Due date

Had a mist miscarrage in November at 9 1/2 weeks. It was twins. Growth stopped at around 6 weeks 3 days. Just went into the Dr's for an ultrasound to date/confirm my new pregnancy. We got a heartbeat of 118!! Which is awesome! But I am measuring 6 weeks. I track my ovulation and feel like I should be 7weeks 2 days. Trying not to panic but........ So happy so see a heartbeat but I am afraid that the growth is lagging and the same thing will happen again. Last time we never had a ultrasound until the 9 1/2 weeks when we found out we lost them so I don't know what they looked like at this point.