Obstetric ultrasound — growth and wellbeing
Fetal biometry, liquor volume, presentation, placental site, umbilical artery Doppler
When to use it
- Assessment of fetal growth and wellbeing.
- Reduced fetal movements, suspected growth restriction.
Technique
- Curvilinear transducer of roughly 2-6 MHz with an obstetric preset; the patient is semi-recumbent with a left lateral tilt to avoid aortocaval compression, and the bladder need not be full at this gestation.
- Biometry comprises head circumference, biparietal diameter, abdominal circumference and femur length, each acquired in its defined standard plane — the transventricular plane for the head, the plane showing the stomach bubble and the portal sinus without the kidneys for the abdomen, and the full length of the ossified femur perpendicular to the beam. Estimated fetal weight is calculated from these and plotted on a growth chart, ideally a customised or population chart used consistently.
- Amniotic fluid assessed by deepest vertical pocket or amniotic fluid index according to local protocol; the two are not interchangeable and the method should be stated.
- Umbilical artery Doppler in a free loop of cord during fetal quiescence, reporting the pulsatility index and explicitly whether end-diastolic flow is present, absent or reversed. Middle cerebral artery Doppler and the cerebroplacental ratio are added where growth restriction is suspected, and ductus venosus Doppler at earlier gestations where umbilical flow is already abnormal.
- Presentation and placental site recorded, including the relationship of the placental edge to the internal os where a low-lying placenta was noted earlier — this needs a transvaginal measurement to be reliable.
- Serial growth measurements should be at least two weeks apart; measurement error over a shorter interval is large enough to manufacture both apparent growth arrest and apparent catch-up.
- Keep exposure as low as reasonably achievable, particularly with Doppler over the fetus: thermal index rises substantially in spectral and colour modes compared with greyscale.
Clinical questions that reach this study
Contrast
None