Is There a Difference in Sonographic Estimation of Amniotic Fluid Volume When Measuring With the Probe Perpendicular to the Floor Compared With Perpendicular to the Uterine Contour?

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.

Amniotic Fluid Volume Assessment: Eight Lessons Learned

Actual AFV can be determined by a dye-dilution technique or be directly measured at cesarean. This allows investigators to correlate estimated and actual AFVs. Lessons learned by assessing the relationship of estimated to actual AFVs. 1) Ultrasound estimates normal actual AFVs well, but abnormal AFVs poorly. 2) Quantile regression is a better statistical methodology to create a normal AFV curve across pregnancy. 3) There is no difference in the accuracy of the subjective (visualization without measurements) compared with the objective (visualization with measurements) technique in identifying normal and abnormal AFVs. 4) Color Doppler use leads to the over-diagnosis of oligohydramnios. 5) Intravenous hydration increases actual AFVs. 6) The estimation of AFV can be done with the transducer held perpendicular to the floor or perpendicular to the uterine contour. 7) The single deepest pocket should be used for identifying low AFVs. 8) The AFI should be used for identifying high AFVs.