How do breasts change during one's menstrual cycle?

You may find that your breasts feel fuller and are more sensitive when you're ovulating; and, in fact, they actually do get bigger because of water retention and additional blood flow. This is also when they're most likely to be the same size, however; one study found that breasts have the least amount of asymmetry on the first day of ovulation. During menstruation, breasts shrink again.