BACKGROUNDUltrasound examination of the gastric antrum allows reliable pre-operative assessment of gastric contents and volume in adult patients. However, during pregnancy, the change in the anatomical position of the stomach due to the gravid uterus leads to a change in the measured value of the antral area. Therefore, current mathematical models predicting gastric content volume (GCV) in the adult may not apply in term pregnant women.OBJECTIVETo propose a mathematical model which is predictive of GCV in pregnant women and to assess the performance of an ultrasound qualitative grading scale (0 to 2) for the diagnosis of clear fluid volumes more than 0.8 and 1.5mlkg(-1).DESIGNProspective cohort study.SETTINGHospices Civils de Lyon, Hopital Femme Mere Enfant, Lyon and Assistance Publique - Hopitaux de Marseille, Hopital Nord, Marseille, France.PATIENTSPregnant women in the third trimester of pregnancy.MAIN OUTCOME MEASURESComparison of the GCV as measured by MRI and the ultrasound measured antral cross-sectional area, and an assessment of gastric contents according to a 0 to 2 qualitative grading scale.RESULTSData from 34 women were analysed. A linear model predictive of GCV was constructed with a 95% agreement band of 95ml, with a mean polar angle of -8.7 degrees. Performance of the qualitative grading scale to detect fluid volumes >0.8 and >1.5mlkg(-1) was improved when used in a composite scale including a 505mm(2) antral area cut-off value in the semirecumbent supine position.CONCLUSIONWe report a new mathematical model predictive of GCV in women in the third trimester of pregnancy. Furthermore, the combination of the qualitative and the quantitative ultrasound examination of the gastric antrum might be useful to assess gastric fluid volume in pregnant women.TRIAL REGISTRATIONANSM Register N degrees 2015-A00800-49