Here's some advice from the MedHelp website and our I'm Expecting app on skin changes during pregnancy. It's about stretch marks but there is information on skin darkening, which is normal and typical. Hope this helps. http://www.medhelp.org/pregnancy-health/articles/Stretch-Marks-During-Pregnancy-Your-Symptoms-Manual/2097
I believe it's because our bodies produce more melanin when we are pregnant, which darkens the skin. I'm 38 weeks pregnant with my first child and my nipples, underarms, belly button, under eyes, elbows, (pretty much everywhere) got dark! I don't know if it goes away after pregnancy.