Polyps are non-cancerous growths in your uterus. They occur naturally, like skin tags, and are harmless to your health. Sometimes they even fall away all by themselves, and come out during your period. But if a polyp is big or blocking an important part of your uterus, it could block an embryo from implanting successfully. Because polyps are a little irritating to your uterus, they can also cause spotting between your periods.