Doctors used to think of heart disease as a man's problem. Now they know that it strikes men and women in equal numbers.

The American Heart Association (AHA) reports that heart attack, stroke and other cardiovascular diseases are the No. 1 cause of death in American women. The disease claims the lives of over 400,000 women each year. That's more than the next three causes of death combined, including all forms of cancer.

While deaths from heart disease are declining for men, they are rising for women, the AHA found. Yet women are more likely than men to be both underdiagnosed and undertreated.

ABC New's chief health and medical correspondent, Dr. Richard Besser, held a tweet chat yesterday to raise public awareness for this issue. Health experts from the Mayo Clinic, National Heart Lung Blood Institute, American Heart Association, Every Mother Counts and Montefiore Medical Center in New York City tweeted facts and advice. Olympian and The Heart Truth spokeswoman Gabby Douglas also tweeted her thoughts on the topic, as did women who signed in to share their personal stories.

Just three weeks after having a baby, tweet chat participant Sarah Wassner Flynn, goes for a heart healthy run. Photo credit: Sarah Wassner Flynn.

Here's a summary of four important facts about women and heart disease highlighted in the tweet chat.

Women don't always know they're having a heart attack

Women should understand what a heart attack feels like - and act fast if they experience symptoms. Both sexes may have chest pain, pain that radiates into the shoulders and arms, nausea or dizziness.

"For women, symptoms may be different including shortness of breath, flu like symptoms, jaw, neck & back pain or fatigue," tweeted Dr. Suzanne Steinbaum, a spokesperson for the AHA.

Women don't always associate physical signs and symptoms with a cardiac event. As a result, the AHA reports that women are 15 percent more likely than men to die of a heart attack. And they are twice as likely to have a second heart attack in the six years following the first.

"Doubt contributes. Women think, 'I can't be having a heart attack,'" tweeted Dr. Andrew Arai, senior investigator for the National Heart Lung Blood Institute.

Can you die of a broken heart? Click here for more info.

Women have heart attacks later in life than men.

The average age for a first attack for a man is 66, AHA statistics show. For women, it's 70.

The risk of a heart attack climbs for women after age 55 when they're past menopause and no longer enjoy the cardio-protective benefits of the hormone estrogen. Estrogen seems to decrease the amount of "bad" LDL cholesterol and increase "good" HDL cholesterol and also helps keep blood vessels healthy and relaxed.

Menopause is the great heart disease equalizer. Once it occurs, estrogen levels drop by 90 percent and the type of estrogen in the body changes, with most of the estrogen being produced by fat cells instead of the ovaries. By age 65 women develop heart disease at the same rate as men.

This doesn't mean that younger women aren't at risk for heart troubles. Studies show Latina women are at risk a full decade earlier than most other women. Chatter Eva Gomez tweeted about going through open heart surgery when she was just 39 years old to fix a malfunctioning heart valve.

Health events during pregnancy can be risk factors later in life. Preeclampsia, gestational diabetes and high blood pressure are all conditions that happen during pregnancy, but their effects may linger long after the birth of a child. For example, preeclampsia increases risk of developing heart disease in middle age. Gestational diabetes can lead to glucose intolerance and other pre-diabetic conditions that contribute to obesity.

Reducing heart health risk factors during pregnancy is also essential for the health of your baby.

As the organization Every Mother Counts pointed out in a tweet, "High blood pressure during pregnancy reduces blood flow to the placenta, restricting oxygen and nutrients to the baby." They also noted that hypertension is a top killer of pregnant women worldwide.

Everyone has the same risk factors.

It doesn't matter what sex you are, certain lifestyle behaviors up your chances of developing heart disease. Smoking, high blood pressure, high cholesterol, diabetes, obesity, family history and older age are considered the "big seven" heart health risk factors.

There's nothing you can do about family history or your age, but you can take control of all the other risk factors.

"Simple prevention advice still applies: Walk more, eat less, sleep more," Dr. Randall Thomas tweeted on behalf of the Mayo Clinic. "One of the best 'medicines' for preventing heart disease is exercise."

Gabby Douglas chimed in with this advice about diet, "Eating healthy is important too. I love to snack on fresh fruits & veggies. Blueberries and snap peas are my favorites!"

One thing all the expert tweeters stressed: Prevention should begin early. Start by knowing your heart numbers, including cholesterol values, blood pressure and body mass index. Eat a heart-healthy diet and exercise regularly.

"You're never too young or too old to start thinking about heart health behaviors," Dr. Julie Ramos, a cardiologist specializing in women's health at Montefiore Medical Center.

For a link to the full chat transcript, click here.

Dr. Besser Tweet Chats on Tuesdays from 1 to 2 p.m. ET

Besser hosts a one-hour "tweet chat" on a health topic most Tuesdays from 1 to 2 p.m. ET. His next chat on "End of Life Issues" takes place on Tuesday, Feb. 19. Want to participate? Here's how.