Chances of a missed miscarriage?

Don't know why my anxiety flared up again. But I know there's knowledgable people in here. Last week I heard my baby’s heartbeat, I even saw him move away from my doctor when I tapped on him. Something popped up in my tiktok of this girl saying “oh yeah, I had a missed miscarriage” and she’s obviously in distress but she was saying that she thought she felt the baby move and all that the entire time and I’m a FTM, never been pregnant, I’m probably mistaken the baby moving… or not, I’m just nervous now, I know my therapist was like “last you checked, baby was fine” and I’ve been coping with that and being 18 weeks, now I'm scared I’m doing this wrong all over again. What are the chances of a missed miscarriage? And I know this is a stupid question but how can we avoid it? Is a Doppler even beneficial at this stage?