Perhaps it was this article -- where analysis of age-50+ members of a particular Amish community showed that lifespan of mothers increases linearly up to 14 children, and increases with later age at last birth:

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.)