Like most things menstruation-related, the perception that your period may cause weight gain isn’t all in your mind. In fact, there are various psychological and physiological reasons why menstruation adds weight that may last longer than the week of your period. The big news: With some understanding of the why and how you don’t need to grin and sustain it. Here’s everything you need to know: 1. Constipation Your period influences how well your gastrointestinal tract strives to push out waste from your body. Progesterone causes some relaxation of smooth muscle, leading to a drop in gastrointestinal movement, which may cause constipation and bloating in some women. To overcome both, raise the amount of fiber in your diet, especially in the second half of the menstrual cycle. 2. PMS For a majority of women, their monthly “cycle” starts with at least one of the plenitude of traits known as premenstrual…