This 2011 Finish Study found that mothers who breastfed for 33 months or longer (cumulative lifetime total) had stronger bones than women who nursed for a shorter time. Turns out that the low levels of estrogen during lactation, which keep milk supply steady, also allow for outer bone growth. And when it comes to bone strength, having greater bone diameter is more important than density. This explains why women who have breastfed are less prone to fractures later on in life. Yet another awesome reason to breastfeed. Your bones will thank you!

http://www.springerlink.com/content/u3w2v6x023525604/fulltext.html