Is A 30mm Cervical Length At 18 Weeks Of Pregnancy Concerning, And Should I Start Vaginal Progesterone Now?

HI, currently i am in 18 weeks of pregnancy . I was diagnosed with 30 mm cervical length, i was in duphaston (progestrone) treatment till 16 weeks. I am much worried about cervical length can any one suggest me if my cervical length is ok or can i start vaginal progesterone from this week.