The amount of breast enlargement after pregnancy varies from person to person, and can be affected by personal constitution and genetic factors. The secretion of estrogen and progesterone increases rapidly after pregnancy, and the rise in hormone levels will stimulate breast enlargement and swelling. Nipples may also become bigger, which is a normal reaction to pregnancy. However, due to the different physique of each person, some pregnant women have more enlarged breasts. Some pregnant women do not respond well to changes in hormone levels and may experience only a slight increase in size, which may be less than one cup size. Breast size may be affected by genetic factors. Due to hereditary factors, some women have smaller breasts, and even if they are constantly stimulated by progesterone levels during pregnancy, the increase in size may not be very noticeable. However, if your breasts are fuller, they may increase in size more after pregnancy, perhaps by two to three cup sizes.