Introduction Fetal fibronectin (fFN) is an excellent predictor of preterm birth. Seminal fluid may cross react with the fFN assay. Sexual intercourse on quantitatively measured concentration of fetal fibronectin (fFN) in vaginal swabs from women who have had sexual intercourse has not previously been analysed.

Method Prospectively collected data from 78 women at risk of preterm birth (with ethics/consent) who had quantitative fetal fibronectin testing at the Preterm Surveillance Clinic in St Thomas' Hospital, London, was performed. 39 women with sexual intercourse within 48 hours of testing were compared with 39 controls, matched by previous history, gestational age and delivery outcome. Data was blinded so concentrations were not revealed until after delivery.

Results The mean concentration of fFN in women who had sexual intercourse in the previous 48 hours was 132.1 ng/ml (STD = 171.2) compared to a mean concentration of 9.7 ng/ml (STD = 18.6) in the control group, (p value =0.00004). 17 women had false positive tests in the sexual intercourse group (50ng/ml; 37/40 gestation), compared with 2 in the control group.

Conclusion The concentration of fFN was significantly higher in women who had sexual intercourse in the previous 48 hours compared to women who didn't. Sexual intercourse increases the risk of false-positive fFN test. A history of vaginal sexual intercourse should be obtained from women prior to sampling.