Cervical length

Hi alls, I have a question,  On the 9jan when doc do a quick cervical measurement it was 3.5cm. And than on  12jan i went for my scan apt + cervical measurement frm another department it was 2.8cm. Anybody have similar experience? Oh im pregnant 20weeks +4days with identical twins. This is my 7th pregnancy. Im on vaginal progesterone once daily before bed. And now they up to twice daily.  Thank you for the answer.