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DURHAM, N.C. -- Pregnancy increases a woman's risk of suffering a heart attack, and the threat is even higher for women over age 35, according to a recent study conducted by researchers at Duke University Medical Center. Merely being pregnant increase a woman's risk three- to four- fold, and other factors, such as age, smoking or high blood pressure drive the likelihood higher, they found.

The researchers said their findings suggest obstetrician/gynecologists should now consider heart attack as a possible diagnosis when treating pregnant women with chest pains. They also urged women attempting to become pregnant to take steps to prevent heart attacks by lowering their blood pressure, stopping smoking and attaining a healthy weight.

The researchers emphasize that heart attacks, or myocardial infarctions, during pregnancy are rare, occurring in roughly six out of every 100,000 women. However, physicians have not previously appreciated how dramatically pregnancy increases the risk, said Andra James, M.D., MPH, assistant professor of Obstetrics and Gynecology in Duke's Division of Maternal and Fetal Medicine.

The research findings were published in the March 28, 2006, issue of Circulation. The study was funded in part by the National Institutes of Health.

"Until now, ob/gyns haven't worried about heart attack in pregnancy because women were not old enough for it to be a concern," said James, lead study author. "But, that's changing now because more and more women over ages 35 and 40 are getting pregnant."

Between 2002 and 2003, the birth rate for women ages 35 to 39 rose 6 percent, and it jumped 5 percent for women ages 40 to 44, James said. The birth rate for 40-to-44-year-old women has doubled since 1981, according to a 2001 National Vital Statistics Report.

For their study, the researchers analyzed data on nearly 12.6 million deliveries from 2000 to 2002 through the Nationwide Inpatient Sample from the Agency for Healthcare Research and Quality. Of those deliveries, there were 859 heart attacks – 626 occurred during pregnancy, including during delivery, and 233 occurred postpartum. Approximately 5 percent of women died after experiencing a heart attack during pregnancy.

This research is the first of its kind to analyze a national sample of pregnant women, enabling researchers to identify more factors that increase the likelihood of heart attack during pregnancy, as well as to determine the extent to which previously suspected factors heighten the danger, James said.

Between the ages of 20 and 40, a woman's risk of suffering a heart attack during pregnancy increases by 30 fold. The increase is gradual, and, based on study analysis, the elevated risk is actually noticeable in women after age 30. Thirty-year-old women face a seven-fold increase in their chances of having a heart attack when compared to a 20-year-old. Older women could be more prone to heart attacks because of the existence of greater atherosclerosis and their heart's decreased capacity to handle the stress of pregnancy, James said.

Race and ethnicity play a large role in determining a woman's risk of heart attack during pregnancy, the researchers found. African-American women are more than 50 percent more likely to have a heart attack than white women, James said. African-American women over age 35 faced a five-fold spike in their risk compared to a white woman under age 35 – a previously unknown finding. According to study data, among 35-year-olds, black women had nearly 41 heart attacks for every 100,000 deliveries, compared to nearly 15 per 100,000 for Hispanic women and 22.5 per 100,000 for whites.

"African-American women tend to suffer more from high blood pressure," James said. "This chronic condition puts them at a greater risk for a heart attack during pregnancy."

James' group identified several other risk factors for heart attack, including smoking and transfusion. Prior to this research, physicians did not have enough information to fully assess the role smoking played in pregnancy-related heart attacks, the researchers said. The large patient volume in this study revealed that smoking increases the chances of an occurrence by eight-fold – a risk equivalent to that produced by smoking while using oral contraceptives. The increased levels of estrogen and progesterone in pregnancy (much like those induced by oral contraceptives) react with chemicals found in cigarettes and compound the heart attack risk, they said.

Although transfusions affect only 1 percent of pregnant women and are mostly a life-saving procedure, the likelihood of a heart attack jumps seven times in women who require them, the researchers said.

Other factors that raised risk were hypertension, thrombophilia, diabetes mellitus, postpartum infection and migraines, James and her colleagues found.

Even though patients cannot control all factors that increase the risk of heart attack, such as age or race, pregnant women can take some steps to prevent an occurrence, James said.

"Women should fully understand the potential dangers that come from pregnancy at a higher age, and they should either change their lifestyle to lower their blood pressure or control it with medication," she said. "They should also quit smoking. Also, pregnancy is not the time to lose weight, but achieving a healthy weight prior to conception can lower the chances of cardiac distress."

Two limitations could affect the study, James said. Because the data came from a national database of hospital discharges, detailed information about individual patient diagnoses was not available. Also, 3 percent of patients transferred facilities, so they could have been counted twice.

In addition to James, co-authors are all associated with Duke University Medical Center and are Margaret G. Jamison, Ph.D., Division of Epidemiology, Mimi S. Biswas, M.D., MHS, Division of Cardiology, Leo R. Brancazio, M.D., Division of Maternal and Fetal Medicine, Geeta K. Swamy, M.D., MHS, Division of Maternal and Fetal Medicine, and Evan R. Myers, M.D., MPH, Division of Epidemiology.