To detect a difference of at least 15 percentage units in the proportions of cesaeren sections between high/low AFL levels, with 83% power, we needed to include 160 deliveries with low (< 10.1 mmol/l) and 80 with high (≥ 10.1 mmol/l) AFL values. We assumed that at least 50% of the women included would receive oxytocin, and of these 15% would have a high AFL value, which implied that a minimum of 1444 (160/0.85+80/0.15)/0.5) deliveries had to be included in the study. The recruitment goal was then set at 3000 to allow for missing data or missing AFL samples. Differences among groups defined by operative delivery were tested by ANOVA for continuous variables and chi-square tested for categorical variables. The cut-off value of AFL was set at ≥10.1 mmol/L. This was in line with earlier publications where an increased frequency of operative intervention has been seen in deliveries with an AFL value ≥ 10.1 mmol/L [22–24].To evaluate the predictive capabilities of AFL, sensitivity, specificity, positive (PPV), negative (NPV) predictive values for operative intervention were calculated with 95% confidence intervals (CI). Logistic regression was used to study the association between operative interventions and each of the independent factors: maternal age, education, gestational age, fetal presentation, latent phase duration, use of epidural anesthesia (EDA), AFL, and country of delivery . Our model strategy was as follows: first, unadjusted associations with each factor were studied; second, the adjusted association with respect to the risk factors measured was studied in a multivariable model with all factors included; third, a subgroup analysis was performed to study whether the operative interventions differed between those with high and low AFL with respect to the levels of the other factors. An interaction term between AFL and each of the factors was added to the adjusted model (sequentially). P-values < 0.05 were regarded as statistically significant. Analyses were performed using IBM SPSS 22 (SPSS Inc. Chicago, Illinois, USA).