Objective To study the effect of pregnancy on the immune response against Trichinella spiralis infection in mice. Methods Six pregnant mice were orally infected each with 300 muscle larvae of T. spiralis，and the serum anti-Trichinella antibodies at different time after infection were detected by ELISA. The mice were sacrificed 6 weeks after infection and the carcass was digested to observe the muscle larval burden（larvae per gram，lpg）. The ability of sera from infected pregnant mice to mediate the death of pre-encapsulated larvae（PEL）were assayed in an antibody-dependent cell-mediated cytotoxicity（ADCC）. On 6th，8th and 12th day after infection，the infected pregnant mice were sacrificed to examine the intestinal worm burden and the fecundity index of female worms in vitro. Six virgin mice injected with progesterone were infected with T. spiralis，the serum antibodies and muscle larval burden were detected 6 weeks after infection. Results The absorbance value of sera from pregnant mice（0.113）were significantly higher than that from virgin mice （0.078） at 2 weeks after infection （F=21.390，P＜0.05）. The muscle larval burden in pregnant mice（1 251±450 lpg）was significantly lower than that of virgin mice（2 310±1 123 lpg）6 weeks after infection（t=2.419，P＜0.05）. The ability of sera to mediate the death of pre-encapsulate larvae in ADCC was significantly higher in pregnant mice（42.6%）than in virgin mice（26.9%）at 2 weeks after infection（F=1.195，P＜0.05）. The difference of intestinal worm burdens on 6th，8th and 12th day after infection have no statistical significance between pregnant and virgin mice （Z6=-1.185，Z8=-0.149，Z12=-0.298，P＞0.05），so did the difference of fecundity index of female worms in vitro on 6th and 8th day after infection between the two groups（Z6=-0.149，Z8=-1.043，P＞0.05）. Serum absorbance value of progesterone injected virgin mice（0.299）was significantly higher than that of no-injected virgin mice（0.191）（t=2.955，P＜0.05），but the difference of muscle larval burden between the injected （1 457±551） and no-injected virgin mice （1 235±439） showed no statistical significance（t=0.726，P＞0.05）. Conclusion Pregnancy has a synergetic effect on immune response of mice against T. spiralis infection，which may be related with the increased level of serum anti-Trichinella antibody and enhanced ability of sera in mediating the death of pre-encapsulated larvae in ADCC.