What Are The Chances Of Pregnancy While On Oral Contraceptive Pills?

Hi I have been taking the combined oral contraceptive pill for 5 months now. My first 2 periods were relatively normal but shorter and lighter. My periods since then have been even lighter and shorter and its almost just spotting for 3-4 days. Since taking the OCP, my period has come on the exact day every 4 weeks so the regularity is not an issue. Is this normal? Should pregnancy be a concern?