One in 900, said the scan operator at Henny Beaumont’s 12-week ultrasound scan, and that didn’t sound very scary. “You have the same risk of a baby with Down’s syndrome as a 25-year-old mother,” she was told, reassuringly (Henny was 35 at the time). At the next scan, three months later, there was more good news: all four chambers of the baby’s heart were working “beautifully”. “I’d already had two babies and I was quite blase about it all,” says Henny. “I thought, this will be easy. I’m good at it. I can make babies, pop them out and sail on with my life.”

A few weeks later, Henny and her husband Steve Epstein’s third daughter, Beth, was born and for four hours everything seemed fine. The baby was starting to feed and slept quietly; Henny was recovering well. All the couple wanted was to get home to introduce the new arrival to her sisters. And then the midwife asked an innocuous-sounding question, one that would change their lives for ever.

“She said, who does the baby look like?” Henny remembers. “She was asking that because she could see that Beth had all the hallmarks of a baby with Down’s – slanted eyes, a flat head, and poor muscle tone.”

Confirmation that Beth had Down’s came the following day; later that week, a detailed cardiac scan revealed a hole and a faulty valve in her heart, which would need corrective surgery.

While the doctor talked about the risks of heart failure, potential breathing difficulties and how important it was to take Beth straight to hospital if there were any concerns, all Henny could hear were the words: “Life is over.” She didn’t mean Beth’s life, though at that stage she wasn’t even sure if Beth’s heart defects meant she would have a life at all: she meant her own life, Steve’s life, and the lives of their other daughters, Matty and Bridie, then six and three.

A fortnight earlier, Henny had been dreaming of a child who would be bright, quick, popular and achieving, like her older daughters: now she put a big black cross through her dreams and could see only bleakness and failure ahead.

It’s 13 years since that day, but Henny remembers every detail. Now she is consigning her memories to a graphic novel – A Hole in the Heart – that tells the story of what happened next. That’s because, says Henny, the years following Beth’s arrival have not been impossible as she believed they would be – and she wants others, who might one day find themselves in a similar situation, to know that life will not be over.

Life will be different but, importantly, the differences will not be all bad. She has discovered that the fear about raising a child who is different is much more debilitating than the reality. “Beth sailed through the open heart surgery and today she is so much part of who we all are,” she says.

But life with a child with a disability often seemed impossibly complex, especially in the early months and years, and depicting it in graphic form makes the feelings associated with the experience more accessible. “As an art form it seems less weighty, and I also like the idea that comics have traditionally been a male arena, and now we’re claiming them to illustrate more domestic concerns,” says Henny.

A Hole in the Heart packs a powerful emotional punch by pitting ordinary scenes of life with a newborn alongside the devastating reality of the diagnosis of Beth’s condition; it also allows Henny to show her unspoken fears alongside her declared feelings.

Beth’s arrival forced Henny to re-evaluate her ideas of what it meant to be a parent, and what “achievement” and “success” actually mean. She is candid about the fact that she expected to have children who were bright and did well at school; now, she was the mother of a little girl who was not going to be on the top table in class and might not be popular with her peers. “I didn’t know how I’d cope,” says Henny.

One friend told her she was “off the hook” in terms of competitive parenting; Henny took that hard. “I still cared desperately about Beth doing as well as she possibly could. But what changed was the feeling – I think many mothers have this – that everything that went wrong was my fault and that everything that went right was to my credit. Having Beth showed me how wrong and unhelpful that can be.”

Today, as well as writing and illustrating her book (which was shortlisted for a Myriad graphic novel award), Henny is about to start lecturing to health workers who work with people who have disabilities.

What, if anything, could the medical world have done better to support her and Steve through the early months and years with Beth? “They say there’s no right way to give a difficult diagnosis, but there’s certainly a wrong way,” she says. “You need a doctor who is going to stay with your pain. The guy we saw was clearly scared by the diagnosis and said he had to leave almost immediately because he had somewhere he had to be. That feels awful.”

Healthcare staff rarely have direct experience of what it’s like to raise a “different” child and their views, like the view of so many people, are coloured by lack of experience.

Finding yourself the parent of a child with a disability leaves you ranting about the prejudice of a world out of tune with disability, while at the same time acutely aware that until recently you, too, were part of that world; and perhaps that realisation makes you crosser still. “For a long time I was so angry that disabled people were so invisible in society, and I was angry about the way they are referred to and the language around them,” Henny recalls.

She still cares, but today is much more relaxed about it. Where once she’d have been offended or angry, she is more likely to be amused; and meeting difficulty with humour is a sure sign of being on top of things.

There is one dark dilemma at the core of Beth’s story, about which Henny is honest. “If we had found out antenatally, we would have had an abortion. I’m absolutely pro-choice, but I’m so grateful we didn’t know before we had Beth. I couldn’t imagine life without her. I adore her.”

They are a family of six now – four years after Beth was born, Henny and Steve went on to have the fourth child they had always planned. “We had a CVS test to check for Down’s syndrome because that gives a conclusive result, not a risk factor like the ultrasound test,” says Henny. Would they have terminated if she’d been carrying another baby with Down’s? “The truth is, I don’t know,” she says.

The new baby was Karl, now nine, who has been a wonderful companion to Beth. “They have been great mates to one another – for a long time they were at the same stage developmentally and then Karl overtook Beth.”

Karl has always adored Beth, as do her two sisters, now 19 and 16. These days, she says, she doesn’t worry too much about the future and what will happen when Beth grows up. Experience has taught her that things will probably work out just fine.

• More information about A Hole in the Heart at hennybeaumont.com