Cervical lenght

13 weeks my cervix length was 3.7cm..16+5 weeks length 3.4cm.my obgyn nothing to say about this.i m very worried about this..like why did it decrease ...previously At my 8week scan it was 2.5 cm doc prescribed progesterone and it increased to 3.7cm

At 13w I stopped progesterone

Now I’m 17weeks

And worried about my cervix getting short