Uterine Artery Scan
Uterine artery Doppler is a promising technique for assessment of the level of risk of pre-eclampsia and IUGR. Doppler ultrasound provides a non-invasive method of assessing the utero-placental circulation. In normal pregnancy , impedance to flow in the uterine artery decreases with gestation and this presumably reflects the trophablastic invasion of the spiral ateries and their change into low resistance vessels. The uterine artery is a branch of the internal iliac artery close to the bifurcation of the common iliac. Colour flow Doppler is the method of choice for accurate screening of uterine artery waveforms.
The resistance index (RI) is the most commonly used index to measure the uterine artery flow as it is unlikely to have absent or reverse end Diastolic (EDF) flow. Although the pulsatility index (PI) can also be used. High resistance waveforms in the uterine artery with early diastolic notching in the second trimester are asssociated with the later development of pre-eclampsia, IUGR, placental abruption and intra-uterine death.
Patients are initially screened at around 20 weeks. Anyone with bilateral notches or a unilateral notch with a mean RI > 0.60 is screened again at 24-26 weeks. We feel that patients with a unilateral notch and a mean RI < 0.60 should still have serial growth scans at 24, 28, 32 and 36 weeks.
Uterine artery Doppler has proved to have the best predictive value in high risk pregnancies complicated by growth restriction (IUGR) and/or hypertensive disorders.
What does an ABNORMAL Uterine Artery Scan mean?
After an abnormal Uterine Artery Doppler Scan the chance of later developing IUGR varies between up to 38%, and and pre eclampsia upo to 31%.
The scans predict up to 47% pregnancies that will develop IUGR and up to 79% that will develop pre-eclampsia.
What does a NORMAL Uterine Artery Doppler Scan mean?
If the scan is normal however there is a 90% likelihood that neither IUGR or pre-eclampsia will occur.
Pregnancies with bilateral uterine artery notches combined with a poor past obstetric history indicate a greatly increased risk of pre-eclampsia and IUGR.
At present it is thought that notches of the uterine artery waveform with a high RI, hold the most risk of complicated pregnancies, although it is still to be established whether the notch or the high RI is the best indicator of poor outcome.
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