What Causes Spotting Before Menstruation?

I am a 35 year old woman who has always had irregular periods. They have actually gotten closer to normal since I had my little girl, almost 3 years ago. In the past few months, I have started spotting for 2-3 days before I start my period. My husband and I are considering trying for another child and I was wondering if this is something I should have checked out before we start trying. (It took us many years and a couple of miscarriages...