Irrespective of whether you are religious, spiritual, agnostic or atheist, having your first child is an act of faith.

No matter how much you learn or how well you prepare, no first time parent knows what they’re getting themselves into. You don’t know if you’ll make a good parent, or how you’ll cope with the lack of sleep, or the crying, or the screaming, or if your relationship will survive the stress. You don’t know what it’s going to be like changing nappies, or feeding, or bathing, or dressing, or being entirely responsible for another life in its physical, emotional and developmental needs. It’s the equivalent of being led blindfold to the edge of a cliff and then jumping off and trusting you’ll survive the fall. It’s not rational at all.

So why do we do it? Unless we play fast and loose with contraception, we do it because we’re driven to do it, without rhyme or reason. We do it because a couple of billion years of evolution have programmed it into our DNA to ensure our genetic legacy. And we do it because our hearts are crying out for completion, for something more to love.

Having a second child is nothing like that. It’s not such a leap into the unknown as you pretty much already know what it is to have and raise a baby. You know how your lives have changed, how your relationship has altered, and therefore how a second baby is likely to affect this fledgling family dynamic. As a result, discussions about a second baby are less to do with the heart than they are with the head.

‘I want Izzie to grow up with a sibling so she has someone to play with, learns to share, and won’t be lonely. I think an age gap of two to three years is best – with Izzie at pre-school there’ll be less disruption, and they’re close enough in age to get along. And I’m better with toddlers, you’re better with babies, so you can look after the new baby while I look after Izzie. It’s the perfect division of labour.’

So says my partner Lizzie. It all sounds very logical, and rational, and clinical, but logic had nothing to do with why I had Izzie. I had Izzie because my entire being was crying out to become a dad. There was a gap in my heart that I knew only a baby could fill.

Izzie filled it. It might change in the future, but right now I feel complete. My heart is whole. I don’t feel the pressing need to have a baby that I did before. So surely, then, it wouldn’t be right to have another baby purely because I can justify it intellectually?

And there are other considerations. As I wrote yesterday, Izzie was our miracle baby, a gift from the gods. How ungrateful would we be to take that miracle and demand another? And the journey to her birth was so long, and moving, and life-changing that how could a second baby possibly compete?

‘This is our daughter Izzie. After years of fertility treatment and events conspiring as though Nature itself determined that we should become parents, we were gifted with her presence. And this is our son Gregory.’

[pregnant pause]

‘We thought Izzie might like a playmate.’

Now I know that our children aren’t meant to compete, and I know that every child is a miracle (No, says the biologist, it’s a natural process resulting from the coming together of two gametes), but Izzie has set the bar pretty darned high. Even the reason for having a second baby – for Izzie’s personal development – means even before it’s born it’s in her shadow, not desired or considered in its own right the way Izzie was. And that’s just wrong.

It’s wrong to Izzie too. I love her so much and we’re so close, I feel like having another baby would be something of a betrayal. It’s like saying to her, ‘You’re great, and all, but we need more. And you can’t provide it. So there. Sucks to be you.’

And, in all honesty, I am afraid of having a second baby. My heart is full. People say that you always worry you won’t love the second child as much as the first, and then it arrives and your heart grows to fit all the love you feel and you don’t know what you were worrying about. You discover your capacity for love is boundless, and blah, blah, bollocks.

But what if you don’t? What if you discover that, heaven forfend, you have a limited capacity for love, and wouldn’t you know it, you’ve just hit your limit? Do not pass Go. Do not collect £200. Go to the back of the class.

I have specific reasons for my doubts. Because of my Asperger’s, I’ve always struggled to manage feelings and relationships. If I had a friend, I couldn’t be friends with anyone else because not only would it be a betrayal (I know it’s not, but I can’t help feeling it is), I couldn’t find the mental space to consider the needs of more than one person at a time. And when I have a partner, like I do now, the very thought of wanting to spend time with anyone else just makes me feel dirty. This is a lifetime pattern of behaviour. I’m a U2 kind of guy (one-love, one-life).

I loved the fish until we got the chickens; I loved the chickens until we got the cat; I loved the cat until we got the dog; and I loved the dog until we got Izzie. What if, by having another baby, I transfer my love to it and can no longer care about Izzie or manage to consider her needs in such a way that I go from being a good dad to merely an adequate one? I don’t want to turn my attention and my heart away from her towards anything else and let her down. The very thought of it is heartbreaking.

This is all a very long-winded way of saying I’m afraid of having a second child. I’m afraid I don’t have enough love to encompass two children. I’m afraid that my relationship with my daughter will irrevocably change. And I’m afraid if I’m spread so thin I’ll lose my ability to be a good, caring, attentive dad.

So in a way, I guess having a second child is a leap of faith. You’re not sure you’re going to love it – you don’t feel that you can – but you have it anyway, trusting that it’ll come good in the end. I said before that you can’t live your life imprisoned by fear, or else you deny yourself the chance of something good, and perhaps this is one of those things.

But not right now. Right now, I don’t feel the desire for a second baby, not on its own terms. My heart isn’t crying out for something to love. And until it does, I can’t even think about bringing another child into this world.

It’s explaining this to Lizzie that’s the hard part.