Umbilical arterial systolic / diastolic ratio

Last revised by Rohit Sharma on 18 Feb 2024

Umbilical arterial systolic/diastolic (S/D) ratio is a parameter used in obstetric imaging as part of umbilical arterial (UA) Doppler assessment. It is the ratio between the systolic velocity and the diastolic velocity.

Practical points

The reference range varies through the stage of pregnancy. If there is an increase in the resistance to blood flow through the umbilical artery, this is seen as an increase in the systolic/diastolic (S/D) flow ratio.

The S/D ratio mean value decreases with fetal age 1:

  • at 20 weeks, the 50th percentile for the S/D ratio is 4

  • at 30 weeks, the 50th percentile is 2.83

  • at 40 weeks, the 50th percentile is 2.18

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