Pregnancy and breastfeeding are major factors reducing breast cancer (BC) risk. A potential mechanism for this effect might be changes in mammographic density, but other factors might be involved. The aim of this study was to investigate factors influencing changes in breast size and breast stiffness after pregnancy. Of a consecutive cohort of 5991 women who gave birth between 1996 and 1999, 559 replied to a questionnaire including questions about breast changes. The women completed their own assessments of changes in breast size and stiffness since their last pregnancy. Factors being investigated regarding their predictive value for these changes were: BMI before pregnancy, weight gain, age at first full-term pregnancy (FFTP), number of pregnancies, breastfeeding, and BMI of the children's fathers. A decrease in breast size was reported in 21.8% of the participants and an increase in 35.1%. With regard to the breast stiffness, 66.4% reported a decrease and only 5% reported an increase. Independent predictors for increased breast size were age at FFTP, increase in BMI since last pregnancy, BMI before pregnancy, and time since FFTP. Factors predictive of greater breast stiffness included age at FFTP, BMI before FFTP, time since FFTP, breastfeeding status, and number of pregnancies. Breast changes after pregnancy depend on several variables, which are described as BC-risk factors. Individual reaction of the female breast to a pregnancy leads to different outcomes with regard to breast size and stiffness. Further studies are needed to clarify whether these individual responses interact with the effect of pregnancy on the BC risk.