AIM To measure MCA and CA153 concentrations in maternal serum (MS) and amniotic fluid (AF) paired samples during normal pregnancy, in order to evaluate the usefulness of these markers in monitoring pregnant patients with a history of breast cancer. PATIENTS AND METHODS Serum and AF MCA and CA153 values were measured in 20 pregnant women during the 1st trimester, 29 cases in the 2nd, 26 in the 3rd and 20 at parturition and compared with those of 20 healthy, age-matched, non pregnant women (controls). RESULTS MS values of MCA increased significantly with gestational age (p < 0.0001), being higher in the 3rd trimester and in labor than in control values (p < 0.0001). MCA values in AF were remarkably higher than those in MS and increased significantly with advancing gestation (p < 0.0001). In contrast, CA153 values in AF, which were marginally higher than in MS, did not differ significantly with the progression of pregnancy. CONCLUSIONS Maternal serum MCA values are significantly influenced during pregnancy. Thus, this marker seems to be reliable only during early pregnancy. In contrast, CA153 remains a useful marker in monitoring pregnant breast cancer patients.