After ectopic pregnancy

I ended up having the methotrexate shot for an ectopic pregnancy on January 11. As of today, January 26 I have an hCG of 105 so hopefully soon I will be down to zero. They said I can can try after two menstrual cycles. I was told by the doctor that next time I get pregnant they will have me come in for an early scan to make sure that the baby has implanted in the right place. When is the earliest time for an ultrasound that can be done to make sure that has happened??