Ultrasonographic assessment of the amniotic fluid index in normal pregnancies from the 12th to the 42nd Week of Pregnancy - Revista Brasileira de Ginecologia e Obstetrícia

Purpose: to determine the amniotic fluid index (AFI) through ultrasound assessment in normal pregnancies and produce a curve of normalcy for the AFI from the 12th up to the 42nd week of pregnancy. Methods: the study involved 471 measurements on 256 pregnant women, all undergoing normal pregnancies. In pregnancies of more than 20 weeks an estimation was made of the sum of the largest vertical diameters of the amniotic fluid pockets in the four quadrants into which the uterus was divided. In the pregnancies of 20 weeks or less, the sum was obtained from the largest vertical diameters measured in the two halves into which the uterus was divided. Results were expressed in centimeters. Results: AFI was measured (471 measurements) and the results were stratified and grouped by weeks of pregnancy (every two weeks), except the 12th week which was analyzed alone. From an average of 4.7 cm (limits 3.8-5.9 for the 5th and the 95th percentiles) at the 12th week of pregnancy, the AFI grew progressively up to the maximum mean of 14.6 cm at the 32nd week (limits: 7.0-2.5 cm). AFI presented stable measurements from the 21st up to the 40th week. After that, AFI measurements suffered a sharp decrease. The AFI cutoff point occurred at the 21st week of pregnancy. The percent increase of AFI obtained at the 32nd week, when compared to the 12th was 197.7%, and 2.9% at the end of pregnancy when compared to the measurement of the week taken as reference. Conclusion: AFI varied during pregnancy. It increased progressively up to the 21st week and then stabilized up to the 40th week. After that, it experienced a sharp decline. The maximum measurement of the AFI occurred at the 32nd week. By establishing a normalcy curve for AFI it becomes easier to detect changes and allows for a better follow-up of the pregnancy period.