The hardest thing about having a baby alone isn’t the expense, the fear or the loneliness. It isn’t the process of getting pregnant, with its cycles of raised and dashed hopes, or the term “sperm donor”, with its unsettling connotations. It’s not even the queasy feeling that what you are doing sets you apart from other people and that the reason you are doing it is not that you are a powerful, rational, resourceful woman, but, as a friend of mine put it after considering and rejecting the idea of having a baby alone, that “I couldn’t get anyone to shag me”.

No. The hardest thing about having a baby alone is making the decision to do it.

“So are you going to do it then?” says Rosemary. It is late summer 2013 and we are drinking whisky in a hotel bar in Edinburgh.

“Yeah, probably,” I say. “I mean, I might. Are you?”

“I don’t know.”

I haven’t seen Rosemary for months and it is only after a lot of whisky, and with a casualness that belies the cold terror underneath, that we reach the main order of business: our ongoing discussion, part lament, part spur to action, over what to do about having children. That is: if, when, how and with whom, or, since we are both, for the purposes of this conversation, single, “with” “whom”.

I have always known I wanted children. From the time I was old enough to conceptualise my future, motherhood made sense to me. It was always one child in my imaginings and never part of a fantasy about marriage, and while everything else in my life changed over the years – the country I lived in, the kind of work I did, the gender of the people I dated – the distant outline of a child remained steadfast. On the rare occasions I allowed myself to inspect it directly, the idea that it might never happen made me feel giddy with loss.

***

I met L two years after moving to New York. On the surface of things, we looked very different – me, English, lefty, fundamentally unkempt; she, New Yorker, centre-right, well put together. On any given day we could disagree about everything – fact or fiction, subway or car, Republican or Democrat – so that, in the months after we met, it felt like being on safari in each other’s alien worlds.

If falling in love is, partly, a question of finding a docking station for one’s neuroses, I knew I was home when L told me that, after her building was evacuated during 9/11, she went straight to an off-licence and bought hundreds of dollars’ worth of booze in case civilisation collapsed and the world reverted to a barter economy. Come the zombie apocalypse, this is a woman you want on your side. But there was this, too: the house she grew up in would one day have to be sold, she said, and what she would miss most were the things you can’t take with you, like the sound the stairs made when they expanded at night. Somewhere in my system, a pilot light flared.

She was three years older than me and told me from the outset that, in the near future, she was planning on trying to get pregnant. Logistically, this made sense; it would be madness to forestall while we flapped about for another two years trying to decide what we were doing. Emotionally, however, it stumped me. According to every relationship model I knew, you could either be with someone who’d had kids before you met, have kids together and separate down the line, or split up and have a baby alone. There was no such thing as being with someone who had a baby on her own. It sounded like a terrible deal: all the stress and anxiety without the substance of motherhood.

I didn’t want to be dating people with children when I had none of my own, wishing I’d had the courage to act

At that stage, the strongest terms in which I could have put my own long-held but dormant desire for a baby were that I didn’t want not to have one. If there was, behind this impulse, a larger, less tangible longing, I didn’t want to look into it too deeply lest it unleash a full-blown baby hunger I couldn’t get back in the box. But I started to notice small, unsettling changes in myself. When somebody asked me, “Do you have children?” – a question that, until recently, I had responded to in my head with versions of, “Are you mental? I’m about 11” – it started to sound less neutral, more unfriendly. I had always believed that, medical issues aside, most women without children had acted through choice, but my faith in this weakened. I watched as a number of friends missed out on having children because their boyfriends broke up with them when they were in the vicinity of 40, before having children with younger women. I watched as women six, seven years my senior finally met someone new and went through round after punishing round of IVF.

I didn’t want to be alone at 45, or 50, and on Tinder, dating people with children when I had none of my own. I didn’t want to be 70, the age my mother was when she died, lying on my deathbed without the image of my child’s face in my head. Above all, I didn’t want to look back on this period and wish I’d had the courage to act.

I also didn’t want to “help” another woman raise her baby. Unless I was Mother Teresa (I’m not), the only way it would make sense for me to stick around in the event of L having a child was if our relationship became a more conventional union, or if I had my own baby independently, too.

It’s not that L’s pregnancy made me more broody (I defy any woman to see another woman’s early pregnancy up close and think, “Hey, that looks fun!”) and I wasn’t bound by her decisions. We didn’t live together. In fact, an infantile strand of my personality deliberately wanted to make different decisions. If we were going to suffer the deprivations of single parenthood, we might as well realise all the advantages, too – in my case, starting from scratch and doing precisely what suited me and my notional baby.

All I had to do was figure out what that was. Would I use a friend as a sperm donor, or a stranger? If the former, who? If the latter, how would I make that choice? Would I move back to London for free treatment on the NHS (which, to the horror of the rightwing press, now offers fertility services to single women and lesbians) or stay in America and spend thousands on something that might not even work?

In the event, I choose the path of least resistance: America will never really feel like home, but it is where I live, where L has her baby and where, eight months later, I am sufficiently panicked to finally get moving with my own.

One of the things you have to get used to when you are a British person embarking on fertility treatment in the US is the pace. In Britain, the law of supply and demand is such that there are more women wanting sperm than there are donors, so even private clinics have waiting lists. In America, where no one with adequate resources waits for anything, you have a chat with your doctor, schedule a date, call the donor bank, which bikes the sperm round to the clinic, and off you go. You might have spent six months or six years deciding to do this; but you could, potentially, be pregnant within a month of first seeing your doctor.

I want someone clever, with dark hair. I want someone whose favourite film isn’t Titanic

That is, if you have made what feels at the time like the hardest decision: how to pick a donor. This question probably cost me six months of concentrated flapping, during which time I asked a male friend if he’d do it, because it seemed more “normal” than the alternatives, and was achingly relieved when he said no, before eventually deciding to find an anonymous donor.

This is a tricky part of the story for me. There may come a day when it is as regular as milk to share details of one’s sperm donor – when there is a language less alienating to describe it than this, and that feels less compromising of one’s child’s privacy. But we are not there yet, and I’ve no idea how to calibrate this choice. Is it the biggest of my life, or essentially meaningless? Underplay the donor and you risk turning the guy into the elephant in the room; go on about him too much and you risk pathologising your child’s background.

Scrolling through profiles, I look for characteristics that align with my own. I want someone clever, which here means educated. I want someone with dark hair. I want someone whose favourite film isn’t Once Upon A Time In America or Titanic. In the absence of a metric for gauging a man’s humour or internal beauty or moral worth, I want someone tall and basically symmetrical. A choice is superficial only if it is made at the expense of deeper considerations and so, although I reject sperm donors on criteria that would outrage me if applied in real life by men to women, I tell myself I’m not doing anything wrong.

Facebook Twitter Pinterest ‘The idea that motherhood might never happen made me feel giddy with loss.’ Photograph: Sophia Spring/The Guardian

It’s a mistake to see this exercise as equivalent to friendship or dating. I keep reading articles about sperm donor or egg-freezing “parties”, as if having a child this way were not a series of sober decisions but some mad hen night. The donor banks are just as bad, all called things like Infertility Solutions, making them sound as if they have a sideline in targeted killings. But when you visit the websites, most are set up to look like quasi dating services, reinforcing the lie that you are choosing a husband, co-parent and the progenitor of exactly 50% of your child’s face and personality. They go to great lengths to avoid the word “catalogue” but that’s what it is, pages of donor profiles with vital statistics and photos. Some websites even have a little shopping basket icon in the right-hand corner and an option to “check out” – entirely for show, given that you can’t do any of this without making at least one phone call.

Everything is extra: $35 for the guy’s baby photos; $50 for an audio file. Guidelines vary, but in New York you can see photos of him only as a child. Some donor banks offer a “silhouette” of him as an adult, which would be hilarious if it weren’t so creepy. What next – his breath in a jar to rule out halitosis?

I don’t listen to the audio files. I don’t try to find the guy, even though there is so much information, it would probably take me less than a day. This is not gene selection; it is the selection of the story of how my child came to be, and, through a combination of vital statistics, familiarity of background, a subtle implication that he is a Democrat and his use of the word “tremendous”, which signals to me a certain wryness and enthusiasm, I make my choice. In other words, on nothing substantive. What matters is it’s my choice and I make it.

I pay extra for ID disclosure, enabling any child to trace the donor when they turn 18. I decide how much to buy – enough for three cycles – then fill in a form and return it, along with payment for almost $2,000. When I call to confirm my request, I half expect the receptionist to laugh and ask what on earth am I doing, trying to buy genetic material over the phone as if it were lunch? Instead, after I mumble, “Need to order some sperm”, she puts me through to the lab, where a technician will check to see if what I want is available.

I give him the donor number. There is a clacking of keys, followed by a short pause. Then, with the smoothness of a sommelier fielding a wine order at dinner, he says, “An excellent choice.”

***

After weeks of monitoring, at the end of 2013, my eggs are ready. This is it, says Dr B. I can come in tomorrow and, after waiting an hour for the sperm to defrost, finally get this show on the road. He asks if I’d like L to be present when the insemination takes place. “Some people find it nice to involve their partners.”

Fertility treatment can be hard and excluding, he says, and involving the patient’s partner, even to the extent of inviting him or her to operate the syringe full of sperm, can give them a feeling of inclusion. I blush. Clearly he’s in favour of L being present, either because it gives him a warm feeling or to neutralise some latent ambivalence he has about helping to create single mothers.

I try to imagine the scene: me, stressed out and half-naked on a gurney; L, holding the catheter and rolling her eyes; the medical staff, trying not to intrude on our beautiful moment. I don’t think I want L there – I don’t want anyone there, it’s embarrassing – and when I imagine asking her, I realise I don’t want to give her an opportunity to say no, either.

There is a cold, mean streak in me that makes me think trying to involve the partner is ludicrous under any circumstances. Surely there’s a dignity in allowing things to be what they are? This is a medical procedure; pretending otherwise risks making the treatment seem sadder, just as choosing a sperm donor will continue to feel sad, or bad, or weird, as long as it’s tied to conventions associated with choosing a spouse.

The next day, a week before Christmas, Dr B breezes in full of good cheer. We chit-chat as he loads the syringe with a substance that is, gram for gram, more expensive than the world’s finest heroin (though less expensive than marrying someone you’re not into in order to have a baby).

The cycle fails, as do the subsequent three cycles, one of which results in a short-lived pregnancy and all of which mean that, by the spring of 2014, I am taking, for the second month in a row and despite producing too many eggs the first time, large amounts of fertility hormones.

It is different this time.

“How do you feel?” says Dr B.

“I feel messed with.”

For five days I have been injecting myself with a preloaded pen, which has bruised me terribly. The skin of my abdomen looks like 1970s wallpaper, all bright purple flowers with a greeny blue border. I feel altered, hideously bad-tempered. I tell myself it’s chemical and will pass. But it doesn’t.

A week after finishing the injections, Dr B looks at my charts and tells me to stop taking the drugs. Ten days later I go in for insemination number five.

Are our children victims of a half-arsed piece of emotional evasion, or beneficiaries of a radical new vision?

“Whoa,” says the nurse doing the ultrasound. “You’ve a lot going on in there.”

I look at the screen: a lot of shapeless dark patches connected by strings.

“They look like spider’s eggs,” I say, and shudder.

I have, once again, overreacted to the hormones. But Dr B says not to worry: not all of them are mature. I could call off the cycle but I say, “Go ahead.”

The sun comes out that weekend, and L and I take a walk with the baby in the buggy. I feel Zen in the face of all possible outcomes. On Monday night I go into my kitchen and crack an egg against the side of a pan for dinner. Two bright yellow yolks slide down. I have never seen such a thing before and stare down at the eggs, feeling bad for the hen. I am so surprised I say it out loud: “Twins.”

***

It’s twins. Of course it is. How could it not be? I am a walking exemplar of the phrase, “Be careful what you wish for.” Over the next few weeks I wait for the idea of carrying twins to normalise, but it doesn’t. For minutes at a time I forget I’m pregnant, then I remember with the force of the original shock. I have lunch with an old friend I haven’t seen for a while. I know he’ll be shocked, too, and he is.

“Wow. Congratulations.”

“Thanks!”

“How’s that going to work?” he says.

And there it is, the question we’ve been avoiding since L’s pregnancy. If I have these babies, what will the babies be to L and what will she be to them? The answer is only partly to be found in the relationship I have with her baby. There is no honorific to describe what I am to him and there is no word for what he is to me. He is at the centre of us, the miracle over whom we both marvel, but I have no moral, financial or legal responsibility for him. Neither do I perform many of the most basic parental duties.

I have always known this lopsided arrangement would be tolerable only until I had a baby of my own. What I hadn’t anticipated is the ways in which its limitations would also prove to be strengths. In the year since his birth, my relationship with the baby has evolved to be oddly free-floating from that with L. He is my buddy, a child in whom I have no stake other than love. That it’s a love I’m not bound – by law or biology – to feel makes it all the more precious.

On the other hand, what am I doing potentially bringing two further children into a situation it takes so long to explain? I can just about rationalise to myself why a woman without a child might want to maintain a degree of separation from a partner with a child, given the vast difference in lifestyle. But two women in separate households with babies of a similar age who hang out on evenings and weekends? If we’re not a blended family, then what on earth are we?

Clearly, at this point, the proper course of action would be to either give up this nonsense of separate households and separate children, and move in together, or else call it a day. There is no middle way. Perhaps it is selfish. It’s selfish to carry on along parallel tracks, denying the children a second parent and creating two single-parent families. It’s selfish, practically, morally, financially and environmentally, to maintain our independence while being together, like driving two cars to a single destination. And while my relationship with L’s baby is full of joy, how can it survive once I have my own children and am unable to travel back and forth to see him?

Facebook Twitter Pinterest ‘I wait for the idea of carrying twins to normalise, but it doesn’t.’ Photograph: Sophia Spring/The Guardian

For the first time I seriously question why I want to do this alone. It isn’t just that L and I have conflicting ideas about parenting – very broadly, I am too mean in her eyes, and she isn’t mean enough in mine – it’s the historical weight each of us puts on those differences and our assumptions about where they might lead us. We both have a highly developed sense of self-preservation, which expresses itself in different ways, except, perhaps, in this one shared belief: that the way one protects children from harm is by controlling who has access to them. The only thing more frightening to me than not having a baby is having a baby in a hostile environment.

One afternoon L sends me an email with a link to an apartment listing that is almost double the rent I pay in Brooklyn. The floor plan looks familiar, as does the view from the window. It’s in her building, the mirror image of her home, but one floor down.

“?!” I reply.

“!!”

“But do we want to live that close to each other? Isn’t it weird?”

“I don’t know.”

I go to see it. The landlord is putting in new flooring and a new bathroom and most of the apartment is under polythene, but because it’s an exact copy of L’s, bar the fixtures and fittings, I don’t have much trouble imagining it. It occurs to me, as I walk around, that he may not even want to rent to a single woman expecting two babies. But in any case, it’s too expensive. Eeven if it’s the kind of building I need, with a mail room and an elevator and a maintenance team on site; even if it would be amazing to have L upstairs when I bring the babies home; even if the very fact that the listing came up in the first place, in a co-op that discourages rentals, is the kind of coincidence that feels like a gift from above – none of that matters, because I can’t afford it.It is, surely, nuts: to sort of live together but not. It feels like cheating, to have L’s support and proximity without the hard work of cohabitation. How would we explain it to the children? Or to ourselves? That we like each other sufficiently to be in daily contact, except on days when we don’t? What would the kids even be to each other? Cousins? Best friends? The victims of a half-arsed piece of emotional evasion, or beneficiaries of a radical new vision?

In those first weeks after moving, we enter a honeymoon period in which the loveliness of living almost together is nothing to the luxury of living sort of apart. The act of leaving my flat and walking up one flight imbues daily visits with the tiny frisson of occasion. When one of us snaps, the other goes home without it being construed as a histrionic gesture. There’s no marriage or joint mortgage, but a commitment has been made. I have the long-overdue realisation that relationships rely on a balance between independence and the right level of curtailment of freedom to liberate one from the burden of choice.

One evening, L sits on the sofa with her son, reading a book about different kinds of families. “‘Some people have two mommies,’” she reads, pointing to an illustration of two badgers wearing earrings with a baby badger in their midst. “‘Some people have two daddies. Some people have one mummy, hasome people have one daddy.’” Her baby, who isn’t a baby any more but a toddler and the most delightful child in the world, isn’t quite old enough to formulate questions and we are off the hook for a little while yet. L and I exchange glances. “Some people have a neighbour,” she says, sotto voce.

***

My final ultrasound of the year falls just after Christmas. I am six weeks from the due date. The technician looks at the screen. He frowns, says something I don’t catch and leaves the room. Someone else comes in. Everyone gathers by the monitor while I look at the ceiling and try to figure out what to have for lunch. A fourth doctor comes in and tells me to get dressed and follow him. I feel a spike of alarm. In his office, my high-risk obstetrician, Dr Y, is waiting.

“They have to come out,” Dr Y says.

“Oh my God.”

The placenta for the smallest baby is working only intermittently; if it stops altogether, she’ll die.

“This is not an emergency,” Dr Y says calmly, “but it is… fairly urgent.” He tells me he has time the following day, New Year’s Eve, or the day after that.

“Let’s do it tomorrow,” I say, trembling.

“Three pm?”

“OK.”

I have been so stringent in ensuring I can do this alone, perhaps the reward is that I don’t always have to

My dad is in London and offers to come straight to New York, but I don’t want him in the air while I’m having surgery; I can’t add fear of his plane going down to everything else. At L’s that night, I tell her to ask her mother to come across town the following day to watch her son.

“I’m so happy you’ll be there,” I say.

“It’s only because everyone else is in England.”

“No, it isn’t. I would want you to be there, whatever.”

As I say this, I realise it’s true. Fear pushes me inward, joy pushes me out, and while I am as frightened of having these babies as of anything, it’s a different kind of fear: not a shrinking but an opening out. I have been so stringent in ensuring I can do this alone, perhaps the reward is that I don’t always have to.

Right up until the last moment, a small part of me thinks, what if all this is a mistake? What if Dr Y turns to me and says there’s nothing in there – of course you’re not pregnant! Did you think that, by signing a few forms and handing over your credit card, you could dodge millennia of evolution, not to mention convention and common decency? Go home, buy yourself a cat and never speak of this again.

But at 4.17 pm the next day, a tiny, fierce cry fills the room. Baby A is removed from the basement of my body. I burst into tears. L grips my hand. A moment later, Baby B comes out and L leaps from her seat in the direction of the babies while Dr Y, turning to his students, holds a quick pop quiz over my guts. Then the nurses bring over the babies.

L gets all of this illegally on camera. It’s not footage I can watch too often. The babies, two flat-faced Glo Worms covered in gel, are blotchy and impossibly alive. I am insane on the gurney, grinning drunkenly at my two girls. Over and over I say it, in the manner of a woman shortly to be given more drugs: “Oh my God, I can’t believe they’re both blond.”

• This is an edited extract from An Excellent Choice by Emma Brockes, published by Faber & Faber at £16.99. To order a copy for £13.99, go to guardianbookshop.com or call 0330 333 6846.

Commenting on this piece? If you would like your comment to be considered for inclusion on Weekend magazine’s letters page in print, please email weekend@theguardian.com, including your name and address (not for publication).