While younger women rarely have heart attacks, those who do face a greater risk of dying while in the hospital compared with men the same age—especially if they don’t present with the classic symptom: chest pain.

That’s according to a study published today in the Journal of the American Medical Association, which found that about 16 percent of women under 65 who had heart attacks without chest pain died in the hospital compared with 12.5 percent of men under 65. Those gender differences disappeared in those over age 65.

The research, which examined more than 1 million heart attack admissions to hospitals from 1994 to 2006, highlights an important challenge doctors confront: it’s tough to recognize heart attacks in younger women with atypical symptoms—shortness of breath, extreme fatigue, nausea, or heart palpitations and no chest pain—but a delay in diagnosis and treatment leads to a greater risk of dying.

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“Chest pain is a hallmark symptom of a heart attack but fewer women present with this symptom than men,” said study author Dr. John Canto, a cardiologist at the Lakeland Regional Medical Center in Florida. Nearly 19 percent of women under 45 who had heart attacks had no chest pain in the study compared with 13 percent of men.

But Dr. Stacy Fisher, a cardiologist at the University of Maryland Medical Center who wasn’t involved in the research, said plenty of men in the study also presented without chest pain, and in both genders, the absence of chest pain was associated with a four- to five-fold increase in mortality.

And nearly half of men and women over age 65 who had heart attacks in the study never felt the telltale chest-squeezing tightness with pain radiating down their arm or into their neck and jaw.

The absence of chest pain symptoms appeared to result in a delay in diagnosis and treatment, the study found. Those without chest pain came into the hospital for treatment, on average, about two hours later and were given EKGs to check for signs of a heart attack about 15 minutes later than those with chest pain.

Younger men without chest pain received their first treatment with a clot-busting drug intended to restore blood flow to the heart within an average of 62 minutes after they entered the hospital, compared with an average of 81 minutes for younger women without chest pain.

“These delays in diagnosis and treatment are huge,” said Dr. Malissa J. Wood, co-director of the Massachusetts General Hospital’s women’s heart health program. “Time equals muscle because muscle deprived of oxygen can be injured or die in the setting of a heart attack.”

Canto agreed that delays in treatment—most striking in the younger women who presented without chest pain—could have contributed to the higher death rates in this group. “There may also be biological differences in younger women who have heart attacks,” he added, “such as hormonal factors that lead to a different mechanism for clot formation, which could make heart attacks more deadly.”

While heart attacks are far less common in women under 65, they are on the rise along with obesity and diabetes—both risk factors for heart disease. “Women need to be aware of their risks and identify the warning signs so they can seek prompt medical attention in the event they have any,” said Wood. “We know from previous research that women tend to head to the hospital about an hour later than men, and this new study shows why they need to get those symptoms checked out immediately.”

Canto pointed out that most patients who have chest pain aren’t having a heart attack and that likewise, most people with atypical symptoms aren’t having one either. “The art is trying to figure out when to seek medical attention.” Younger folks who have one heart attack risk factor—smoking, family history of early heart disease, high blood pressure, high cholesterol levels—should be more on the alert if they’re having atypical symptoms like light-headedness, nausea, sudden dizziness, breaking out in a cold sweat, or unusual fatigue. Along with chest pain that radiates into the arms, back, neck or jaw, these are symptoms that warrant an immediate call to 911, according to the US Department of Health and Human Services Office on Women’s Health. They launched a publicity campaign last year to educate women about the importance of responding immediately to heart attack symptoms.

Related: Advice for preventing heart disease in women