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Fetus

Poster Session 4

(1032) The volume of fluid corresponding to each centimeter of amniotic fluid index changes throughout pregnancy

Wednesday, February 14, 2024
1:00 PM – 2:30 PM  
Location: Prince George's Exhibit Hall CDE, Lower Atrium Level
Objective: The most frequently used metric to estimate  amniotic fluid (AF) volume is the amniotic fluid index (AFI). Normal values range from 5-25 cm, using these same thresholds throughout pregnancy to define normal AF or oligo/polyhydramnios. However, a similar AFI may correspond to different AF volume at different gestational ages. Our aim was to estimate the amount of fluid corresponding to1 cm of AFI when a known amount of fluid is infused in the amniotic cavity as treatment of fetuses with anhydramnios due to bilateral renal agenesis or severe dysplastic kidneys.

Study Design: Serial amnioinfusions (AIs) were performed to promote lung development and reduce fetal thorax compression. AFI was measured before and after AI and the difference (increment) was calculated in relation to the amount of infused fluid (Infused fluid/(AFI final-AFI initial). Mean and standard deviation were calculated and  5th, 10th 50th, 90th and 95th percentiles estimated using regression models. 

Results:

Results 279 AIs in 19 fetuses were performed (median/case n=15, range 3-22).  The volume of infused fluid ranged from 250 to 500 mL at 20-35 weeks, 700 to 800 mL at 27-29 weeks, and 800 to 1000 mL at 30-32 weeks. There was a mild increment in volume per AFI cm from 20 to 30 weeks, with a subsequent mild reduction at 34 weeks (Fig 1). The mean volume of fluid per cm of AFI at 20 weeks was (mean) 25.1 mL (10th 9.0 mL, 90th 41.2 mL), whereas at 30 weeks 1 cm of AFI corresponded to (mean) 40.6 mL (10th 12.8 mL - 90th 68.4mL). There was 66% increment in AF fluid per cm of AFI from 20 to 30 weeks of gestation. Extrapolating these calculations, a normal AFI at 20 weeks (5-25 cm) could range from 125 mL (AFI=5 cm) to 627 mL (AFI=25 cm), and at 30 weeks from 203 mL (AFI=5 cm) to 1015 mL (AFI=25 cm).

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 66% in fluid volume from 20-30 weeks of gestation