What is the normal FL/AC ratio??

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What is the normal FL/AC ratio . & What is the relevance of  FL/AC ratio in predicting FGR ?  From 21 weeks this ratio is 22. For any reason if ratio is > 23.5 then suspicion of FGR but this ratio is not helpful in symmetrical FGR:--.However the HC/AC ratio or FL/AC ratio are poorer predictors of FGR.  Better predictor will be AC/EFW.