Girls, I have another problem?? 19 weeks, from the ultrasound they sa…

Girls, I have another problem?? 19 weeks, from the ultrasound they said the cervix is 31mm, they said it's an opening?? I read on the internet that there should be 40 mm-35 mm until 20 weeks, I don't know what to think. We wanted to put them in the hospital, but I refused.