Technically you aren't pg until implantation. Implantation can happen at a very wide range of days. Normal is believed to be something like 6dpo-12dpo. However it can happen earlier or later I've read some women don't implant until after 14 dpo even when most women's AFs are due, which probably accounts for most of the late BFPs people have. So if you implanted on the later side the baby would be behind your expected dates according to your LMP or even going by O. A few days makes a big difference early on bc the babies grow at fast rates. I can tell you that with 2 of my successful pgs I measured anywhere from 3-7 days off my first u/s at 6w1d and 6w3d. We saw a hb both times that was 105 one time and 115 the other the other pg, both of which were considered normal. With both babies they had caught up w/ their edds by the next u/s. GL to you and lots of prayers!!!
Christa- what have the doctors said?
The implantation thing definitely makes sense. That's why many doctors do a "dating ultrasound" these days. Because implantation can vary. And then your due date can be adjusted. I am not sure about the heart rate part but remain that stress is bad for the baby. <3 keeping you in my thoughts/prayers