Amniotic Fluid Volume Estimation from 20 Weeks to 28 Weeks. Do You Measure Perpendicular to the Floor or Perpendicular to the Uterine Contour?

OBJECTIVE: Two different techniques are described in the literature for measuring amniotic fluid pockets to estimate amniotic fluid volume. This study was undertaken to determine if ultrasound estimates using amniotic fluid index (AFI) and single deepest pocket (SDP) techniques should be measured perpendicular to floor vs perpendicular to uterine contour in pregnancies between 20 and 28 weeks. STUDY DESIGN: Amniotic fluid was measured using AFI and SDP techniques in low risk pregnant women undergoing an indicated ultrasound between 20 and 28 weeks of gestation. Measurements of both AFI and SDP were made holding the ultrasound transducer perpendicular to the floor and then perpendicular to the uterine contour. Pearson’s correlation coefficient was used to assess the association between estimated amniotic fluid volumes determined by the transducer perpendicular to the floor versus transducer perpendicular to the uterine contour; intra-class correlation coefficient was used to test agreement of the two techniques. RESULTS: Measurements were collected on 160 women between 20 and 28 weeks. For pregnancies between 20 and 28 weeks, the level of correlation for AFI was 0.67 (95% CI 0.57–0.74) [moderate] and for SDP was 0.47 (95% CI: 0.34–0.58) [poor]. CONCLUSION: In pregnancies between 20 and 28 weeks, the correlation of AFI values perpendicular to floor and perpendicular to the uterine contour remains moderate, either measurement can be used to estimate amniotic fluid volume. The correlation for SDP is poor and it remains uncertain which technique, perpendicular to floor or perpendicular to uterine contour, should be used for estimating amniotic fluid volume.