Where does my extra blood volume go after giving birth?
The extra volume of fluid is due to the fluid retaining effects of high oestrogen secretion in a pregnant human female. After this high level of oestrogen subsides the extra fluid is mainly eliminated as:


urination
some is lost during parturition (labour)
lactation to some extent as well


Reference:A Textbook of Medical Physiology (South Asian Edition) Guyton and Hall