Ninde - not sure if this helps, but I've always consulted this webpage (which is super detailed about early u/s) http://www.brooksidepress.org/Products/ ... cannin.htm
Crown Rump Length: Gestational age = 6 weeks plus (CRL x days). This relies on the growth of the normal fetus of 1 mm per day after the 6th week of gestation. For example, a CRL of 16 mm would correspond to a gestational age of 8 weeks and two days (6 weeks plus 16 days = 8 weeks and 2 days).
So a CRL of ~4mm means 6w4d. A heartbeat at this point is great. If you are 7 weeks, that is only 3 days behind, which is not fantastic but not awful.
Are you sure you are calculating your weeks correctly? The day of the retrieval is "2 weeks" pregnant. This website is great for calculating IVF due dates http://ivf.ca/duedate.php
Just to share my story -- with my miscarriage, everything measured great at 6w2d - but at 7w2d, the baby hadn't grown and the heartbeat had slowed down. At 8w2d, the heartbeat had stopped and the baby still measured 6w2d.
With my boys, I had a major bleeding scare at 6w2d (gushing and gushing blood - I was sure we had lost them and was worried about bleeding to death!). I had 3 u/s in 4 days -- and I was really nervous when they didn't appear to grow appropriately (1mm a day is normal growth). But things were FINE. Sometimes techs don't get the best measurements.
So bottom-line -- I won't lie - it's not fantastic news if the baby is measuring small -- but it's not necessarily bad news. Maybe the baby was in a weird angle and the tech didn't get a great measurement. Maybe the baby is really a few days behind, but fine. Maybe you are a little off with your dates.
Try to relax and focus on the little baby growing inside you. Waiting is sooooo hard. I'm sorry.