Abstract: Objective To investigate the growth and deve lopment of fetus and placental coefficient of BALB/c mice infected with Toxoplasma gondii during the second trimester pregnancy.Methods To establish the animal models by in trap 2 eritonea l injection(.ip.)method at the 8th day of pregnancy,then separate the placentas and fetus at the 12^th,14^th,16^th,18^th day,respectively.The survival rate of fetus was observed by anatomical microscope.The weight of placentas and fetus were weighted by electronic balance,and the body length and tail length were measured by sliding caliper.Results It was showed that the number of absorption fetusinc reased appa rently a long with that of the pregnant days in the experimental group,of which the death of fetus and abortion were more than that of the normal control group.In the normal control group,the weight,body length and tail length of fetus were increased obvious and the placental weight was increased first then decreased,which caused the decrease of placental coefficient.However,the change of fetus and placenta of the experimental group were not obviously compared with the normal control group.At different time points,there was significant difference in each group(P<0.05).At the same time points,the results of the experim ental group w ere significant different compared with that of the normal control group except the weight of the placenta at the 12^th day,placental coefficient at the 14^th day,and body leng that the 14^th day(P<0.05).Conclusion The infection of Toxop lasm a gondii during the second trimester pregnancy can cause disorder of placertal function and inhibition of the growth and development of the fetus,which will lead to the dealth of fetusor abortion.