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Updated: Jul 17, 2026
High Frequency Ultrasound for the Analysis of Fetal and Placental Development In Vivo
Published on: November 8, 2018
Curve of amniotic fluid index measurements in low-risk pregnancy
Maria Regina Machado1, Jose Guilherme Cecatti, Fabiana Krupa
1Department of Obstetrics and Gynecology, University of Campinas (UNICAMP), Campinas, Brazil.
Objectives:
To establish a curve of amniotic fluid index [AFI] measurements from the 20th to the 42nd week of pregnancy.
Methods:
A prospective and descriptive study was performed, in which an independent sample of 2,868 low-risk pregnant women were studied using routine ultrasound, including fetal biometry and measurement of AFI. Data were analysed using multiple linear regression, and constructing a curve using 2.5th, 10th, 50th, 90th and 97.5th percentiles of the AFI measurements, according to gestational age, and after submitting values to smoothing process using quadratic polynomial adjustment.
Results:
There was a significant variation in AFI measurements at the different gestational ages. The 50th percentile remained practically constant at approximately 150 mm between the 20th and 33rd week, after which there was a decline in volume, which became evident after the 38th week. At the 40th week, the 10th percentile was around 62 mm and the 2.5th percentile around 33 mm.
Conclusions:
The curve of percentiles of AFI measurements in low-risk pregnant women showed significant decrease with gestational age, especially after the 33rd week. These data should be validated for a diagnosis of increased or decreased volumes of amniotic fluid at specific gestational ages.
