Abstract Objective: We conducted a prospective comparative cohort study to determinate the influence of coitus on quantitative fetal fibronectin test results under normal pregnancy conditions. We also compared values obtained in cervical and vaginal secretions. Methods: In a population of women with normal singleton pregnancies between 22 and 28 weeks gestation, we have performed (cervical and vaginal) quantitative fetal fibronectin tests in two separate groups classified according to timing after coitus (one group of women had intercourse within 24 h before sampling and the control group had intercourse >24 h before sampling). The main outcome measures were the proportion of positive tests in both groups and the correlation between cervical and vaginal values through the Pearson correlation coefficient. Results: Both groups were similar in terms of general characteristics and pregnancy outcomes. The proportions of positive results in the vaginal secretions were 7.5% and 25.0% (P=0.007) in the control and coitus group, respectively. In the cervical secretions, the proportions of positive tests were greater, but not statistically different (39.7% and 40.0%, respectively). The Pearson correlation coefficients were very low (<0.3) indicating poor correlation between both sampling locations. Even if the cervical values were generally greater than the vaginal values, they were lower in 26% of the women. Conclusions: Coitus definitely interferes with vaginal fetal fibronectin test results. In cervical secretions, the positive rate was so high that coitus had no influence, but cervical sampling in this location should be avoided.