Miscarriage

Okay. So this is my 2nd pregnancy. I'm currently 7 weeks. I had a scan at 6 weeks thinking I was 8 weeks but anyway. Some reason this pregnancy I am TERRIFIED of having a miscarriage. Some reason I feel like I'm gonna go to the doctor and get a ultrasound & they are going to say I am miscarriaging. So I guess my question is. Did anyone know they were having a miscarriage? What are the symptoms? Or did you go to the doctor thinking everything was okay, and they told you were having a miscarriage?