Measuring 6 days behind

Cheri
Anyone ever measure behind in early pregnancy and have things turn out to be ok? I had a scan on Friday. Saw both my twins and both had heart beats (rates Unknown), however both twins were only measuring 6w2d's when I should in fact have been 7w1d. My specialist was concerned & had to prepare us for the fact that we may lose them both due to being 6 days behind in growth. I did <a href="https://glowing.com/glow-fertility-program">IVF</a> so my ovulation dates cannot be off.  Any advice?