Their sizes can fluctuate

You can tell when her period is with just a glance. According to the National Institute of Health, women's levels of the hormones progesterone and prolactin increase about a week or two before she menstruates. This causes her to retain more water and her breast tissue and milk glands to grow, and with them, her bra size. Some women can grow as much as a cup size bigger. But be careful: If she is about to be on her period, stimulation can be more painful and decrease sexual arousal, says Leah Millheiser, M.D. at the Stanford School of Medicine.