Dear Kris:
I have one article that might be related.  It talks about how in
hunter-gatherer societies, there are lower rates of reproductive cancers and
that the women have lots of babies, nurse long-term, walk a lot etc...  Some
have hypothesized that the lower levels of estrogen caused by pregnancies,
later menarche, extended nursing etc... have decreased the risk of cancer.
 There reference is:
Foole, S. (1993). Darwin takes on mainstream medicine. Journal of NIH
Research, 5, 64-66.

I hope this helps.  My gut tells me that this doc is off-base with his
suggestions.
Kathy