https://aeon.co/essays/why-pregnancy-is-a-biological-war-bet...
https://aeon.co/essays/why-pregnancy-is-a-biological-war-bet...
If the mother's body leaves fetal cells alone, they have can act as essentially free stem cells, helping repairs.
I forget the exact journal, but women that had like two or three kids, live longer on average than those that didn't.
https://www.ncbi.nlm.nih.gov/pubmed/16510865
(Interesting... it also shows a positive -- but lower/noisier -- effect for fathers as well, so there may be more at work than just stem cells.)
An evolutionary theory based guess is that healthy long lived individuals are more attractive in the mating arena and therefore have more children.
Can't have 3 kids if you died at age of 10.
I reposted it before seeing your comment because it has been 4 years. My how time flies.