“This person came out of the woodwork and said, ‘I want to study trophoblastic inclusions,’ ” Dr. Walker recalled. “Now I’m fairly intelligent and have been an obstetrician for years and I had never heard of them.”

Dr. Walker said she concluded that while “this sounds like a very smart person with a very intriguing hypothesis, I don’t know him and I don’t know how much I trust him.” So she sent him Milky Way bar-size sections of 217 placentas and let him think they all came from babies considered at high risk for autism because an older sibling had the disorder. Only after Dr. Kliman had counted each placenta’s inclusions did she tell him that only 117 placentas came from at-risk babies; the other 100 came from babies with low autism risk.

She reasoned that if Dr. Kliman found that “they all show a lot of inclusions, then maybe he’s a bit overzealous” in trying to link inclusions to autism. But the results, she said, were “astonishing.” More than two-thirds of the low-risk placentas had no inclusions, and none had more than two. But 77 high-risk placentas had inclusions, 48 of them had two or more, including 16 with between 5 and 15 inclusions.

Dr. Walker said that typically between 2 percent and 7 percent of at-risk babies develop autism, and 20 percent to 25 percent have either autism or another developmental delay. She said she is seeing some autism and non-autism diagnoses among the 117 at-risk children in the study, but does not yet know how those cases match with placental inclusions.

Dr. Jonathan L. Hecht, associate professor of pathology at Harvard Medical School, said the study was intriguing and “probably true if it finds an association between these trophoblast inclusions and autism.” But he said that inclusions were the placenta’s way of responding to many kinds of stress, so they might turn out not to be specific enough to predict autism.

Dr. Kliman calls inclusions a “check-engine light, a marker of: something’s wrong, but I don’t know what it is.”

That’s how Chris Mann Sullivan sees it, too. Dr. Sullivan, a behavioral analyst in Morrisville, N.C., was not in the study, but sent her placenta to Dr. Kliman after her daughter Dania, now 3, was born. He found five inclusions. Dr. Sullivan began intensive one-on-one therapy with Dania, who has not been given a diagnosis of autism, but has some relatively mild difficulties.

“What would have happened if I did absolutely nothing, I’m not sure,” Dr. Sullivan said. “I think it’s a great way for parents to say, ‘O.K., we have some risk factors; we’re not going to ignore it.’ ”