Babies born more than three months before their scheduled due dates have a three times greater chance of showing early signs of autism, according to a new study.

Researchers from Boston University School of Medicine and Boston Medical Center based that finding on a checklist test called the M-CHAT that looks for signs of autism in toddlers.

The M-CHAT is used when parents or doctors are concerned about a child's development. It includes questions for caregivers on 23 behaviors. Checking any three items or two of six critical items as "unable to perform" leads to a positive screen on M-CHAT.

About 1,000 preemies were screened when they were 2 years old, and more than 21 percent screened positive. In the general population, the rate is 5.7 percent, according to a news release on the study.

"Even after excluding those children with motor, vision and hearing impairments, we still found 16 percent screened positive," said lead author Dr. Karl Kuban.

He said, however, that a positive score on this test did not necessarily mean that a child would develop autism. Making a diagnosis of an autism-spectrum disorder often occurs after many years of symptoms.

These findings appear in the January issue of Journal of Pediatrics.