I was told at my 6 week scan that they couldn't see a gestational sac, and that my pregnancy was non viable. Meaning that I would MC. At my 8 week scan, they could see a sac, but no fetal pole, and I was told again that my pregnancy was non viable, and I should have a D&C. I refused.
My non viable pregnancy is now a 20mo toddler snoring next to me in bed. 

By my 12 week scan, everything had caught up, and he was measuring 12 weeks, and my EDD was adjusted BACK to what it should have been in the first place. DS3 ended up being 8 days late though.
So not all stories like this end badly, my DS3 is none the worse for being slow growing in the early stages of my pg, and I personally believe that this happens more than medical people realise, but isn't obvious because not every woman has such early scans, and by 12 weeks, most will have caught up.
I'm now (despite having had previous MC's and a SB) of the opinion that early scans are often not entirely useful in determining the viability of a pg.