The Volume of Fluid Corresponding to Each Centimeter of Amniotic Fluid Index Changes throughout Pregnancy: A Prospective Cohort Observational Study

– Introduction: The normal values of the amniotic fluid index (AFI) throughout pregnancy have been established as 5–24 cm. However, a similar AFI may correspond to a different AF volume at different gestational ages. The aim of this study was to calculate the amount of fluid corresponding to each cm of AFI when a known volume of fluid is infused into the uterine cavity. Methods: This was a secondary analysis of a prospective cohort observational study. Serial amnioinfusions (AIs) were performed between 10 and 34 weeks of gestation in 19 fetuses with anhydramnios due to severe renal damage. The AFI was measured just before and after AI, and the volume of fluid/AFI cm was estimated as infused fluid (mL)/AFI final-AFI initial. Mean and standard deviation per gestational week were obtained, and amniotic fluid volumes corresponding to AFIs of 5 cm, 18 cm, and 24 cm were calculated. Results: A total of 279 AIs were performed (median/case n = 15, range 3–22). The mean volume of fluid per cm of AFI at 20 weeks was 25.08 mL (SD 5.72 mL), 40.61 mL (SD 27.4 mL) at 30 weeks, and 34.64 mL (SD 14.8 mL) at 34 weeks. There was a 61% increment in AF fluid per AFI cm from 20 to 30 weeks of gestation, and a reduction of 14.7% between 30 and 34 weeks of gestation. Extrapolating these calculations, the volume of fluid corresponding to an AFI of 18 cm at 20 weeks was 451.4 mL, at 30 weeks 731.0 mL, and at 34 weeks, 623.5 mL. Conclusion: The volume of amniotic fluid per cm of AFI changes during gestation. The definition of poly or oligohydramnios based on the same AFI can vary up to 61% in fluid volume from 20 to 30 weeks of gestation.