Abstract: Objective: To establish the experimental mice model about the genital infection with UU during the course of cyesis and observe the effects of ureaplasma urealyticum(UU) pregnant infection on body weight of newborn mouse.Methods: Pregnant mice were infected by serotype 4 UU during the period of pregnancy through vagina and observed the body weight of newborn mouse,and the possible mechanism was discussed by the methods of UU culture,pathology and immumohistochemical staining test.Results: The everage weight of newborn mice by UU infection was 1.518 g,and the control group was 1.924 g,there was significant difference between two groups(P<0.001).Conclusions: UU infection during the period of pregnancy can affect the embryonic development and result in the low body weight of new life through vertical transmission.