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, R(2) = 0.17, P = 0.03) in patients with polyhydramnios (n = 28). Drainage of 2000 ml fluid (range 700-3500 ml), reduced AFI from 42 cm to 21 cm (P<0.001). Their pre-procedural cervical length did not change at one (mean Delta:-0.1cm, 95%CI, -0.4 to 0.2) or 24 hours (0.2 cm, -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.