NEW YORK - In a new study from a single California community, women who had been pregnant at least four times were less likely to die from cardiovascular disease than those who'd never been expecting.
Researchers said that could be due to the protective effects of pregnancy-related hormones as well as the extra social support that comes with having more children -- or the fact that women who are able to get pregnant more often are healthier to begin with.
It's not clear that getting pregnant more often protects women's hearts, necessarily.
"It's just one more little piece of the puzzle that maybe physicians should be aware of or think about," said Donna Kritz-Silverstein, from the University of California, San Diego.
"Heart disease is one of the biggest problems facing women today. It's the leading cause of death among women in the United States," Kritz-Silverstein, who worked on the study, told Reuters Health. But, she added, "Many women don't perceive themselves to be at risk."
According to the Centers for Disease Control and Prevention, heart disease kills more than 600,000 Americans a year, about half of them women.
The new study, led by Marni Jacobs at UCSD, involved close to 1,300 women from Rancho Bernardo in Southern California. In the mid-1980s, when participants were in their early 70s, on average, researchers asked them how many times they had been pregnant and given birth. They also surveyed women about lifestyle habits related to heart health, such as smoking and exercise, and measured their cholesterol, blood pressure, height and weight.
For the next 19 years, the researchers brought women in for regular clinic visits, sent them annual questionnaires and used death records to track their diagnoses.
Over that follow-up, 707 women, or about 55 per cent, died of various causes. Just under half of those deaths were due to heart and vascular diseases, including heart failure, heart attacks and strokes.
Compared to women who had never been pregnant, those who reported at least four pregnancies were about 35 per cent less likely to die of any heart or vascular disease, including half as likely to be killed by a stroke or another condition related to artery build-up and high blood pressure. That was after taking into account women's age and weight.
The trend held up when the researchers looked specifically at the number of times women had given birth, and not just how often they'd been pregnant -- but in that analysis, they couldn't be sure the lower rates of cardiovascular deaths that came with more kids weren't due to chance.
The authors wrote in Fertility and Sterility, that finding may simply be due to the smaller sample of women who had four or more kids -- 131 of them -- compared to the 240 that had been pregnant at least four times in their study.
On the other end of the spectrum, 316 women said they had never been pregnant and 406 had never given birth.
There was no link between the number of times women were pregnant or gave birth and their chance of dying from all causes combined during the study.
The researchers said the findings may not apply to postmenopausal women in general, as the women in their study had especially long life spans, were relatively well-off financially and had good access to health care.
But they proposed a few explanations for the link between the number of times women had been pregnant and their chance of dying from cardiovascular disease. For example, women who are pregnant more times produce more estrogen over the long run, and longer lifetime exposure to the hormone may have protective heart effects. Alternatively, heart-related deaths could be reduced in women who have lots of grown children and social support around them as they age.
Finally, Kritz-Silverstein and her colleagues wrote that higher fertility may just reflect better general health -- although their associations stood up when they took into account blood pressure and cholesterol readings as well as lifestyle habits.
"It's really hard to tease out" the cause, she said. "We're just speculating -- there's no real way that we can know that from our data."
Kritz-Silverstein added that future research into the link between pregnancy history and cardiovascular disease could help get at some of the differing heart risks and protective effects in men and women, including the "not-so-obvious factors."