Our objectives were to assess the plasma coenzyme Q10 (CoQ10) levels in normal pregnancy, in pregnancy with a spontaneous contractile event, in spontaneous abortion and in threatened abortion. Six hundred and fifteen CoQ10 levels were analyzed in 483 pregnant women: 350 patients were employed to design a normal curve; 66 patients with spontaneous contractile activity underwent two or more CoQ10 analyses in different trimesters; 49 patients presented spontaneous abortion, and 18 patients threatened abortion. The normal curve of plasma CoQ10 levels rises during each trimester of pregnancy, while there is a correspondence between a low CoQ10 level and spontaneous abortion. Furthermore we found a statistically significant difference between the plasma CoQ10 value in spontaneous contractile activity, mainly in the third trimester. We found an increase in the plasma CoQ10 level in relation to the contractile activity of the uterine muscle. Further studies are necessary to explain the involvement of this marker on pregnancy in clinical practice.