MCA (Mucin like carcinoma associated antigen) is used as a marker for the follow-up control of breast cancer patients, and high levels of MCA have been observed during the course of pregnancy. The aim of our study was to determine the value of MCA as a tumor marker in the follow-up of young breast cancer patients with subsequent pregnancy. We determined the MCA concentration in the sera of 201 pregnant women in the course of gravidity in order to find the degree and time of increase MCA shows during normal pregnancy. 121 serum samples of healthy, non-pregnant women were analysed for MCA in comparison. Up to the 19th week of gestation, the concentration of MCA did not show any differences in comparison to non-pregnant healthy women (n = 121, median: 7 U/ml: 25% percentile: 7 U/ml: 75% percentile: 10 U/ml (control collective). Median: 6 U/ml: 25% percentile: 5 U/ml: 75% percentile: 10 U/ml (gestational week 5-19)). After the 23rd week of gestation, the MCA concentration was elevated. After the 35th week of gestation, all women had MCA levels which were significantly higher than in healthy, non-pregnant women. Median: 100 U/ml: 25% percentile: 71 U/ml: 75% percentile: 147 U/ml (gestational week 36-42). Therefore the tumor marker MCA can be used in the followup of breast cancer only until the 19th week of gestation. After the 23rd gestational week very high MCA concentrations are observed in some cases and can not be distinguished from MCA augmentation due to advanced breast cancer.