Like every parent, from time to time I disagree with the father of my child about how our four-year-old son should be brought up. There have been moments, for example, when I've told Louis that I think it's absolutely fine for Fergus to have some chocolate or chips. In turn, Louis's most common refrain is that I'm too slapdash – and I should be making our child eat more healthily. I then usually respond he's being too uptight.
The difference is that, just as we start to drive each other mad, I can say: "You've really annoyed me now. Right, I'm off home." There's no bitterness or recrimination. We never get to the point where we secretly want to kill each other, because Louis and I have never been a proper couple. We've only ever been best friends – best friends who sometimes used to sleep together, and by accident ended up with a baby. In fact, the night Fergus was conceived was the last time Louis and I ever had sex.
When I met Louis I was 26, working as a feature writer on Company magazine, and generally lapping up the media life as a single girl in London. Louis was a photographer six years older and not the settling down type. Neither of us was looking for a relationship, just partners in crime to have fun and get drunk with. When I moved to a job at Marie Claire as a feature writer, we would fly around the world together on magazine assignments. It was a dream life.
But while we never fancied each other, there was a huge mental spark between us. Louis was unconventional, interesting and I loved talking to him. He was my best male friend in the world. Perhaps with this thought in mind, we inevitably ended up in bed together one drunken night a year after we met. We got on so well as mates, I guess we thought: "You never know until you try."
When we woke up the next morning at Louis's flat, I didn't particularly think: "Oh my God, I wish we hadn't done that." It was more like: "Hiya. Fancy a cup of tea?" We got up, had a fry-up and went back to being friends. We didn't analyse it. In fact, we carried on having an occasional sexual relationship for the next three years. We'd be at the same parties, single, sometimes lonely in London, in need of a cuddle – and it would turn into something else. People would say we looked good together and friends constantly asked us why we just didn't go out with each other properly. But it just never felt right.
Then I fell pregnant. Looking back, we were being reckless. If I ask myself now why we didn't use contraception, maybe there was some part of me that needed to take it to the next level to see if a relationship would work. Emotionally, we were also too involved with each other to see anyone else. I was stuck an emotional cul-de-sac and knew I needed something to change.
I found out I was pregnant in March 2004 in the toilet of a Starbucks at Blackfriars on my way to work. My period was suspiciously late and when the thin blue lines came up, nothing can describe my absolute horror. In shock, I immediately rang Louis, who said I must be reading it wrongly. That lunchtime, he came to meet me at work. As we sat his car and stared at the test, his first words were: "Whoops, there are two lines, aren't there?"
But still, I hadn't expected him to be so delighted. At 37, he knew this might be his one and only chance to become a dad. I, on the other hand, was still too stunned to feel anything.
From the start, Louis was very pragmatic and persuaded me the world hadn't come to an end. He said: "Look Katy, you can't control everything in your life. Worse things have happened to people. We'll just have to deal with it."
For my part, there was a little wobble over whether I was going to keep the baby, but I didn't feel I could justify an abortion. I was nearly 30, had a good salary – and after all, my best friend was supporting me through it. Nevertheless, I did feel a huge sense of displacement. The rollercoaster of emotions I went through that year aged me about a decade.
Until then, I had always imagined having a baby with the "the one" – the man I loved and was going to settle down with. I had also imagined people's joyful reactions to the news I was going to have a baby. Instead, general bemusement was the standard response. When it got to the stage when I had to break the news to my colleagues, the colour drained from their faces and one actually piped up and said: "Who's the father?" I felt like I was starring in a comedy film of my own life.
When I was five months pregnant, I moved out of the flat I had been sharing with four other people and moved into Louis's flat in Camberwell, south London. It felt like we were playing at being happy families. Inevitably, we ended up doing the things that couples do, such as going to Sainsbury's, and out to dinner. Like any expectant couple, we also bought baby books and signed up for NCT classes But we drew the line when we were told it was a good idea for the man to rub almond oil into his partner's perineum. In the parenting groups, other loved-up parents would say: "And how did you meet?" I'd say: "Well, we work together." People were intrigued and wanted to ask questions. I just felt like a freak.
Then at the end of the day we would return to Louis's flat and sleep in separate beds. We had enough on our plates dealing with the pregnancy without trying to fall in love too. And even now he was the father of my baby, to have sex with Louis again would have been too intense. It would have made us proper couple, and neither of us wanted that. We both knew we couldn't force a relationship out of a night of illicit sex we shouldn't have been having.
Even with Louis's support, there were times when it felt very lonely. I remember crying over a meal in a gastropub, wailing to Louis: "People don't get pregnant by their friends!"
On 6 December 2004, the day that Fergus was born, Louis was like any other expectant dad, in the birthing pool in his boxing shorts and shouting at me to push. We felt bonded by the total miracle of having a child. Before the birth, I had wanted to see if having a baby together would make us fall in love. But it either happens or it doesn't – and after Fergus arrived, it still didn't. It was a shame and I admit I have shed many tears over it, because I knew then we would never be a "proper" family.
Immediately after the birth, I suffered some post-natal depression. But gradually I gave up on the fantasy I had always had of the perfect life, and together Louis and I started enjoying our child. And at least I didn't have to worry whether or not Louis would fancy me with a saggy tummy. We continued to be like a couple, except that Fergus would sleep in with my room with me and Louis would be in the other bedroom. We even went on holiday together and slept in twin beds.
Eventually, when Fergus was eight months old, we decided I should move out. I felt I was prolonging the inevitable and I had to get on with the single mum thing. But Louis and I didn't want to be far apart and I found a flat on the same street and worked out an arrangement where Louis had Fergus half the week and I had him the other half. The neighbours would see me walking down the road to his house in my slippers, carrying a cereal dish in the mornings and probably think: "There goes the Ross and Rachel couple again!"
Now we find we have a far better relationship than couples who have been together and then broken up. We weren't together in the first place so there's no huge reason why we split – and no bitterness. There are plus points, too: Fergus sleeps at his dad's house half the week, so I get to go out more than most mums, and we live so close we can both see our son even when it's not our turn to have him.
Out of all this also came a column in Marie Claire, "Then There Were Three", which turned into a blog, and now I have based my debut novel, One Thing Led To Another, on my experiences. Because it's what the fiction market expects, I gave the book a happy ending. The couple in the book are just like us – best friends who have a baby by accident– except that ultimately they end up together. In a way, I admit that part of me wanted to play out in fiction what hadn't happened in real life. I found it very cathartic. In fact, Louis was the first one to read the book. He knows ideally what I would have liked to have happened and that I naturally wanted to be in love with the person I was having a baby with. But in my own life, having a baby with my best friend, not my partner, has taught that it's possible to be happy in what is not the perfect situation. I have seen other so-called couples who were supposed to have the text-book fairtytale endings, but did not survive. My couple friends also observe that Louis and I talk to each other more than they do – and we don't take each other for granted as much as they do.
Most of all, I take great comfort from the fact that our son Fergus will never have to go through the pain of seeing his parents divorce. Nor do I have to worry about Louis falling out of love with me – or having an affair. There is nothing to go horribly wrong. It can only go horribly right.
A few months ago, Louis and I both moved to the same town in Hertfordshire, as we both wanted to get out of London. We chose Berkhamsted together. I don't see him quite as much now. We made a decision that we had to stop living in one another's pockets and make a marked effort to move on. So instead of living on the same street, we decided we should live a healthier distance apart, and now Louis is ten minutes away.
Both of us know we need to find partners. When Louis meets a new woman, there will be no twinge. I will genuinely feel delighted for him. I want him to be happy. In the meantime I have my own happy ending – just not the one I expected.
One Thing Led To Another by Katy Regan (Harper Collins, £6.99). Interview by Tanith Carey