Gestational stress is associated with many maternal and child complications, however, this association must be taken with care, since there are studies that find inconsistent results between stress measures and maternal complications. It is believed that the lack of convergence is due to the way in which gestational stress is evaluated. The aim of the present study was to design and validate an instrument based on a bio-psycho-social model of gestational stress. The design and validation process of the inventory was divided into four phases: (a) Construction of the items bank and content validity, (b) construct validity, (c) inventory reliability and (d) convergent and discriminant validity with psychometric instruments that have been used in other investigations to evaluate gestational stress. A valid and reliable Stressors and Modulators of Gestational Stress Inventory (SMGSI) conformed by two scales was developed: (a) Gestational stressors, which is formed by two factors, the psychological stressors and social stressors with a variance of 48.5% and 51.8% and a reliability of .79 and .67, respectively; and (b) gestational stress modulators integrated by 8 items that explain 55.2% of the variance and with a reliability of .92. In conclusion, a valid and reliable tool was obtained that measures gestational stress from a bio-psycho-social perspective. This inventory allows for the identification of allostatic and pantostatic stress, making it useful as a diagnostic tool to prevent maternal and childhood complications that are associated with chronic gestational stress.