Why some doctor decline to perform a cerclage at 17 weeks?

Depends. There are certain clinical characteristics involved in the decision to perform a cerclage or not. In general in it performed after a previous second trimester loss that was not caused by ruptured membranes or preterm labor. In some cases it is performed when the cervical length is found to be short. If the right conditions are present a doctor should not decline and a 2nd opinion may be needed.