Amniotic fluid index in low-risk, post-dates pregnancies

To determine normal values for amniotic fluid index (AFI) in uncomplicated post-dates singleton pregnancies and compare it to current reference ranges.

Four hundred and forty-eight women with singleton, uncomplicated pregnancies, presenting for post-dates assessment (gestation 40+0 to 42+0 weeks) between January 1, 2004 and June 30, 2005.

Ultrasound assessment of liquor to calculate the AFI.

The mean and standard deviation for AFI in the sample was 9.86 (SD 3.4), which is significantly different to the mean of the currently used reference range (mean 11.6, SD 3.9, p < 0.0001). The 5th percentile in this population was 4.6 cm.

This study has demonstrated lower mean and 5th percentile values for AFI in post-dates pregnancies than current reference ranges. It has the largest published UK study population, and can therefore be used as a reference range in similar populations, which is likely to reduce unnecessary obstetric interventions.