“Why do I always get a cold when I have my period?”

Around ovulation (which is when estrogen levels drop-off), your immune system is down regulated by progesterone in order to prevent immune cells from attacking a fertilized egg and from preventing implantation. Testosterone, depending on the phase, will encourage or suppress regulation of different immune cells. Although this theory feels intuitive, we don’t currently understand many of the biological mechanisms behind the menstrual cycle, pregnancy and menopause, and as such more research is needed to understand how the hormonal changes these stages of life present interact with the immune system. Every woman is different as to what kind of symptoms they experience during menstruation and the extent to which their immune system is affected at all.
Take care and stay healthy!