This usually happens due to an increase and high level of progesterone in a pregnant woman's system. (checkpregnancy.com)
During pregnancy the body produces high levels of progesterone which causes sodium and water retention. (newkidscenter.com)
However, if a woman has been having her menstrual cycle regularly, past pregnancies should not leave high levels of progesterone or estrogen in the body. (checkpregnancy.com)