---

Background & aim: The effect of stress on the nervous system activity may be due to alterations in the structure of the nervous system. In this study, the effects of maternal stress was examined on the seizure threshold and hippocampal structure of their offspring during pregnancy. Methods: In this study, thirty pregnant mice were divided into two groups. 1) No stress group and 2) Stress group that received one hour immobilization stress from 1 to 14 day of pregnancy. The seizure threshold test was performed in offspring by injection of Pantilen tetrazol (PTZ). To study of the hippocampus development, the mouse offspring were divided into three groups: The control group no received immobilization stress on pregnant rat and their offspring also no received PTZ. The Sham group no received immobilization stress on pregnant rat but their offspring received PTZ. and experimental group that pregnant rat received immobilization stress and their offspring received PTZ. At the end of the experiences, all offsprings were killed by chloroform and their hippocampus fixed in and processed and 5µm sections were prepared and stained by the H&E method. The pyramidal and granular layers thickness in the Hippocampus were measured using Motic software and the number of cells in these layers and the number of blood vessels in the molecular and polymorphic layers were counted. The data were analyzed using ANOVA and T test. Results: The results showed a significant increase in seizure threshold in offspring whose mothers were under stress, compared with offspring whose mothers were not under stress. Average thickness of hippocampal pyramidal and granular in the experimental group than in the control group showed a significant increase (p<0.001). in addition, mean thickness of pyramidal and granular layers of hippocampus significantly increased in the experimental group compared to control group. The number of cells in hippocampal and granular layers significantly decreased in the experimental group when compared with control and sham groups (p<0.001). Conclusion: Prenatal stress can cause an increase in seizure threshold and also impair offspring's development and their hippocampal structure.