Objective: To observe the immunoreaction of offspring mice by ovalbumin (OVA) re-exposure after their mothers exposed to OVA during pregnancy. Method: A prospective controlled study was conducted to observe mice after repeated OVA exposures at 6-8 weeks.Their mothers were exposed to OVA during different stages of pregnancy.The symptoms were recorded and scored.The levels of OVA-specific IgE in serum, interferon-γ(IFN-γ) and interleukin-4(IL-4) in supernatant of spleen primary lymphocytes in vitro were measured by ELISA.The results were analyzed by single factor analysis of variance or rank sum test. Result: All the mice in each group had acute diarrhea.The diarrhea happened earliest (2 days) and most severe in the late pregnancy group (early pregnancy group 7.0±1.0; middle pregnancy group: 7.1±1.1; late pregnancy group: 9.9±2.2, P<0.01). The levels of absorbance of OVA-specific IgE in the pregnancy groups were higher than those of the control group.The absorbance of OVA-specific IgE in late pregnancy group was the highest (control: 0.27±0.06; early pregnancy group: 0.51±0.13; middle pregnancy group: 0.50±0.09; late pregnancy group: 0.63±0.13, P<0.01). There was no significant change in IFN-γ expression in cultured supernatant of spleen lymphocytes in each group (control: (133±7) pg/ml; early pregnancy group: (133±4) pg/ml; middle pregnancy group: (134±6) pg/ml; late pregnancy group: (132±4) pg/ml, all P value >0.05). The expression of IL-4 in the experimental groups was higher than that in the control group, especially in late pregnancy group(control: (25.3±2.4) pg/ml; early pregnancy group: (32.4±4.4) pg/ml; middle pregnancy group: (35.0±5.4) pg/ml; late pregnancy group: (47.1±5.8) pg/ml; P value all<0.01). Conclusion: The allergic reaction of the OVA re-exposure progeny whose mothers were exposed to OVA in the late pregnancy period was most severe, suggesting that late pregnancy period might be the high risk stage of intrauterine sensitization, or"window period".