With hindsight, Nicky Clark says early signs of autism were present in both her children. The elder one, though very bright, had a love of routine and was not interested in fantasy games like other children. The younger one liked to line things up in rows and would watch the same video clip over and over again for hours. When she got the diagnosis it came as a huge shock, as it would be for any parent. But there was an additional reason why it was unexpected – both her children are girls.

Autism is an overwhelmingly male diagnosis – it has been described as the "extreme male brain". Boys with the diagnosis outnumber girls by between 10 and 15 to one. The typical high functioning male, if he is lucky, finds a secure post in a university where he can use his exceptional powers for academic study, shunning most social contact as the "eccentric professor", and relaxing at home with his train set in the attic.

But in the developing story of autism – interest in which has increased hugely in the last decade – girls have been neglected. That omission will be remedied this week with the first conference on autistic spectrum disorders in women and girls. One aim will be to examine whether the condition has been underdiagnosed in females – and what links there may be with eating disorders.

According to Janet Treasure, professor of psychiatry at the Institute of Psychiatry, King's College, London, around a fifth of girls diagnosed with anorexia have autistic spectrum features and 20 to 30 per cent may have exhibited rigidity and perfectionism in childhood. Anorexia has been called the female Asperger's (the mild version of autism).

Professor Treasure says: "When I was training at the Maudsley 30 years ago, anorexic girls were treated as little more than malfunctioning machines. The view was that it was an illness that mainly affected middle-class intelligent white girls and was little more than an awkward phase of adolescence. Today there has been a huge change in the understanding of the disease. People with eating disorders find it difficult to change self-set rules and learnt behaviour once fixed in the brain. They also see the world in close up as if looking through a zoom lens, and get lost in the detail. There is a strong similarity to autistic spectrums."

She will tell the conference that there are two aspects to the link. First, people with autistic spectrum disorder are more at risk of getting anorexia. "If girls are obsessed with systems and rules then the rules governing eating become very attractive. They grab them very much."

Second, being undernourished and underweight as a result of an eating disorder exaggerates any autistic traits. The effect of starvation on brain function impairs set-shifting – the ability to think flexibly and to multi-task instead of focusing on one thing – and the ability to read other people's minds. "They become more socially isolated, withdraw more and more into their own world and become cut off and lonely," she says.

The task for therapy is to bring them out of their obsession with eating, or avoiding it. But they have to make "big choices that require a lot of courage". The autistic features are reversible in the majority of sufferers once they regain full weight but it requires "a leap so they stop listening to their eating disorder voices".

Girls whose autistic traits predated their anorexia face a harder task, even supposing they conquer the eating disorder. "For those with high functioning Asperger's they can work out what they have to do – to remember to smile, to ask, 'How are you?' They need to work at it because autistic traits make them uncharming and can cause heartache throughout life. If they remember the social rules it makes them a bit more charming," Professor Treasure says.

Nicky Clark's daughters do not have eating disorders but, like all those with autistic traits, they struggle with the complex rules that oil the wheels of social intercourse. The eldest, Lizzy, 15, is bright, able and verbally confident. She was progressing well at school until the age of 10, when she was ostracised by her peer group and bullied.

Nicky says: "I had always known she had to have a routine. She never really played with dollies or liked imaginative games like other girls. She asked very complicated questions, constantly seeking and clarifying her perception of the world. I had to be very careful with expressions like "he will bite your head off" because she would understand it literally and be frightened."

"She was my first child and I assumed all children were like this. But it became more apparent when she was 10 that there were differences. She moved to secondary school but the bullying became much worse and she started self-harming – biting the backs of her hands till they were raw because of her frustration. Things really spiralled down."

It took 18 months for her to be diagnosed with Asperger's Syndrome – "we really had to fight for a diagnosis," says Nicky – and by that time she had developed head nodding and a mild version of Tourette's syndrome.

Eventually Nicky and her husband Phil, who come from Shrewsbury, moved Lizzy to an independent school, with small classes where she has since thrived. In 2008 she appeared in the award-winning BBC film Dustbin Baby, playing Poppy, a character with Asperger's.

Nicky says: "She has a small group of friends; they realise her take on things is different and they back off. That is all it takes. She has the maturity to say, 'This is the way I like to do things. I am who I am and you do things differently from me.'"

Emily, aged 12, was diagnosed as a toddler – long before Lizzy – with autism and learning disabilities. Nicky says: "She wasn't speaking and my mum, who was a health visitor, said I should take her to the GP who referred her to the child development centre. She was diagnosed the day before her third birthday. I didn't see it coming and all I could think of was Rain Man, the movie with Dustin Hoffman. Nothing the doctors were saying to me was going in. I left the consulting room and collapsed.

"Emily was terrifically affectionate and interested in things but had no language – that was the only symptom. She went to mainstream school and gradually her language started to come through. But around the age of five her friends started to drift away."

They moved her to a special school where things improved. "She is still affectionate but she has grown and matured. Puberty has set in – she is becoming a teenager and there is a natural withdrawal. As long as she has her Teletubbies and Thomas the Tank Engine DVDs she is happy. But there is the huge challenge of her learning disabilities, which can lead to temper tantrums and what I call serious meltdown."

Visitors to the Clark household are often surprised when they learn the girls are on the autistic spectrum. "People say, 'Isn't that a boy thing?' or want to know if she can draw Westminster Abbey. Someone once said they couldn't have autism because it only affected boys."

Ignorance about the condition in the female sex extends to professionals, adding to the distress it causes. Research suggests that even when girls are screened autistic traits are not picked up. In one study of 60 patients at a psychiatric hospital in England, none were diagnosed as autistic but 11 were later shown to have autistic traits. They were diagnosed with other conditions such as personality disorder and schizophrenia.

Charities say a growing number of adult women are coming to them, having recognised themselves in autistic characters on television or in print. Often they are relieved to discover why they are "different". The explanation for their delayed diagnosis, in addition to medical ignorance, is that girls tend to be better at masking the condition. They are better with language, more advanced than boys, less disruptive and better able to compensate for their problems.

Judith Gould, psychologist and director of the Lorna Wing Centre of the National Autistic Society, says: "We are definitely seeing an increase in women and girls being diagnosed. Girls are diagnosed later than boys at age 12 upwards. It is when they hit puberty that it becomes more obvious. Often they are on the periphery of social groups. They have learnt their social skills by intellect, not naturally or instinctively. Teenage girls are very socially demanding on each other and can be marginalised, teased and bullied. Often girls do not show difficult or aggressive behaviour but are shy or passive and depressed."