Aberrant complement activation can induce “thrombo-inflammation” attacks to host tissue. Beside kidney and blood vessel, the placenta is also susceptible to these attacks. Complement dysregulation is recently classified as one of the new mechanisms leading to pregnancy disorders. Studies have indicated that dampening complement activation can ameliorate pregnancy outcomes. During pregnancy, the mother’s immune system is finely domesticated to accept the semi-allogeneic fetal antigens. As an important part of the innate immune system, some interesting changes have also taken place in complement system during pregnancy. The complement proteins are highly expressed in placenta, and their split products are increased. They are tuned in maintain placental immunity and structural homeostasis. An abundance of evidence shew that complement protein deficiency lead to autoimmunity disease and pathological pregnancy marked by excessive inflammation. Although complement suppressing strategies have been proven effective in treating some pathological pregnancy in individual case studies. we should take the dual role of the complement into consideration that fully and completely inhibit of complement may not be a wise choice.