Objective To investigate the effect on the growth and development of fetus and placental coefficient of BALB/c mice infected with Toxoplasma gondii during the second trimester of pregnancy. Me-thods The animal model was established by intraperitoneal injection ( i. p. ) of tachyzoites of T. gondii at the 8th day of pregnancy, then the placentas and fetuses of the infected mice were separated at the 12th, 14th,16th, 18th day of pregnancy, respectively. The survival rate of fetus was observed under anatomical micro-scope, the placentas and fetuses were weighted with electronic balance, and the body length and tail length were measured with sliding caliper. Results It was showed that the number of absorbed fetuses increased appar-ently along with the pregnant days in the experimental group, of which the death of fetus and abortion were more than that in the normal control group. In the normal control group, the weight, body length and tail length of fe-tus increased obviously and the placental weight increased at first then decreased afterward, causing a decreased placental coefficient ( weight of fetuses : 40.69 ± 11.63, 173.43 ± 30.19,495.56± 73.97, 967.62 ± 83.34,body length of fetus: 11.53±0.95, 16.83 ±1.15, 20.86 ±1.25, tail length of fetus: 5.26±0.56, 8.08 ±0.98, 10.54 ± 0.58, weight of placentas : 48.62 ±11.87, 87.14 ± 14.05, 94.44 ± 12.29, 93.33 ± 13.00,placental coefficient : 1.30 ± 0.52, 0.51 ± 0.09, 0.20 ± 0.05, 0.10 ± 0.02 ). However, the changes of fetu-ses and placentas in the experimental group were not obvious compared with that in the normal control group ( weight of fetuses: 51.11 ± 13.89, 148.16± 36.09, 296.11 ± 58.63, body length of fetus: 11.51±1.42,14.04 ±1.59, tail length of fetus : 4.04 ±0.95, 6.87 ±0.87, weight of placentas: 49.31 ± 9.42, 66.41 ±13.28, 52.22 ± 12.15, placental coefficient: 1.03 ± 0. 32, 0.48 ± 0.15, 0.18 ±0.05). At the same timepoints, the parameters detected in the experimental group except the weight of the placentas on the 12th day,the placental coefficient on the 14th and 16th day and the body length of fetuses on the 14th day, were signifi-cantly lower than that in the normal control group (P < 0.05). Conclusion The infection of T. gondii dur-ing the second trimester of pregnancy could interfere with placental function and inhibit the growth and develop-ment of the fetus, and may lead to the death of fetus or abortion.