cervical length at 30 weeks?

Well my cervix is now at 2.2 in length. Doc days he thinks my son wll be born at 35-36 weeks. What is the normal cervival length for 30 weeks? Anyone have experience with this ? By the way my first son was born a 35 weeks but th other two at 39 weeks   thanks

My cervix was the same at that point. I went into preterm labor at 31 weeks, but he held on until 38 weeks.