I had an ultrasound last Thursday and they said it looked to be about 6w along even though I should have been closer to 7 based on my ovulation date but we saw the heart flicker so they told me to come back in a week or so. I went back and today they said measuring 6w with a good heartbeat. The sac seems to have doubled in size but the baby hasn't. So does this mean the baby isn't growing? Should I be concerned?