NEW YORK — “How could she?” It’s the headline du jour whenever a horrific case emerges of a mother killing her kids, as Lashanda Armstrong did when she piled her children into her minivan and drove straight into the frigid Hudson River.

Our shock at such stories is understandable: They seem to go against everything we feel about the mother-child bond.

But mothers kill their children in the U.S. much more often than most people would realize. By conservative estimates it happens every few days, at least 100 times a year.

Experts say more mothers than fathers kill their children younger than 5 years.

And some say our reluctance as a society to believe mothers would be capable of such acts is hindering our ability to recognize warning signs, intervene and prevent more tragedies.

So the problem remains.

“We’ve learned how to reduce auto fatalities among kids, through seat-belt use. We’ve learned how to stop kids from strangling on the strings of their hoodies. But with this phenomenon, we struggle,” said Jill Korbin, an anthropologist at Case Western Reserve University in Cleveland who has studied such slayings.

How common is filicide, or killing one’s child, among mothers? Finding accurate records is nearly impossible, experts say. One problem is classification: The legal disposition of these cases varies enormously.

Also, many cases doubtless go unreported or undetected, such as very young mothers who kill their newborns after hiding the entire pregnancy.

“I’d say a mother kills a child in this country once every three days, and that’s a low estimate,” said Cheryl Meyer, co- author of the book “Mothers Who Kill Their Children.”

Several databases track such killings but do not separate mothers from fathers. At the Department of Health and Human Services, the National Child Abuse and Neglect Data System reported an estimated 1,740 child fatalities — meaning when a child dies from an injury caused by abuse or neglect — in 2008.

Meyer and co-author Michelle Oberman interviewed women at the Ohio Reformatory for Women. They found that of 1,800 women at the prison, 80 were there for killing their children.

It’s also a crime that defies neat patterns: It cuts across boundaries of class, race and socioeconomic status.

Oberman and Meyer came up with five categories: filicide related to an ignored pregnancy, abuse-related, neglect-related, assisted or coerced filicide (such as when a partner forces the killing) and filicide with the mother acting alone.

Different as these cases are, though, there are factors that link the poor teen mother who kills her baby with an older, wealthier mother, and one of them, experts say, is isolation.

“These women almost always feel alone, with a total lack of emotional support,” said Lita Linzer Schwartz, professor emeritus of psychology and women’s studies at Pennsylvania State University, and co-author of “Endangered Children.”

Schwartz said women are often not checked for mental illness after their crimes.