I'm not sure who originally posted about the mom eating chalk during her pregnancy/lactation.  According to "Nutrition in Pregnancy and Lactation" by Bonnie Worthington-Roberts and Sue Rodwell Williams:
 
"Some patients with iron deficiency are found to exhibit a behavior called pica.  Eating starch, dirt, or clay has been implicated as a cause of iron defiency in pregnancy; the opposite viewpoint has also been proposed-that is, iron defiency is the etiologic factor behind pica." 

I also asked the pediatric nutritionist that I work with who said that as far as she knows, eating chalk would be a sign of low iron, not calcium.

Hope this helps.
Mary Lussier BSN IBCLC
Ct. Childrens Medical Center

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