Jackie,

This is from the abstract from LLL's 1997 Facts about Breastfeeding.  The
full paper can be found in  Am J Clin Nutr 1996; 63(4):526-31.  The title
is Intestinal calcium absorption of women during lactation and after
weaning.  Authors:  Kalkwarf, H.J. et al.
I assume this study was done on healthy mothers, but maybe it will help.

The fact sheet stated the following:

Increased efficiency of calcium absorption from the intestine is one
maternal physiologic adaptation used to meet the demand for calcium
during lactation, but this response is only apparent after weaning or the
resumption of menses.  The increased fractional calcium absorption
postweaning may be related to a gain in bone mineral density of the
lumbar spine within 6 months of weaning.

Hope this helps.

Aloha,
Gloria Thai  LLLLeader Hawai'i