Cause of dark Nipples during pregnancy?

I am 25 years old and 20 weeks pregnant.This is going to be my first pregnancy and I am noticing this strange colour change in my nipples.They were quite light in colour just before my pregnancy but recently have gone real dark I dont know why?My husband is also noticing the change and advicing me to have a doctor’s openion on it.Before consulting a doctor I want to know from you is this some kind of hormonal change which I am going through?Does someone know the real cause of dark nipples during pregnancy?