First ultrasound

Tomorrow at 9:30 am I have my very first ultrasound. According to this app I'm 9 weeks. I haven't had a doctor appointment yet but my doctor decided to do an ultrasound to see how my little baby is doing. I can't help but I been worrying so much. Before my daughter I had two missed miscarriages and I just so scared this might happen again. I have all the symptoms and I'm even showing but I know it's still possible something is wrong that's why they are called missed miscarriages. What if something is wrong? How and I going to explain to my 3 year old something is wrong with the baby. She's so excited and always kisses my stomach and says she loves her sister or brother. I just want everything to be okay.