Pregnancies and breastfeeding are two important protective factors concerning breast cancer risk. Breast volume and breast volume changes might be a breast phenotype that could be monitored during pregnancy and breastfeeding without ionizing radiation or expensive equipment. The aim of the present study was to document changes in breast volume during pregnancy prospectively. In the prospective Clinical Gravidity Association Trial and Evaluation programme, pregnant women were followed up prospectively from gestational week 12 to birth. Three-dimensional breast surface imaging and subsequent volume assessments were performed. Factors influencing breast volume at the end of the pregnancy were assessed using linear regression models. Breast volumes averaged 420 ml at the start of pregnancy and 516 ml at the end of pregnancy. The first, second and third quartiles of the volume increase were 41, 95 and 135 ml, respectively. Breast size increased on average by 96 ml, regardless of the initial breast volume. Breast volume increases during pregnancy, but not all womens' breasts respond to pregnancy in the same way. Breast volume changes during pregnancy are an interesting phenotype that can be easily assessed in further studies to examine breast cancer risk.