Your cervix is still a normal length at 3.3 cm, but the internal opening has widened from 2.9 mm to 6.6 mm between your two scans, which is exactly what doctors watch closely this early in pregnancy. A cerclage stitch is usually suggested in this situation, to support the cervix and lower the chance of it opening further or labour starting too soon.
Next Steps
Waiting without the stitch means relying on repeat scans to catch any further change in time, which isn't assured. Since your own obstetrician has already suggested the cerclage based on this trend, going ahead with it is usually the safer path here.
Health Tips
Please see your obstetrician before the timing is finalised, since they can weigh this alongside your subchorionic clot and placenta position.