Fetus two weeks behind

I went for ultrasound for the first time. I was told by the doctor my baby is too small. Actually 1-2 weeks behind. Fetus is actually 6 weeks. But the doctor said 4 weeks n some days. He asked me to come back after a week to see if fetus is growing. I still have pregancy symptoms and cravings. Do you think it is misscarriage? Is there anyone who was told the same thing and baby actually grew?