Cervix length

Megan. • TTC since April 2014, 1 MC at 13.5 weeks.
Hi guys just a question , I had an ultrasound at 16 weeks  and they said my cervix was 4cm , yesterday I had my anatomy scan at 19 weeks and my cervix measured 3cm . I know that it's still a normal length but is it normal for it to drop by 1cm that quickly?? Any information would help , thank you:)