Two years ago, I received a text no one would ever want to receive. Sent by my ex-boyfriend Ethan, it was a screenshot of my profile on a DNA-testing website. Above it he'd written the horrifying words: 'You are my sister.'

I was utterly devastated. I couldn't sleep for days. I kept having flashbacks to the special times we'd shared . . . it was all tainted. Of course, when we'd got together we'd had absolutely no idea we were related – or that there was even the remotest possibility we could be.

Teenage sweethearts, we'd had a sexual relationship for a year between the ages of 17 and 18. We'd only split up when we went to university because we felt too young to settle down. To be honest, in the years that followed I'd always harboured hopes that one day we might be reunited. When we bumped into each other at a friend's wedding in our mid-20s, I was struck by a wave of emotion and wondered whether we should rekindle our relationship.

Thankfully nothing happened, and I met my husband Ben shortly after. But things could so easily have gone the other way. What if Ethan had become my husband and father of my children?

It's unthinkable. The idea that I've been intimate with my own half-brother is torturous enough.

So how did this happen? A month before that devastating message, Ethan and I had discovered our mothers had undergone fertility treatment with the same doctor, conceiving us via sperm donation.

Such was the aversion to discussing such treatments in the 1970s and 1980s that fertility doctors would advise patients to go home and have sex after the procedure. That way, if they did get pregnant, they would never know for sure who the biological father was. They were advised never to tell their children, too.

DNA tests weren't available then, so paternity could not be proven either way.

Victoria discovered she had been conceived via a sperm donation one month before finding out Ethan was her half-brother

The doctor the mothers went to – Dr Burton Caldwell, once a professor of medicine at the prestigious Yale University and doctor at Yale New Haven Hospital, whose private fertility clinic was situated near the hospital, with some treatments carried out in Yale buildings – had told both of them they'd received sperm from an anonymous donor. But that donor had in fact been him.

It turned out he had been secretly donating his own sperm to his patients for 20 years.

Not only is this morally reprehensible, flying in the face of any code of ethics, but there have been grave consequences for me, my family and his many other offspring.

New Haven, Connecticut, where Dr Caldwell's clinic was based, has a current population of just 135,300. At the last count, I had 25 half-siblings – 14 sisters and 11 brothers – and consequently there are more than 50 first cousins in our vicinity.

But more are always coming forward. In fact, as well as Ethan, I've recently learned I had two other half-siblings at our school – one sister and another brother. This increased my chances of unwittingly having a relationship with a close relation.

It's a serious issue for my children, aged eight and three, too. Should they stay in the area, they will need to ask potential partners to have DNA tests to avoid unwitting incest with unknown cousins.

And to think I could have lived my whole life without knowing the truth.

In 2019, aged 34, I was hospitalised with a fever, and spent months struggling with weakness and pain. Doctors couldn't get to the bottom of it, so in 2020 I decided to do a DNA test to see if it would highlight any underlying genetic conditions. I'd always been interested in my heritage, because my father's family is German; I wondered what percentage of my DNA came from there.

Victoria and Ethan were high school sweethearts and dated for around a year

I grew up in Connecticut with my mum Maralee, a legal secretary, dad Roy, an electrician, and my brother, who's eight years older. My parents divorced when I was ten, and although my relationship with my dad has had its rocky moments, we're on good terms now.

My mum and I are so close that she lives in a self-contained flat next door to me and my family. But when I told her about the test, her reaction took me by surprise.

'Please don't do it, Victoria,' she begged. 'Don't sell your private information to some company.' She wouldn't let it drop.

'Honestly, Mum, it will be fine,' I assured her.

I paid the £80 fee, spat in the tube provided and sent off the sample to 23andMe, a popular DNA analysis and ancestry company that has 15 million users worldwide. I thought so little of it I didn't even mention it to my dad.

A month later, an email with my results arrived. When I scanned the health page, my heart sank. There was nothing helpful at all to explain my illness.

Reading my results in more depth, I thought: 'That's weird,' looking at the page where they listed my geographical heritage. 'There's no Germany.'

Even stranger, on my relatives' page – where you are informed if you are a genetic match with any of 23andMe's other users, with links to their user profiles – there were several complete strangers listed as 'half-siblings'.

As well as Ethan, Victoria recently learned she had two other half-siblings at her school – one sister and another brother

But like the absence of my German heritage, I put it down to inaccuracy, assuming they were distant cousins who had been listed incorrectly, telling myself: 'What a waste of money!'

Then I noticed multiple messages in my inbox.

'Hi, I know what you're seeing might be alarming,' read the first one, from a woman called Sarah. 'Please reach out if you want to know more.'

'Now I'm getting spam!' I thought crossly.

I clicked on the next one. It was also from Sarah, this time asking if my parents had gone to Yale for fertility help. I froze. How on earth did she know that?

When I was 17, Mum had told me that when she and Dad struggled to conceive a second child they had gone to a fertility clinic, where Dad's sperm had been artificially inseminated, resulting in my birth. But I hadn't thought much of it.

I dialled Sarah's number, feeling suspicious and confused. That's when she dropped the bombshell.

After a brief hello, Sarah said: 'I'm sure you're curious about why you have eight other half-siblings. Did your parents need fertility help?'

The genetic ancestry app 23andMe helped uncover the truth about Dr Burton Caldwell's horrific actions

My brain skipped over the first part, unable or unwilling to understand what it might mean. Eight half-siblings? I just replied: 'Yes.'

'I don't think your dad is your biological father,' said Sarah. 'I think it was your mum's fertility doctor, Dr Burton Caldwell. I'm so sorry.'

I actually laughed. Impossible! Dad was my dad.

But Sarah said she'd taken her own DNA test previously and had found herself linked to Dr Caldwell's relatives, along with a number of half-siblings.

'I started investigating it,' she explained, 'and discovered all of our mothers went to Dr Caldwell for fertility treatment. I advise you to speak to your parents. I'll be here any time for anything you need.'

When the call ended, I sat in silence, utterly bewildered. Surely my parents wouldn't lie to me?

I shouted for my husband Ben and blurted everything out. His face went from laughter to utter shock when he realised I was deadly serious.

'I need to talk to Mum right now,' I said, heading for her flat next door.

DNA tests revealed Victoria and Ethan had the same father

'Mum,' I said. 'Do you know the name of your fertility doctor?'

Panic flashed across her face.

'Was it Dr Burton Caldwell?' I asked.

Mum went white as a ghost. 'Victoria, please sit down,' she said gravely.

Mum admitted they hadn't used Dad's sperm but that of an anonymous donor – a medical student the clinic had supplied. They'd followed Dr Caldwell's advice not to tell me and made a pact to take the truth to their graves.

By this point we were both in tears.

Then I realised Mum only knew half the story; she still thought my biological father was that unknown medical student.

When I told her Dr Caldwell had used his own sperm, she recoiled and shook her head vigorously. He'd been her doctor for years and she trusted him. She didn't believe it. Instead she turned her attention on me: 'Are you upset that I never told you?'

The answer was yes. For as long as I could remember I'd felt different in my family, like a puzzle piece that didn't quite fit. I didn't look anything like my dad. 'At least we share the same hands!' I'd joke. 'Maybe I'm the postman's daughter.'

'Don't worry, you look like your mother,' he'd always reply, quickly changing the subject.

Confused and distressed, I hurried back home.

Unable to sleep, I spent the night Googling Dr Caldwell. All I found was one old, blurry photo. If there was a physical resemblance, I couldn't see it. Somehow I made it through the next few days. I swung wildly from feeling grateful my parents had tried so hard to have me, to anger at how they'd handled it – and disbelief at Dr Caldwell's actions. I was a mess.

Telling Dad was hard. I sent him a text ending with the words: 'I'm sorry you've had to carry this for so long. I love you.'

He replied, confessing his fears I'd find out and it would end our relationship. He'd always known he wasn't my biological father, but he was appalled by Dr Caldwell's lies.

Mum, too, was struggling to come to terms with it. How dare a doctor do such a thing without her consent? To me it was medical rape.

Days later, I found myself doing something risky. Having discovered his address online, I drove to Dr Caldwell's house to confront him – something none of my new-found siblings had yet done.

I knocked and his wife answered. As I babbled like a lunatic, she began to close the door. 'Wait,' I said. 'I think your husband is my biological father.'

The next minute I was sitting in her dining room alone. My heart pounded as I heard footsteps approaching. I didn't have a plan; what was I going to say?

Then there he was. Incredibly tall, in his 80s he seemed very old and infirm. But the instant I saw his greyish blue eyes – my eyes – it all became real.

As I told him everything I knew, he didn't flinch.

'I haven't thought about it in many years,' he said simply. 'I was in the business of making babies.'

I hadn't come for an emotional reunion – he would never be a father to me – but his cold manner, the way he spoke as if he hadn't done anything wrong, was so shocking.

When I asked how many times he'd done it, he chuckled, as if to say he had no idea.

'Are you expecting something from me?' he asked coldly.

'I just needed to see you to make it real,' I replied. 'And for you to supply me with your medical history.'

He told me nothing. (Though he later provided me with medical information, it held no clues to my illness.)

We only spoke for eight minutes but it felt like an hour. I could see there was no remorse.

On the drive home, I had to pull over in tears. I looked at my daughter, so tall for her age; did that come from this man? What on earth was I going to tell her as she grew up? The ramifications seemed never-ending.

Every few months another half-sibling would happen to take a DNA test and learn the horrible truth, showing up on my 23andMe relatives page.

But it was about to get much worse. In 2023, I attended a school reunion, where I saw Ethan for the first time in years.

By then, I was used to telling people my story. Along with therapy and journalling, it helped me process what had happened. And I was sure Ethan would react with shocked laughter.

Far from laughing, however, his face drained of colour.

'Mum recently told me they went through fertility treatment to have me,' he said, looking appalled.

I quipped: 'Oh yes, I'm sure we're siblings!' Then the fact this was a real possibility hit me like a truck.

Ethan texted his mum to find out the name of her doctor, but she didn't reply before it was time to leave. I felt sick.

As I was drove home, Ethan rang to say his mum had also been to Dr Caldwell. 'I need to take a DNA test,' he said.

I could tell Ethan was clinging to the idea this was all a mistake. But I already felt sure what those results would show.

In tears, I thought back to the instant connection we'd had as teenagers. Our love had been real and pure. Was it because we were siblings all along? All those memories were corrupted by what Dr Caldwell had done.

Throughout this whole rollercoaster, Ben has been my rock. Even with this horrifying possibility, his love was unconditional.

Three weeks later, Ethan's test confirmed our worst fears. Then came the waves of disgust. I'd been intimate with my sibling. I'd unknowingly committed incest.

Ethan and I met up a couple of days later. When we saw each other, we realised that connection was still there. The bond we'd had hadn't vanished just because of this awful discovery.

I know that's hard to understand. But if you discovered tomorrow that your husband or wife was a sibling, would you instantly love them less?

Ethan's partner said she didn't want us to see each other again. Initially I was angry; I felt blamed for something outside of my control. Then I realised I had a choice: to turn my repulsion inward and let it overwhelm me, or to give myself compassion. Why should we hate ourselves? It wasn't our fault.

Now 40, I'm still struggling. Dr Caldwell died in February this year, aged 86, a year after his actions became public knowledge. I'm not sorry he's gone but I am sorry he got off scot-free.

Meanwhile other cases of fertility fraud, including where doctors use their own sperm without consent, are being discovered all the time.

In a licensed clinic in the UK, a donor's sperm can only be used to create a maximum of ten families. In America, however, each state makes its own regulations. There are 'guidelines' on how many donations a single donor can make but it's not enforced in law.

Last year my mother and I were among those who testified before the Connecticut legislature's Judiciary Committee for a proposed Bill to prohibit doctors from using their own sperm to inseminate patients without consent. Shockingly, it did not become law due to a lack of support. So even now what Dr Caldwell did isn't considered criminal.

Several of his patients – including my mum – bravely launched civil cases against him before he died.

Mum's case, which is still ongoing, also lists the Yale University School of Medicine, Yale New Haven Health and the Yale New Haven Hospital as co-defendants. This is because at the time of her treatment she believed Dr Caldwell was affiliated with Yale, and he told her his sperm donors were Yale medical students.

What Dr Caldwell did changed everything: my sense of self, my relationship with my parents, my fears for the future. It's also complicated the lives of my children.

But this is about so much more than just one man. We need proper laws, because the fertility industry is like the wild west.

I can't be the only one who's accidentally had a sexual relationship with a close relative. It's just so taboo that the world doesn't want to hear about it.

I know people expect me to stay silent and ashamed. But I'm placing all the disgust and disgrace firmly on Dr Caldwell's shoulders. Him and all the other doctors like him who think they can play God.

I don't know if I'll ever see Ethan again. I hope so. Until then all I can do is speak out to try to bring about change. One day I want to write a book about what I've been through. If sharing my story helps to create legislation, protect vulnerable families and prevent anyone else going through this nightmare, it will be worth it.

Representatives for the Yale bodies and Dr Caldwell's attorney were approached for comment. Only the Yale New Haven Health responded and its spokesman said: 'We are aware of the allegations. We maintain there is no evidence of Yale New Haven Health's involvement in the conduct alleged against Dr Caldwell.'

Ethan and Sarah's names have been changed

As told to: KATE GRAHAM