When Should The Cervical Cerclage Be Done?

I'm 32 years and conceive after 4 years of marriage and i delivered a baby boy in 6weeks and 9 days (premature baby) and my son only live 23 days and his past away. Now its almost 5 months since i delivered and i had consult my gynae and his advice me to try again to conceive. The same time he advice that i need to take bed rest once i conceive until i deliver the baby because i still have the risk. My gynae also advice to do cervical cerclage on 3rd month pregnancy. my actually problem is my servix is weak and it open 4cm at 25 weeks and at the 26 weeks and 9 days i no have choice because i frequently have contraction. so advice me that is it safe to do cervical cerclage for my case.