6 week scan showing 4 week sac - anyone else had this?

Emily

It’s my first pregnancy and yesterday I went for an early scan at 6 weeks on the nose for reassurance - which has done the total opposite.

They could only see a 3.3mm sac, no yolk or anything else. Apparently I am only 4 weeks but I am sure of my dates as we used OPK sticks.

I’m terrified I’m going to misscarry, but I’m still having symptoms like sore boobs and I’ve not had any bleeding or cramping.

What do you think? Do I have a chance or should I prepare for the worst?