Objective To test the hypothesis that cervical shortening in polyhydramnios reflects the degree of excess amniotic fluid, and increases with normalisation of amniotic fluid volume. Study Design Prospective cohort study of 40 women with monochorionic twins undergoing interventional procedures between 16–26 weeks. Cervical length was assessed via transvaginal sonography pre-procedure, 1 and 24 hours post-procedure, and results compared between amnioreduction and control procedures. Amniotic fluid index (AFI) was measured pre- and post-procedure. Results Pre-procedural cervical length correlated with AFI (linear fit = 5.07 -0.04x, R2 = 0.17, P = 0.03) in patients with polyhydramnios (n = 28). Drainage of 2000ml fluid (range 700–3500ml), reduced AFI from 42cm to 21cm (P<0.001). Their pre-procedural cervical length did not change at one (mean Δ:−0.1cm, 95%CI, −0.4 to 0.2) or 24 hours (0.2cm, −0.1 to 0.6) after amnioreduction. There was no change in cervical length at control procedures. Conclusion Cervical shortening in twins with polyhydramnios does not appear to be an acute process; cervical length can be measured before or after therapeutic procedures.