Does a fetus measuring small mean miscarriage?

When I went for an ultrasound at 6 weeks and a few days, they told me no heartbeat or fetal pole was detectable so I was scheduled to come back in 2 weeks. In what should have been 8 weeks and a few days they finally did see a fetal pole and heartbeat, but the baby was measuring about 8 days behind what I KNEW it should have been. My husband and I had been trying for a baby for 3 months so I KNOW exactly when I ovulated, I (obviously) knew when we had and I knew when this baby was conceived. I got my bfp on November 2nd so when they tried telling me that my ovulation dates must have been a little off, I knew that wasn't the case. Sure enough at 12 weeks I had found out that I had a missed miscarriage and I was devastated. Could this have been predicted from the start since the baby was measuring small even though a heartbeat was detected? I've read SO many other stories where it seems the same thing has happened to a lot of women. I don't mean women who aren't sure when they ovulated and their dates could very possibly be wrong, I mean when you know for a fact when you ovulated and the dates just don't match up with the growth of the fetus.