When I met my husband Anthony, one of the most intriguing discoveries I made about him was the fact he had a family coat of arms. I spotted it on a signet ring, worn by another relative – three stags' heads and the motto, "facta non verba": "deeds, not words". I also discovered he had a huge family tree, full of well-to-do landowners who could be traced back to the 14th century.

Part of the reason why it intrigued me was the fact that my family are from much more humble origins. They are a mix of Irish immigrants who left after the potato famine, Jewish East Enders, Welsh craftsmen and an Indian grandfather who arrived in London in the Thirties and made his fortune by setting up a vinegar factory.

I've always been interested in family history. In 2011 I became the one of first British journalists to sign up to the company 23andMe, which promised to tell me a bit more about who my ancestors were – and the health issues I may have inherited from them.

All I had to do was send off a sample of saliva to the firm's California headquarters. Six weeks later, I logged onto the site.

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As well as telling me that I have the gene for smelling asparagus in urine and wet ear wax, it also correctly identified that I am, indeed, the mother of my children (and that my husband is their father) – as Lily, Clio and Anthony also sent in their samples anonymously. So there was no doubt in my mind that the service worked.

23andMe also allows you to see details of other individuals who have used the service and with whom you share DNA. Over time, I received more and more notifications, usually telling me about distant cousins whose Irish ancestors had headed for America.

However, a couple of weeks ago I had a big catch-up on the site and spotted that one of the names on the list of 500 distant relatives was none other than my husband.

As the popularity of genealogy grows, keeping family secrets will become much harder.

According to the analysis, it turns out we share a section of DNA on the 18th chromosome, due to a relative four or five generations back.

In other words, we were fourth or fifth cousins. As our families haven't exactly been moving in the same circles, this immediately led to all sorts of fevered speculation about illicit Upstairs, Downstairs-style affairs.

This wasn't the first time 23andMe had opened a can of worms for me. Four years go, its ancestry service told me that the woman I'd always known as my late grandmother's much younger sister was, in fact, her daughter. She'd been born out of wedlock – and raised by my great-grandmother instead. Even my late father believed the woman was his aunt. Cue all sorts of family drama about what had really happened, and who should and should not be in on the secret.

With ancestry services becoming more popular – thanks to companies such as Ancestry and Me, 23andMe, Living DNA and MyHeritage DNA – it seems that uncomfortable DNA bombshells are becoming more common too. More than 12 million people around the world have used one of the home-testing kits provided by these firms (which cost between £60 and £120), decanting their saliva into a test tube or taking a swab from inside their cheeks.

However, it seems that idle curiosity about whether we are descended from a Viking or Native American can often throw up more than we bargained for. These kits are uncovering secrets that would otherwise have gone to the grave. In fact, so many are surfacing that there is a now a global network called the NPE Friends Fellowship – NPE stands for "Not Parent Expected" – to support individuals who have discovered their family relationships aren't what they were led to believe.

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The network's founder is Catherine St Clair, who was given an Ancestry DNA test for her birthday in 2017. When Catherine's brother took a test too, she was stunned to discover that the results revealed he was her half, rather than full, sibling.

It meant that the late father she always known as "Daddy", hadn't fathered her after all. She managed to track down her biological father, a man her mother had once worked with, who had also since died, and she later went on to find two half-sisters through the site.

St Clair says the NPE Friends Fellowship, whose Facebook group has more than 2,500 members from 12 countries, helps individuals come to terms with a wide variety of scenarios – people who may have been born as a result of a one-night stand or extra-marital affairs or who didn't know they were adopted. She helps those whose family members struggle to come to terms with the revelation. She estimates that more than half of NPE members have had a door slammed in their face by a relative.

St Clair says: "People don't realise that having an NPE discovery can really be a significant trauma. It makes them question everything."

To be fair to 23andMe, the company knows this can be an issue. For this reason, 23andMe's spokesperson Andy Kill tells me, signing up to site's "DNA Relatives" feature is only an optional feature. "We do alert customers to this fact, and this is exactly why you must make an active choice to participate in the tool as it may uncover unexpected information or relationships," he says.

Adam Rutherford, a British geneticist and author of the book A Brief History of Everyone Who Ever Lived points out that with such services on the rise, "this ethical space is unprecedented". However, while a home DNA kit may have uncovered an uncomfortable truth in my family's recent past, Rutherford tells me I am probably wasting my time digging for any scandals that might help explain how Anthony and I are related. And our shared DNA is not so shocking after all, it seems.

"DNA tests are good at identifying close relatives like mothers, sisters, brothers and cousins," he says. "But after the second or third cousin, the usefulness of DNA massively drops off. So while it has great potency for identifying missing close relatives, and parents of adoptees, beyond that it's a bit meaningless. The chances of any two British people being fifth or sixth cousins is very high."

Robin Smith, a senior researcher for 23andMe, points out that I share about 0.1 per cent of my genome with my husband. "Generally speaking, fourth cousins – of which the average person is expected to have about 1,000 – share a set of third great-grandparents, people born about 150 or so years ago. We all have 32 third great-grandparents, so there are a lot of possibilities [for uncovering distant relatives]."

But while my relationship with Anthony is less scandalous than we thought, DNA discoveries about the more recent past will continue to change family dynamics and rewrite family histories.

After all, the post-war generation, whose secrets are now being revealed, could never have foreseen their children and grandchildren would find it so easy to open their Pandora's box. As the popularity of genealogy grows, keeping family secrets will become much harder.

Yet St Clair hopes that this will diminish the culture of shame that many of our forebears had to live with. "It was a different era 50 to 70 years ago, and no one imagined that these secrets could ever be revealed so easily. But with the advent of this new and easily accessible technology, I'm hopeful that the stigma of 'illegitimacy' will eventually disappear."