Initially, all the participants were subjected to through routine antenatal history taking and examination and by routine ultrasound scan using (Voluson, 730 Pro V, GE Medical System), AFI was assessed. All participants were instructed to take regularly the daily water requirement, especially in the two days before starting the study and throughout the study. Participants were randomly scheduled into two equal groups by computer-generated blocks into a control group received placebo vaginal tablets and a study group received 20 mg isosorbide mononitrate (IMN) vaginally once daily until delivery (Effox, Mina Pharma Co, Egypt; under license of Schwartz Pharma, Germany). Treatment allocation was concealed by using sequentially numbered opaque sealed envelopes, opened sequentially by a third person (study nurse). Amniotic fluid index was rechecked after two days and followed up weekly till delivery by one person to avoid interobserver variability. Drug treatment continues until the volume of the fluid reach the normal value or until delivery. Antenatal vitamins were continued.