Introduction Fetal fibronectin (fFN) is associated with cervical shortening and membrane exposure, and is used a screening tool for Preterm birth. However its use following rescue cerclage for cervical dilatation and bulging membranes is not well described. The mechanism of action of rescue cerclage is unknown.

Methods We performed a retrospective analysis of women at St Thomas' Hospital who underwent rescue cerclage (indicated by cervical dilatation & bulging membranes) between 2003 and 2010.

Results Rescue cerclage was performed in 31 women, with fFN testing in 21 of these (63%).

12 (57%) had a single fFN test following cerclage:

• ▶ 83% (10/12) were positive: 70% (7/10) delivered <30 weeks, 80% (8/10) delivered <34 weeks, 20% (2/10) delivered > 37 weeks.

• ▶ 17% (2/12 women) of women with a single fFN test were negative; none delivered at <30 weeks, 1 woman delivered <34 weeks & all delivered <37 weeks.

(43%) women underwent longitudinal fFN tests:

• ▶ 56% (5/9 women) went from positive to negative; of these, 1 woman (20%) delivered at <37 weeks (34+2) & the remaining 80% (4/5) delivered at >37 weeks.

• ▶ 1 (11%) went from negative to positive & delivered at 38+0 weeks'.

• ▶ 1 (11%) remained positive & delivered at 35+1.2 (22%) remained negative; both delivered at > 37 weeks'.

Conclusion The negative prediction of fFN is maintained in women following rescue cerclage. Over 50% of the women with longitudinal tests went from Positive to Negative, suggesting that cerclage may reverse exposure of membranes and fibronectin leakage.