If I need a cerclage, when will the doctor insert it?

After 12 weeks. Usually women who need a cerclage have a history of lossing a pregnancy around 16 to 22 weeks. Generally women with an incompetent cervix will dilate painlessly. If a woman has such a history she may receive a cerclage or a stitch around the cervix to prevent it from opening. Generally such a cerclage would be placed after 12 weeks when the risk of a miscarriage is fairly low.
13-16 weeks. Generally, prophylactic cerclages are placed between 13–16 weeks gestation, after an ultrasound shows a normal fetus with no abnormalities. Urgent cerclages can be placed up to 24 weeks for women who have ultrasonographic changes consistent with a short cervix or the presence of funneling.