How long is your cervix ?

For those who have had a transvaginal ultrasound for cervical length ... How long was your cervix when you were 17 or 18 or 19 weeks ?
I had an ultrasound two days ago and they said my cervix measured 3.2 and said that is very good . 
I had a pre-term labour with my first at 33 weeks and this time they are monitoring me.
Plz share with me your experience ... My dr also told me " you are more than likely will deliver a pre-term again" which I find offensive cos I might and might not! As I know my friend she had a pre-term labour with a premie but next pregnancy she passed her due date and was induced so not the case for every one .