possible blighted ovum

I went in for an ultrasound today because I've been having dark brown discharge. I am six week today according to my LMP which was on 2/4/17. The doctors could not see anything except for the start of a pregnancy. I was hoping they would at least see the embryo. Perhaps it's delayed or still too early? Has anyone experienced anything like this? Could it be ectopic or a blighted ovum? Please leave your responses below, as I would love to read them.