Girls, hello. Has anyone else experienced this? I had an ultrasound to…

Girls, hello. Has anyone else experienced this? I had an ultrasound today at 20-21 weeks (2nd screening) - the cervical length is 34 mm. At 18 weeks, I had an ultrasound and it was 39 mm. Isn't that too quick, a 5 mm shortening in just a couple of weeks? ? This is my third pregnancy. My first child was born at 30-31 weeks, and the cervical length was also 34 mm at the second screening. Then it suddenly shortened without any symptoms. But back then, no one monitored anything for me, so that situation occurred. My second child was born at 40 weeks. At the second ultrasound with my second child, the cervical length was 43 mm. I don't even know if I should start worrying yet or not. I'm thinking of doing another cervicometry in 2-3 weeks. Maybe someone can share their experience.