Why do I stay so worried ..

I had my first US four weeks ago Monday , I go in for my 12 week scan on the 29th . At my first scan I was 7 weeks , we saw the heart beat and everything looked great ... But I've been so worried about Miscarrying ! I can't stop worrying ! I have no idea why ? Anyone else worried about this too . Hoping for a healthy baby to bring home in 7 months from now ! ?​