It's from the enormous increase in blood volume that happens just before month 7. PLEASE don't cut your sodium intake. Along w/ this increase in blood volume you need a HIGHER sodium (and all electrolytes) intake, not lower. It's very imortant to get make sure you are getting higher amounts of soduim, magnesium, calcium, etc, than you needed before to keep up w/ this greater blood volume. Not to mention your blood is feeding the quickly growing baby now, who also is taking more minerals, etc, from your blood on top of what you need for the increase.