Hello Doctor, My Wife Was Pregnant And She Was Bleeding

Hello Doctor,
My wife was pregnant and she was bleeding like she bleeds on periods (one sanitary pad used and the second one with hardly any spots) on the 27th day of pregnency (that is 41th day from the 1st day of her last periods). I searched the internet for similar problems and found out that it can happen due to the process of Implantation, so to be safe and sure we visited the doctor and got an ultrasound done. The doctor could not find any thing during the ultrasound and hence we were told that we have missed the child.
I am confused wherther an ultra sound can detect a featus which is almost smaller than 1mm in diameter (as it was 4 weeks old). We are still hopeing that the featues is still there and the ultrasound could not detect it.
Should we still hope that we have not miss carried the child and the featus will be detected later after 6 to 8 weeks of pregnancy?