Once the conclusion has been reached that cervical cerclage is likely to benefit the patient at high or medium-high risk of cervical insufficiency, the question arises how and when to perform it.
Figure 1 shows the three main levels/types of cerclage: (1). regular transvaginal cerclage at the junction of cervix and fornix, (2) high-transvaginal cerclage after opening the fornix, and (3) transabdominal cerclage at the level of the internal cervical os. The effectiveness of these levels of cerclage has not been systematically studied. From a clinical/mechanical point of view, cervicoisthmic cerclage is superior to other cerclages as it is inserted at the level of the internal cervical os and therefore prevents funneling (opening of the cervical canal from the internal os). As illustrated in Figure 2, the presence of funneling is disadvantageous because any increase in intra-uterine pressure may, in the presence of funneling, exert a dilating force while the short remaining cervical length acts less as an antibacterial barrier and offers less mechanical strength. In contrast, from a surgical point of view transvaginal cerclages have the advantage over transabdominal cerclage, as the surgery is shorter and less challenging, the hospitalization is shorter, and there is no need for delivery by cesarean section as in transabdominal cerclage. Transabdominal cerclage should probably be performed only if adequate transvaginal cerclage is considered technically unfeasible or hazardous because of severe cervical defects .
Not only the position relative to the cervical canal, but also the position relative to the cervical tissue itself might affect the strength of the cerclage. Figure 3 shows three types of suturing: 1. around the cervix (modified Shirodkar), 2. with a series of small bites (modified McDonald), and 3. with 4 large bites of cervical tissue (4-steps). In one comparison, no significant difference in effectiveness was found between the Shirodkar and the McDonald technique . From a mechanical point of view, the 4-steps method would seem to provide the better strength, because the band passes deeper through the tissue and the cervix is least likely to tear should uterine contractions appear.
Based on timing, one may differentiate between 1. prophylactic cerclage prior to conception, 2. prophylactic cerclage in pregnancy, 3. urgent cerclage after shortening of the cervix, and 4. emergency cerclage after exposure of the membranes. In high-risk patients, prophylactic cerclage has the advantage that one will not be surprised by yet another fetal loss or sudden shortening or opening of the cervix that requires urgent or emergency cerclage. An additional possible advantage of prophylactic cerclage could be that it may serve as an early warning system, as any serious force on it is likely to induce pain or blood loss, which would allow time to start tocolytic therapy.
Prophylactic transvaginal cerclage generally is an easy procedure, and morbidity is limited to hospital admission, mild pyrexia and tocolytic therapy . Prophylactic transabdominal cerclage is more challenging, as one operates near the uterine vasculature. Some authors favor performance of transabdominal cerclage before conception, and the use of laparoscopic technique  has the advantages of minimally invasive surgery. However, an obvious disadvantage of preconception cerclage is that pregnancy may not occur, either deliberately or involuntarily, and published experience provides no evidence that preconception transabdominal cerclage is surgically easier or has fewer complications than transabdominal cerclage performed between 12 and 16 weeks gestation .
Emergency cerclages have traditionally been associated with a high risk of chorioamnionitis (up to 37%) and/or rupture of the membranes within 2 weeks of the operation (up to 65%), as a result of cervical shortening and exposure of the membranes to vaginal bacteria . For that reason, a policy of serial cervical length measurements is an insecure alternative to prophylactic cerclage in high risk cases. However, recent small studies seem to suggest that emergency cerclage, in combination with antibiotics, tocolysis and bedrest, may be more effective than previously thought [12, 13], with neonatal survival of up to 96% with cerclage as compared to 57% without it . Further studies are needed to determine if serial cervical length measurements plus emergency cerclage if needed, is as safe a policy as prophylactic cerclage in high risk cases.