Women in the U.S. have an average of two to three children, and that, it turns out, is in the sweet spot for women when it comes to their cardiovascular health. According to a research paper presented at the American College of Cardiology's 63rd Annual Scientific Session, women who have more than one but less than four children are least likely to show early signs of cardiovascular disease. Oddly, women who give birth to four or more children — as well as those who had either no children or just give birth to one — are much more likely to show early signs of cardiovascular disease (such as plaque in their hearts or a thickening of their arteries).

"Our findings add to the growing body of evidence that the changes associated with pregnancy may provide insight into a woman's future cardiovascular risk,” said Dr. Monika Sanghavi, chief cardiology fellow, University of Texas Southwestern Medical Center, and lead author of the study.

To conduct the study, Sanghavi and her colleagues enlisted the help of participants in the multiethnic Dallas Heart Study. The researchers selected a total of 1,644 women from that study based on whether a participant had relevant imaging study data available and whether she had self-reported her number of live births. The average age of all the chosen participants was 45. And, slightly more than half (55 percent) were African-American.

Next, the researchers wanted to see which of these women showed any early signs of possible cardiovascular disease. The team of researchers measured coronary artery calcium (CAC) using CT scans and aortic wall thickness (AWT) using MRI scans in order to determine whether or not the women had atherosclerosis — build-up of plaque made of fat, cholesterol, and calcium — in their heart or artery walls.

What the researchers discovered was that women who had given birth to four or more children had an approximately two-fold increased risk of having abnormal CAC or AWT. "Pregnancy has been called 'nature's stress test,' and for good reason," Sanghavi said in a press release and went on to explain how pregnancy causes changes that can place more strain on a woman's cardiovascular system. For example, the volume of blood being pumped through the heart increases by 50 percent, while, in many cases, increasing insulin resistance and raising cholesterol levels.

What comes as a surprise, though, is the researchers also discovered that women who had no babies or just one live birth also were more likely to show evidence of atherosclerosis. Not entirely sure why this may be the case, Sanghavi speculated that women who have a condition that prevents them from carrying a first or second pregnancy to term may also be what predisposes them to cardiovascular disease. Women with, say, polycystic ovarian syndrome have menstrual irregularities and trouble getting pregnant, and concurrent with these symptoms, they may also experience weight gain.

Altogether, then, this study suggests a U-shaped relationship between the number of babies a woman has and early signs of cardiovascular disease. Clinicians can use these results, then, to better and more easily estimate women who may have a future risk of heart disease. "The benefit of pregnancy is that it occurs relatively early in a woman's life and allows for early intervention for those at higher risk," Sanghavi concluded.