well if no cardiac activity is seen at 6 weeks u shoul get it repeated after 15 days and see for cardiac activity again.
perisac bleeding is seen in many patients but still it has to be treated with bed rest
usg scan next time will determine the weeks of gestatin approximately once caridac activity is seen.
at times patients conceive a little late than their last period of menses.in such case dates might be confusing.
but the best prediction can be made with usg only.