Changes in the regulation of serum osmolarity is just one of the many chances that occurs during the normal physiologic response to pregnancy. Beginning with the first trimester, the plasma osmolarity level, serum Na, and osmolarity thresholds for the thirst and ADH responses are decreased by about 10 mosm/kg, a phenomenon which generally lasts through term.
Later on in pregnancy, however, levels of circulating ADH actually decrease--due to production of the enzyme vasopressinase, which drastically enhances the turnover of ADH. In most cases, the presence of vasopressinase counterbalances the decreased threshold for ADH secretion. However, in individuals with gestational diabetes insipidus--a rare disorder in which vasopressinase production is excessive--can develop polyuria, polydipsia and excessive thirst, usually manifesting during the third trimester.
Gestational diabetes insipidus, if severe, can be treated with DDAVP--a synthetic peptide analogue of ADH (pictured above) which is resistant to endogenous vasopressinase activity.