Cerclage is something you need to worry about only if you've been diagnosed with incompetent cervix. If you have, chances are you've already lost a pregnancy in the second or third trimester because your cervix dilated at a time when you weren't having contractions. In that case, your doctor may recommend that you have a cerclage procedure to prevent that from happening again.
Cerclage is a surgery in which the doctor will sew your cervix shut so there's no chance of dilation. As you get closer to your due date, the cerclage stitches will be removed so you can deliver your baby.