We were lucky, my wife said. Back home, drinking coffee after the scan, after seeing the baby's heartbeat for the first time, the word "lucky" hung in the air. We talked in the future tense now, rather than the conditional.

Lucky. The word stuck in my throat. I knew I was destroying the mood, the good vibes after the scan. I didn't feel lucky, I said. I refused to accept that there was anything lucky about my life right now.

On 2 April this year, at the age of 36, I was diagnosed with stage-three bowel cancer. A couple of weeks later, I had an operation to take out a large chunk of my colon and the cancerous lymph nodes. The doctor said there is a 50% chance of the cancer coming back. In May, I started six months of chemotherapy to kill any rogue cancer cells that might be floating around my body.

A second baby hadn't been in the game plan, but then neither was cancer. We had talked in the past about a brother or sister for our two-year-old, Tommy, but there were careers, money worries. Then, one night, after my diagnosis and before surgery, in a grotty London hotel, in a haze of worsening news and the spectre of metastasis, my wife and I clung to each other. There was urgency now, a need to do something life-affirming. There was no discussion, no agreement, just a shared and unquestioned assumption: with the threat of a life being taken away, we would create a life.

In the kitchen after the scan, my wife, Marketa, pressed on. We were lucky because there were plenty of people who couldn't have children. People lost babies and here we were with a healthy one growing inside her. Even with cancer, even having to undergo the degradations of chemotherapy, I could perhaps understand that someone, looking at our lives, might see us as lucky: to have conceived so flippantly, so undeservedly, like some reckless teenage couple on Jeremy Kyle.

But still, I didn't feel lucky. There was nothing lucky about our situation.

In those first tentative weeks, I told a few people that Marketa was pregnant. I talked about it as a source of hope, a ray of light in dark times, but these were just words, inadequate metaphors that didn't remotely describe the complexity of how I felt. Then, as the weeks went on and we found out we were probably having a boy, the whole thing became infinitely more real. Not an "it" any more, a medical abstraction, a cluster of cells, but a person with hands and feet, one I could imagine holding in my arms and watching him grow.

Luke with his first son, Tommy, when he was nine months old.

But as my son has become more real, so have my fears. Beyond the immediate practicalities of how we will cope – a pregnant wife, an overactive toddler, a husband enduring chemotherapy – I am desperately scared of the future, especially if my cancer progresses. I think of a boy growing up with a sick dad, not like all the other dads. Chemo Dad: withered, hairless, on his last legs. I think of the possibility of endless treatments and chemotherapy and future surgeries spoiling the first years with our new son. Nerve-damaged hands too painful to pick him up. A battered immune system keeping me away from his cot. Overwhelming fatigue and despondency tainting the joy.

Even worse, I think of a boy growing up without a father. To my son, I would be a photograph, a recorded video on each and every birthday. I would be a presence still – someone talked about every now and then – but I would be an abstraction, the way he was to me once, just a cluster of cells.

I imagine moments, rites of passage – learning to ride his bike, first time at the football, school plays, graduations – but it is a picture from which I am absent. I like to think that he would think of me quietly at such times, but never show it, never shed a tear, up there on that podium, emboldened by a stoicism that comes from losing your father when you're young, not to be beaten, leaning into the wind, just like his dad would have done.

I have noticed – or perhaps it is my imagination – that when I tell people about Marketa's pregnancy, their reaction is cautious. I think back to when she was pregnant for the first time and how people responded then – a blur of glass-chinking and talk of good swimmers and well in, my son, well in. Now when I tell people, I see a moment of hesitation. I try to read their faces to see if I can see in them a little of the same doubt that sits deep within me. Are they thinking the same? A child without a father? That this is a pregnancy laden with doom?

With cancer, you fight these feelings, trying to "stay positive". I implore myself to stop thinking like this, focus on that beautiful baby, that miracle. But despair has a habit of rushing in like a tide, catching you out, leaving you stranded when you least expect it. It is the good moments that spawn the darkest thoughts, as you are reminded of what you might lose. Cancer taints everything. I cannot watch my son play happily in the garden now without feeling an anticipatory sense of loss.

My wife was right, of course. I knew it from the beginning, from the very first time she mentioned the word lucky. I was being obstinate, too caught up in myself, too angry about having cancer, to see anything good in anything.

Now I am sitting with the ultrasound images in my hand. In the grainy pictures, I can see the faint shape of my wife's lower lip. I can see our son glugging for a hungry breath. He seems to hold himself placidly, almost thoughtfully. When I see the swell of Marketa's belly now and her joy at being pregnant again, I feel lucky.

I'm lucky because of all the many wonderful things my wife has given me, this is the most generous. Not only is it the gift of a future life, but it was a vote of confidence in me, in our life together. A promise that said, I know you will get better, you have to get better.

When I look more closely at the ultrasounds, when I put the scans in a sequence, there is something comforting about the division of my son's cells. In its steadiness, it is the antithesis of the haphazard and impetuous divisions that have been taking place within my body. I take comfort in his orderly growth, his weekly progression in centimetres, his adherence to a strict timeline.

There is nothing random or reckless about the way he grows. Everything is preordained. The molecules in his body will not betray him. He will not suddenly mutate, or try to leave the womb. He will grow where he is meant to grow, safe and happy in the warm.

• Read Luke Allnutt's blog at theoneeyeddog.com