Our son and daughter were conceived at the same time, while we were in the cafe at Ikea Croydon (a convenient place for brunch after an early-morning egg-retrieval operation). But while our fertility clinic implanted our little girl into my womb three days later, our little boy went into their freezer for two years. Now, she is five and he is three, but technically they are fraternal twins.

Felix first started to become aware of his unusual origins when he was about two, when we found some home video footage of his big sister at her first family Christmas. Watching her chuckling in glee while bouncing on Granny’s knee, he pointed out our familiar red sofa and his older cousins laughing in the background, then asked: “Where was I?”

“Well, that was before you were born.”

“But where was I?”

We’ve always been open about the medical intervention we needed to have our children. So we told him the truth, something along the lines of: “When the doctors put tiny Miranda into Mummy’s tummy to grow into a baby, they put tiny Felix into a special freezer to wait until it was your turn. When that video was taken, you were waiting in the freezer.”

We told him how special and amazing that was. And, in the usual way of toddlers presented with special and amazing things, he simply accepted it and asked to watch more videos.

How different would our children be if he had been born first and she had spent two years in the freezer?

Since that conversation, though, the realisation that he existed in suspended animation for two years while his sister was having fun, with Mum and Dad all to herself, has been a big thing for our little boy to get his head around. Some days, he is full of questions. “Was it our freezer?” he wants to know. “What else was in the freezer with me?” We have to explain that, no, he wasn’t tucked away among the fertility doctors’ fish fingers and frozen peas. “What did I look like?” he asks. I suspect he is imagining some sort of miniature baby-shaped ice cube.

“You were just a blob,” his dad explains. We find some pictures online to show him. He was frozen five days after conception, as a blastocyst, a ball of 200 or so cells, the inner mass of which would become the embryonic Felix, the outer shell his placenta.

“I was a blob!” he repeats, sounding excited and reassured. “Did I feel cold? Did I have eyes?”

Recently, he has wanted to know: “How did I get into your tummy?” and: “How did I get out again?” His solemn assessment of the answers to these questions has been priceless. But there is one question he hasn’t yet asked: “Why me?”

Surely, one day he will wonder why his sister was born first and he had to wait. When the question comes, will we tell him the truth? That his irregular and fragmented cells meant he was classified as an inferior-quality pre-embryo? That the pair given the top two gradings (out of the four used in IVF treatment) were placed together in my body first? That if they had grown into twins, instead of one disappearing as the other developed into our daughter, his fate might have been very different?

Cuddling his robust little body, admiring his ceaseless curiosity, helping him enrol Darth Vader into his sister’s make-believe school of cuddly cats, it is difficult to imagine the alternatives.

Practically and financially, a third baby wasn’t on the cards. Would we have offered him to medical researchers? Donated him to an infertile couple? Would anyone have wanted the runt of our pre-embryonic litter? Would he still be there, frozen expensively in time, like an estimated two million other “frosties” worldwide?

As much as I am thankful to medical science for enabling us to have our family, I am relieved we didn’t have to make such an agonising decision. The alternatives are all the more heart-rending now we know that minuscule ball of cells, just 0.2mm in diameter, as a bouncy, impossibly stubborn, brown-eyed boy who adores superheroes, trumpets and all things yellow.

Every day, I watch him copying his sister’s drawings and dancing, joining in her games of princesses and picnics, and wonder how their characters have been shaped by a very 21st-century twist of fate. How different would our children be if he had been born first and she had spent two years in the freezer – or if they had been identical twins? How will their beginnings affect them as they grow up?

The differences aren’t just environmental, but biological, too. While IVF babies are generally a little smaller than average at birth, those frozen as pre-embryos tend to be heavier. Our family bears this out.

Despite being classed as inferior on day five of their existence, our son weighed 2lb (0.9kg) more than our daughter at birth – she was just under 7lb, he was just under 9lb. His delivery was easier and he fed better, grew faster and slept for longer. While he was a bump, I worried constantly that this low-grade creature from the freezer couldn’t possibly be normal. In fact, he couldn’t have been more perfect.

Surprisingly, freezing is associated with better outcomes for babies and their mums than IVF with freshly fertilised eggs. Defrosted embryos have a lower risk of premature birth and perinatal death, as well as the all-important heavier birthweight associated with better academic performance and protection from conditions such as coronary heart disease, diabetes and stroke.

Perhaps it was Miranda, transplanted into a body battered by the surgeries, stresses and hormonal excesses of fertility treatment, who had a tougher time post-conception. But then, as our eldest child, she benefited from our undivided attention for the first two years of her life. Firstborn girls tend to do better educationally and have higher aspirations than their siblings.

It seems the hand that reproductive science has dealt our children may have slight benefits for both of them. (Note to self: remember these for when the “Why me?” question crops up.)

As for personality, our kids’ place in the family might not be as character-forming as we might think. In 2015, a major multinational study found “no birth-order effects on extraversion, emotional stability, agreeableness, conscientiousness or imagination”. Meanwhile, researchers studying 377,000 US teenagers concluded that birth order has an “infinitesimally small” effect on personality.

I am sure there will be times when they envy one another’s position in the family (is there anyone with a sibling who hasn’t occasionally wished they could swap places?), but for now Miranda and Felix seem content in their roles as big sister and little brother. And, if resentment rears its ugly head, I will tell them that the important thing is not who was born first, but that they were born at all.

We chose the name Felix because it means “lucky”: lucky to have been our sole pre-embryo to make it to the blastocyst stage in vitro; lucky to have survived freezing to -196C and thawing without being damaged by intracellular ice crystals; lucky to have implanted successfully and grown healthily in the womb.

Some evenings, after I kiss him goodnight, he snuggles up to me and says: “Tell me the story of how I came in our family.” Every time I do, I remind myself that, after so many years of thinking we would never have children, to be wondering about the impact of their birth order is a wonder in itself.

@ElyssaCB

Choosing Childcare by Elyssa Campbell-Barr is published by Cross Publishing, £9.99. To order a copy for £8.49, go to bookshop.theguardian.com or call the Guardian Bookshop on 0330 333 6846