inescapable. But sometimes I would sing to my bump. I sang “We're Going to be Friends” by the White Stripes, one of the last bands I'd seen before my life had changed (“Fall is here, ring the bell / Back to school, show and tell … I can tell that we are gonna be friends”), and then I carted myself and the small stranger inside me to a seminar.

Getting pregnant was easy. Becoming a mother came slowly, and becoming a family likewise – these are things that must be learned, and sometimes the learning is error-strewn and painful. Sometimes it was harder because I wanted so badly to be perfect, to have the kind of pregnancy and motherhood that would show the invigilators of my ring finger that I was more than competent. (I wasn’t, of course. I needed all the help I could get.) The baby bucked my expectations in one more way and turned out to be a boy. But the plan went on: me and my boyfriend both got firsts, and in a fitful, scratchy way our careers began. Now our son is 11 and his sister is seven, and after years when it seemed dreadfully possible we might become stuck, we’ve started to discuss the travelling that we might do as four rather than two. Life multiplies and branches to its own logic. We stand beneath the trees, look up and see the light, and think how lucky we have been.