Ultrasound biometry of normal human amniotic fluid index in a Nigerian population

Abstract Background The amniotic fluid index (AFI) closely represents the actual amniotic fluid volume (AFV) with measurements below and above the 5th and 95th percentiles describing oligohydramnios and polyhydramnios respectively. Objective To create a sonographic reference range nomogram of the AFI for a Nigerian population using polynomial regression, and to test for reproducibility of AFI measurements. Methods Five hundred and sixty patients between 16 and 40 weeks fetal gestational age (FGA) were sampled for the study. The FGA was estimated sonographically by combining the fetal biparietal diameter, abdominal circumference, and femoral length measurements. The AFI was obtained by measuring and summing up the deepest pool of amniotic fluid devoid of fetal parts and umbilical cord in each quadrant of the maternal abdomen. To test for reproducibility of AFI measurements, two sonographers with more than 8 years of obstetric clinical experience independently measured the AFI of 40 patients. Polynomial regression, Pearson's correlation, and intra class correlations were used for data analysis. Results The mean ± SD of the AFI obtained in the second and third trimester respectively is 15.2 ± 4.0 cm, and 16.0 ± 5.8 cm. There was a steady but insignificant decrease in the AFI at all the FGA studied. A log transformed linear model adequately fitted the relationship between the AFI and FGA, given as: A F I c m = 1.1461 + 0.00091 × F G A ( R 2 = 0.002 ) . Our normal AFI range is from 8.6 cm to 27.0 cm. Conclusion Oligohydramnios and polyhydramnios can be satisfactorily diagnosed in the study population when AFI measurement is below or above 8.6 cm and 27.0 cm respectively.