Morénike Giwa Onaiwu was shocked when day care providers flagged some concerning behaviors in her daughter, Legacy. The toddler was not responding to her name. She avoided eye contact, didn’t talk much and liked playing on her own.

But none of this seemed unusual to Dr. Onaiwu, a consultant and writer in Houston.

“I didn’t recognize anything was amiss,” she said. “My daughter was just like me.”

Legacy was diagnosed with autism in 2011, just before she turned 3. Months later, at the age of 31, Dr. Onaiwu was diagnosed as well.

Autism, a neurodevelopmental disorder characterized by social and communication difficulties as well as repetitive behaviors, has long been associated with boys. But over the past decade, as more doctors, teachers and parents have been on the lookout for early signs of the condition, the proportion of girls diagnosed with it has grown.

In 2012, the Centers for Disease Control and Prevention estimated that boys were 4.7 times as likely as girls to receive an autism diagnosis. By 2018, the ratio had dipped to 4.2 to 1. And in data released by the agency last month, the figure was 3.8 to 1. In that new analysis, based on the health and education records of more than 226,000 8-year-olds across the country, the autism rate in girls surpassed 1 percent, the highest ever recorded.

More adult women like Dr. Onaiwu are being diagnosed as well, raising questions about how many young girls continue to be missed or misdiagnosed.

“I think we just are getting more aware that autism can occur in girls and more aware of the differences,” said Catherine Lord, a psychologist and autism researcher at the University of California, Los Angeles.

In the first study of autism, published in 1943, Dr. Leo Kanner, a psychiatrist at Johns Hopkins University, identified 11 children — eight boys and three girls — with “the powerful desire for aloneness and sameness.”

It wasn’t until 1980 that autism was officially recognized in the Diagnostic and Statistical Manual of Mental Disorders, the primary classification system used by psychiatrists. But the diagnosis was narrowly defined, requiring a pervasive lack of interest in people, as well as language impairments and particular fixations, all detected before an infant was 30 months old.

Over time, as experts gained understanding of autism as a wide spectrum of behaviors, the D.S.M.’s criteria broadened. Children could have difficulty making friendships or imitating others; verbal or nonverbal communication delays; or restricted or repetitive interests, such as a preoccupation with specific topics.

Most girls diagnosed with autism in those early days had intellectual disabilities, making it easier to identify them, Dr. Lord said.

And many clinicians, she said, did not know that autism could manifest differently in girls who have less noticeable physical manifestations of the condition. Studies since have shown that girls with autism are more likely than boys to camouflage their social challenges, sometimes by mimicking the behaviors of the girls around them. What’s more, girls are often treated differently by adults, such as being told to smile or being encouraged to participate more in group play. Even the toys clinicians used to evaluate children for autism were later criticized for being more appealing to boys.

“There have always been autistic girls,” Dr. Lord said. “I think people didn’t knock themselves out to be aware that girls might be treated slightly differently.”

The most recent edition of the D.S.M., published in 2013, acknowledged an even broader spectrum of behaviors that might indicate autism and specified that autism in girls could go unrecognized because of “subtler manifestations of social and communication difficulties.”

Kevin Pelphrey, a neuroscientist and autism researcher at the University of Virginia Brain Institute, said that more than 15 years ago, when his own daughter began to show signs of autism, even he didn’t recognize them. Pediatricians told him, “‘It’s probably not autism — she’s a girl,’” he recalled.

The brain systems involved in social behavior develop more quickly in girls, he said, which may be a “protective factor” for girls with autism, especially in early childhood.

But as they grow older and social relationships among girls become more complex, girls with autism begin to stand out more and are often bullied, Dr. Pelphrey said.

“That leads to another big difference between boys and girls: Girls can be much more likely to develop anxiety and depression,” he said.

Those psychiatric problems can also obscure the underlying autism and lead to misdiagnoses.

Dena Gassner, 61, an autism rights advocate in Garden City, N.Y., had social and emotional challenges since she was young, but doctors never mentioned autism as a possible diagnosis. Like many girls with the disorder, Mrs. Gassner had been sexually abused, and her emotional problems were later attributed to the abuse. She was also incorrectly diagnosed with bipolar disorder.

She wasn’t diagnosed with autism until she was 40, six years after her son was diagnosed. She was initially taken aback by the diagnosis, she said, partly because her son’s struggles — including language delays and fixations on certain activities and movies — were so different from hers.

“I could never have looked at my son and seen myself in his reflection,” she said.

Mrs. Gassner and Dr. Onaiwu are members of the Interagency Autism Coordinating Committee, a group of federal scientists, academics, parents and autistic adults who advise the Department of Health and Human Services on research and policies.

Now that they have met many other women who were diagnosed in adulthood, both women said they suspect that autism’s true sex gap is smaller than what the data shows.

“They’re not evaluating how many autistic girls exist,” Mrs. Gassner said. “They’re evaluating how many autistic girls we’re finding.”

In a 2017 review of dozens of studies, researchers from Britain estimated that the true sex ratio was closer to 3 to 1. Some online surveys that include people who have self-diagnosed show an even lower skew of males to females.

Although autism is no doubt underdiagnosed in girls, most experts say that it’s more prevalent in boys. Autism has strong genetic roots, and some studies have suggested that the sex differences may stem at least in part from innate biological differences. For example, girls with autism tend to carry larger genetic mutations than boys do. Girls may need a bigger “genetic hit” to be impacted, Dr. Pelphrey said, possibly because they carry protective genetic factors.

The shifting demographics of autism are not limited to sex. The proportion of nonwhite children with autism has also grown swiftly over the past decade. In the C.D.C.’s new report, autism rates among Black and Latino 8-year-olds surpassed those of white children for the first time.

“Autism was this thing that happened to little white boys, and sometimes those little white boys grew up to be Trekkies or Silicon Valley programmers,” Dr. Onaiwu said. “It didn’t happen to the rest of us — but it did.”

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