High AFL value (≥10.1 mmol/l) and occiput posterior presentation of the fetus had the strongest association with operative intervention after adjusting for the other factors (Table 4). No significant interactions between AFL and each of the other eight factors were detected (p > 0.48 for all test of interactions), which implies that the sensitivity for operative intervention among women with a high AFL value (≥ 10.1 mmol/l) was 4.5 times higher compared to women with a low AFL value, irrespective of the levels of the other factors.