My HormonologyStrange, but true: Your breasts change shape throughout the course of your monthly cycle. According to a study in the journal Ethology and Sociobiology, your breasts are more asymmetrical during Week 1 (your period week) and Week 4 (your premenstrual week) when estrogen is low. They’re more symmetrical during Week 2 (which starts 8 days after the onset of your period) when estrogen is high. And they’re at their most symmetrical during ovulation at the end of Week 2 (the mid-point of your cycle). Why the fluctuation in shape? The researchers theorize it may be a subtle cue to men that indicates when you’re approaching the most fertile phase of your cycle.