Getting pregnant despite an IUD is very rare but not impossible. It can occur if the IUD has slipped, has stayed in the uterus too long, or has fallen out. A woman usually notices a pregnancy by typical pregnancy symptoms,  including a missed period. Should you experience light bleeding while pregnant, this is not your menstruation. Rather, the bleeding is most likely triggered by implantation. For more information, go to the profemina article, Period while Pregnant. Note: Seek medical attention if you feel unsure about your symptoms, are experiencing unfamiliar pain, or are spotting. You may be having an ectopic pregnancy, which is more common with IUDs.