In a number of pregnant women ovarian cysts are found incidentally during the routine first trimester scan. These cysts may pose diagnostic difficulties, and the measurement of serum CA125 levels can be used to aid management. In this study we measured maternal serum CA125 levels in 188 women with uncomplicated pregnancies between 11–14 weeks of gestation. All women had morphologically normal ovaries observed on ultrasound examination. The median serum CA125 levels were 23.4 U/mL (range 2.2–166.3 U/mL, 95% reference interval 5.28–70.15) and did not change significantly with gestation. We conclude that CA125 levels are increased at 11–14 weeks of gestation and cut off values which are used to assess the nature of ovarian cysts in nonpregnant women cannot be applied to pregnant women at this gestation.