(WASHINGTON-AFP) - A post-mortem analysis of half a dozen autistic boys showed that their brains were heavier and contained many more neurons than counterparts without the disorder, US researchers said Tuesday.

The study, while small, suggests that brain overgrowth may be occurring in the womb, according to the findings published in the November 9 issue of the Journal of the American Medical Association.

Researchers examined the brains of seven autistic boys, age two to 16, most of whom had died by drowning. The 16-year-old's cause of death was undetermined and one eight-year-old died of muscle cancer.

When they compared them to a control group of six boys without autism who died in accidents, they found that the brains of autistic boys had 67 percent more neurons in the prefrontal cortex and were nearly 18 percent heavier than normal brain weight for age.

"Because cortical neurons are not generated in postnatal life, this pathological increase in neuron numbers in autistic children indicates prenatal causes," the study said.

The researchers suggested that "faulty prenatal cell birth or maintenance may be involved in the development of autism."

The prefrontal cortex is where language and communication is based, as well as behaviors such as mood, attention, and social ability. Typically, autistic children have difficulty in these areas.

However, more research is needed to confirm the link, and to determine if and how brain differences may be tied to the severity of an autistic person's symptoms.

"Factors that normally organize the brain appear to be disrupted," said an accompanying editorial by Janet Lainhart of the University of Utah and Nicholas Lange of the Harvard University Schools of Medicine and Public Health.

Previous studies have suggested that clinical signs of autism tend to coincide with a period of abnormal brain and head growth that usually becomes apparent at nine to 18 months of age, according to the article.

Autism includes a wide spectrum of developmental differences and may range from mild social awkwardness to complete inability to communicate, repetitive movements, sensitivity to certain lights and sounds, and behavioral problems.

As many as one in 110 children is diagnosed with autism. The disorder is three to four times more common in boys than in girls, according to the advocacy group Autism Speaks which helped fund the study.