getting pregnant while breastfeeding

I keep reading that it's more difficult to get pregnant while breastfeeding and it is when you just bottle feed with formula I was wondering if that was true and how many women have gotten pregnant while breastfeeding and how long did it take I am breastfeeding my daughter and she is only two weeks in a few days old and me and my husband plan to start trying for a baby again as soon as we get the clearance from the doctor but I breastfeed and that is it so I was wondering how long it could take