I don't know why, but I find that hard to believe.
I believe that studies have shown that women who get pregnant twice and go full term have the highest life expectancy of any women. The study discussed here supports those earlier studies.
If they give birth 3 or more times, does the mortality rate go back up significantly? Maybe if you have kids, your have more of a reason to live, or don’t engage in as many risky behaviors when you become a mom?