M y childhood fantasy, whenever there was an unexpected knock at the door, was that Charles and Diana had had a breakdown on the A road that ran outside our house and needed a bed for the night. Subsequently, I was always mortified when the door opened on a grinning friend of my parents or some pre-GPS driver lost in the black Oxfordshire countryside. This huge disparity between reality and the grandiose expectations I was able to conjure up in milliseconds has never left me.

The call that Wednesday afternoon was a rare exception. I had just finished teaching my weekly English class at the University of Potsdam and saw a missed call on my phone with a Berlin number. I am always waiting for the call, the one that is going to change my life, so it’s impossible for me to ignore an unidentified number. I always phone back.

“Frau Schw[mumble],” the voice said.

“It’s Ben Fergusson. I had a missed call from this number call.”

“Ah, Herr Fergusson. It’s Frau Schwenk.” Our social worker, I now understood. “Thank you for getting back to me. I’m calling because we have a little boy, four weeks old, who needs a family.”

A woman in our adoption-preparation classes had told us she saw stars when she got the call. For me it was more like pressing the pause button on an old VHS. Everything stopped, went silent, juddered a little. I started taking notes in red pen on the back of a vocabulary card, the scratching ballpoint loud in my ears. “Boy,” I scribbled, “four weeks old.” But that was the sum of the information she could give me over the phone.

“You will need to come to the Senate this evening or tomorrow first thing.” In Berlin, it is rather grandly an office of the State Senate that organises adoptions. “Then, assuming you are fine with the details, you’ll need to meet him tomorrow afternoon. And then if that goes well, you’ll need to be ready to bring him home on Friday morning.” A day and a half to prepare for the rest of our lives.

I knew that my husband, Tom, was at his psychotherapy practice until 6pm. I left him a shaky voice message in the empty classroom bright with fluorescent light. I wasn’t quite in tears, because I wasn’t yet sure it was going to work out. Our social workers had made it clear that we needed to be open-minded when we met our prospective child, in case we didn’t bond. Back home, pinching my bottom lip at the kitchen table, an undrunk cup of tea at my elbow, I watched the grey ticks of my WhatsApp voice message to Tom turn blue. A few seconds later he called back, desperate for details.

“I don’t know. They’ll tell us tomorrow, I think.”

“She said a healthy baby.”

I could hear the cracking of the floorboards in his office as he shifted his weight.

Tom’s practice is outside Berlin, and it would be another two hours before he got home. I tried to read, but found myself staring out into the night sky, the paperback spatchcocked on my chest. I poured myself a Scotch and played a mindless strategy game on my computer for two hours as the night set in.

After many rounds of hugging, Tom and I went through the schedule for the next few days. We had the first meeting at the Senate in the morning, and were meeting our potential son in the afternoon that same day. All going well, we would be picking him up on Friday at 11am. This, we worked out, left us a window of four hours on Thursday to purchase everything a baby might need for its first night at home.

While Tom did a trolley dash around the local drugstore – buying multiple bottles and dummies when he wasn’t sure which type was right – I marched into the nearest baby-clothing shop and said: “I need all the clothes for a four-week-old baby boy.” The friendly but bewildered shop assistant said, “What size is he?”

“Is this your baby?” she asked. Once I’d cleared up the confusion, we set about piling up socks, hats, rompers and sleeping bags on the counter, our search punctuated by her suddenly remembering essentials: “You need hundreds of muslins!” – and offering sage advice – “Too big is always better than just fits!”

That same weekend I returned to exchange half of what I’d bought because I’d guessed most of the sizes wrong and, in my post-arrival stupor, had boil-washed all the woollens.

To be clear, we weren’t simply incompetently ill prepared for the advent of our first child; we were explicitly asked by our social workers not to prepare anything for his arrival. The German care system is heavily focused on foster care, meaning only a few thousand children are put up for adoption each year. There are thus far more applicants than children, and social workers can afford to be picky. And one of the firm criteria for prospective adopters in Germany is that you have to be married. Up until 2017, same-sex couples could only enter into civil partnerships in Germany. Bar exceptional circumstances, like one civil partner adopting their partner’s children, this meant that same-sex couples were largely banned from adopting until the year Tom and I applied to adopt.

Before our son arrived, we often talked about how our social worker could possibly decide whether or not we matched a child that was potentially only a few weeks old. And yet, from the day we brought Theo home, we have been astounded by how perfectly matched he is. Perhaps, blinded by love, we would say this about any child we adopted. But whenever we meet up with other adoptive parents, I look at their children and I look at Theo and I think, it could only have been him. I am shocked by how inevitable he seems to me, how completely familiar.

We met him in the doorway of a building in Mariendorf. Both we and the nurse who had looked after him for the first few weeks of his life arrived too early and the building was locked. We said hello, my heart pounding in my throat at the sight of the car seat with a muslin pegged over the top.

“Could we see him?” Tom said.

We knelt, she lifted the muslin, and we peered in. He was asleep, tiny beneath the folds of his colourful hat and knitted cardigan, his little face waxy and doll-like. The social worker arrived on time and we were shown into a large room. The nurse chattered on about his routine, how much he ate, how much she had enjoyed looking after him during those few weeks. I took down notes on my mobile, intermittently looking up at him. “Who are you?” I wanted to ask him, but he was perfectly still, his tight lips and tight lids inscrutable.

Then he stirred, blinked and woke up. His eyes were black, and the nurse asked us if we wanted to hold him. He started crying the moment he was in my arms. I joggled him around the room, but he was inconsolable, his face becoming all mouth and reddening to a rich crimson as the nurse shouted out instructions like a shepherd to a collie. “Lift him up. Hold the dummy in. Talk to him. Sway! Sway!” Eventually, after 10 minutes that felt like 30, he quietened.

The nurse made up his milk. They tucked Theo into the crook of my arm and gave me the tiny bottle. That was the moment. Tom asked the nurse and the social worker some questions, I listened and nodded, and got lost in his face, his hesitant drinking, the perfection of his fingernails and his creased little palms.

“So,” the social worker said. “We’re going to leave the room now and you can have a think about whether it fits.”

We waited for them to leave and, when we thought they were out of earshot, we looked at each other and said, “Well, yes of course, no?” “Of course.”

I felt as if the momentousness of the decision required more time, more discussion, but what else was there to say? Then we had half an hour just to sit with him asleep in our arms, impossibly small, radiating an impossible amount of heat.

The next day, a cool autumn morning at the end of October, we set out on foot with a mustard-coloured blanket folded up in our new, empty car seat. I thought about the impossibility of fathoming great change. I understood that we were going to pick up our son, but how could I possibly believe that two hours later we would be at home with our baby? Life is filled with change, but there are only a few moments in which we are aware that we are standing in one life and that imminently we will be standing in another. We met up with Theo and the nurse for the second time in a room on the second floor of a large hospital. We chatted about the nurse’s last night with him and how much she and her family would miss him. He started crying; we both looked at him.

“Should I pick him up?”

“He’s your son,” she said, smiling, but looked a little heartbroken as I lifted him into my arms.

Our friend, Konstanze, drove us back through Berlin, showering our sleeping baby with compliments. She stayed for a coffee while I ran out to find a few last things we had forgotten to buy.

It was then, Tom later told me, that he knew. Konstanze had left, I was still trying to fathom what baby-bumpers were, and Tom leaned over Theo on the bed and locked eyes with him. Tom was overcome with a feeling that everything was going to be all right and he heard himself making a silent promise that he was going to look after him forever.

That night, once Theo fell asleep, we placed him in his cot and climbed into bed. We had barely slept the night before in anticipation and now, exhausted, we lay sleepless again. I didn’t know that babies breathe quite erratically in the first few months and I was terrified he was going to just stop. I tracked each breath in the semi-darkness, willing along the next I drifted off in the end, but never really fell asleep.

What I recall of the first few months after our son arrived is constant amber twilight. As autumn moved into winter, the days darkened and shortened and we adapted to our son’s four-hour rhythm. We discovered Theo was soothed by white noise, particularly recordings of rain and waves, and, as we stumbled out of bed for another bottle, I often felt like we were living underwater.

It was an overwhelming time, but also, as the cold hardened outside, an incredibly cosy one. Although no one in their right mind would suggest that children are a good remedy for anxiety, I did find a huge amount of comfort in the immediacy of my son’s needs and the clarity of my task. Childcare requires you to form a radically new relationship with time, existing where once you planned and worried. You move through the hour with primordial slowness, understanding but not really believing that at some unfathomably distant point in the future you will talk to an adult again, you will bathe, you will sleep.

Beyond these flickers of grace, parenthood is a huge challenge mentally. The crucible of most of these issues is sleep. In those first six months, Tom and I would lie on our sofa and ritualistically repeat: imagine doing this alone having just given birth. I was knackered and Tom was knackered, we forgot sentences we had begun, we were regularly startled out of gormless daydreams in the middle of rooms, we napped on chairs and sofas, on the floor. But when my sister got home from hospital after her Caesarean she couldn’t even get up the stairs to her bed or sit up to breastfeed. And two weeks after that her husband went back to work.

Why are we surprised so many women struggle mentally and emotionally after experiencing such profound physical changes, and then are left to care for a tiny life more or less alone? I’m not saying this is the experience of all women when they have children. But what I do think is true is that, as adopters who are both men, we have experienced nothing that comes close to the trauma many mothers experience in the first months of parenthood. However, these reflections were made in the confines of our flat. The reality of how starkly our experience of parenting differed from that of the straight families we knew only became viscerally apparent when we packed our son into his blue felt winter suit and gingerly ventured out on to the streets of Berlin.

This is an extract from Tales from the Fatherland: Two Dads, One Adoption and the Meaning of Parenthood by Ben Fergusson is published by Little, Brown at £16.99. Buy a copy for £14.78 at guardianbookshop.com