Obstetric Growth Scan and Doppler

The obstetric growth scan measures foetal size and wellbeing in the second half of pregnancy. Biometry (head circumference, biparietal diameter, abdominal circumference and femur length) is used to estimate foetal weight with a formula. It is plotted on customised or population centile charts against gestation.

The amniotic fluid volume (deepest vertical pocket or amniotic fluid index) and foetal presentation are recorded, with the placenta (position, maturity, abnormalities).

Doppler ultrasound measures blood flow velocity waveforms in several vessels:

• Umbilical artery: resistance in the placenta, through the systolic to diastolic ratio, pulsatility index or resistance index.
• Middle cerebral artery: brain sparing (low pulsatility index in growth restriction), and peak systolic velocity for foetal anaemia.
• Uterine arteries: placental perfusion, notching, and mean pulsatility index at 20 to 24 weeks for pre-eclampsia and growth restriction.
• Ductus venosus and umbilical vein: foetal cardiac compensation in severe, early onset growth restriction.

The cerebroplacental ratio (middle cerebral to umbilical artery pulsatility index) is a sensitive indicator of foetal compromise. A 3 to 5 MHz probe is used, and the examination takes 15 to 30 minutes.

Foetal growth restriction (estimated weight or abdominal circumference under the 10th centile, with abnormal Doppler or under the 3rd centile) affects 5 to 10% of pregnancies. It is the most important modifiable risk factor for stillbirth (about 30 to 50% of stillbirths are small for gestation).

Large for gestational age (over the 90th centile) occurs in diabetes and obesity. It carries the risk of shoulder dystocia, birth injury and caesarean section.

Ultrasound estimates of weight have an error of about 10 to 15% (the 95% limits of agreement). Doppler of the umbilical artery in high risk pregnancy reduces perinatal deaths by about 29% (in studies of high risk pregnancy).

Indications. Suspected small for dates (fundal height lower than expected, reduced foetal movements, maternal risk factors: hypertension, pre-eclampsia, diabetes, smoking, drug use, renal disease, autoimmune disease, thrombophilia, previous growth restricted baby or stillbirth, multiple pregnancy, low pregnancy associated plasma protein A, abnormal uterine artery Doppler, placental abnormalities).

Suspected large for dates, maternal diabetes, polyhydramnios. Reduced foetal movements. Antepartum haemorrhage. Post-term pregnancy and prolonged pregnancy surveillance. Breech or unstable lie. Monochorionic twin pregnancy (every 2 weeks from 16 weeks). Rhesus alloimmunisation and suspected foetal anaemia (middle cerebral artery peak systolic velocity). Hypertensive disorders of pregnancy.

Suspected oligohydramnios or polyhydramnios. Uterine artery Doppler at 20 to 24 weeks in women at high risk. Not as a routine screening scan in low risk women (no proven benefit).

Contraindications and precautions. None. Doppler is used with the lowest output and the shortest time necessary after the first trimester (thermal index under 1.0). The woman is placed semi-recumbent, tilted to the left to avoid supine hypotension.

Doppler samples are taken during foetal rest, with no foetal breathing or movement, and with an insonation angle under 30 degrees. Counselling is given about the limits of the test and the meaning of the results.

Preparation and technique. Biometry is done in set planes:

• Head circumference: axial view at the level of the thalami and cavum septi pellucidi.
• Abdominal circumference: at the level of the stomach and the umbilical vein portal sinus junction (the most sensitive measurement for growth restriction).
• Femur length: long axis perpendicular to the beam.

Estimated foetal weight is calculated with a formula combining several measurements, and plotted against gestation.

Umbilical artery Doppler is taken from a free loop of cord (the free loop is chosen to avoid the placental and foetal ends). Middle cerebral artery Doppler is taken from the proximal third of the artery at the circle of Willis.

Uterine artery Doppler is taken at the point where the artery crosses the external iliac artery. The ductus venosus is taken in a mid-sagittal or oblique abdominal section.

Normal appearances. Growth tracks along or between centiles, with abdominal circumference and estimated foetal weight between the 10th and 90th centiles (average growth of about 200 to 250 g each week in late pregnancy). The head circumference to abdominal circumference ratio falls progressively through the third trimester, and amniotic fluid is within normal range.

Umbilical artery Doppler: forward end diastolic flow throughout, with pulsatility index falling with gestation (about 1.2 at 24 weeks to 0.9 at 36 weeks) and resistance index under 0.7 at term. Middle cerebral artery: high resistance in mid pregnancy and a fall in pulsatility index near term, with a cerebroplacental ratio over 1.1 to 1.2.

Uterine arteries: mean pulsatility index under the 95th centile for gestation (under 1.5 at 20 to 24 weeks) and no early diastolic notch after 24 weeks. Ductus venosus: forward flow throughout the cardiac cycle, including during atrial contraction. Umbilical vein: continuous non-pulsatile flow.

Interpretation.

• Small for gestational age: estimated weight or abdominal circumference under the 10th centile. Constitutionally small foetuses grow along a low centile with normal Doppler and fluid, and have a normal outcome.
• Foetal growth restriction: estimated weight or abdominal circumference under the 3rd centile, or under the 10th centile with abnormal Doppler, or a fall across two quartiles. Abnormal Doppler means umbilical artery pulsatility index over the 95th centile, abnormal uterine artery Doppler, or low cerebroplacental ratio.
• Early onset growth restriction (before 32 weeks): placental disease, severe, with progressive Doppler changes.
• Late onset growth restriction (after 32 weeks): milder, with reduced cerebroplacental ratio and normal umbilical artery in many cases.
• Doppler progression in placental growth restriction starts with raised umbilical artery resistance. Then come absent end diastolic flow, then reversed end diastolic flow (a sign of severe placental failure with high risk of foetal death within days), with brain sparing (low middle cerebral artery pulsatility index).
• Later changes are abnormal ductus venosus (absent or reversed a-wave: a sign of cardiac decompensation and acidaemia), pulsatile umbilical venous flow, reduced liquor, and abnormal foetal heart rate patterns.
• Foetal anaemia: middle cerebral artery peak systolic velocity over 1.5 multiples of the median (rhesus disease, parvovirus, twin anaemia polycythaemia sequence, foeto maternal haemorrhage) with hydrops in severe cases.
• Macrosomia and large for gestational age: abdominal circumference over the 90th centile (or over 95th), estimated weight over 4000 to 4500 g, polyhydramnios, subcutaneous fat thickening, and asymmetry with large abdominal circumference in diabetes.
• Amniotic fluid: oligohydramnios (deepest pocket under 2 cm: growth restriction, membrane rupture, renal anomaly, post-term), polyhydramnios (deepest pocket over 8 cm: diabetes, anomalies, twin transfusion, idiopathic).
• Uterine artery Doppler: bilateral notching or raised mean pulsatility index (over the 95th centile) at 20 to 24 weeks indicates placental under perfusion and a risk of pre-eclampsia and growth restriction. Positive predictive value is low, and negative predictive value is high.
• Placental findings: placenta praevia, abruption (retroplacental collection), placental infarcts, thick or cystic placenta (triploidy, infection, hydrops), chorioangioma, and accreta spectrum features.
• Twin pregnancy: discordant growth (estimated weight difference over 20 to 25%), selective growth restriction, and twin anaemia polycythaemia sequence (discordant middle cerebral artery peak systolic velocity).
• Twin-to-twin transfusion syndrome: one twin with polyhydramnios and a large bladder, the other with oligohydramnios and a small or absent bladder ("stuck twin").
• Presentation and lie: breech, transverse, oblique, and cord presentation.
• Foetal behaviour (biophysical profile): breathing movements, body movements, tone, amniotic fluid and non-stress test (score out of 10; 8 to 10 is normal, and 4 or less suggests compromise).

Critical values and findings needing immediate action.

• Reversed end diastolic flow in the umbilical artery, absent or reversed a-wave in the ductus venosus, or pulsatile umbilical venous flow: urgent obstetric and neonatal assessment for delivery by caesarean section, with antenatal corticosteroids for lung maturity and magnesium sulphate for neuroprotection before 32 weeks.
• Absent end diastolic flow in the umbilical artery: admission or intensive surveillance with delivery usually by 34 weeks (timing set by gestation, other markers and local practice).
• Middle cerebral artery peak systolic velocity over 1.5 multiples of the median: foetal blood sampling and intrauterine transfusion in a foetal medicine centre.
• Anhydramnios or severe oligohydramnios: assessment for membrane rupture, foetal anomaly and placental failure.
• Abnormal biophysical profile or non-reassuring foetal heart rate trace: delivery assessment.
• Estimated weight under the 3rd centile at term or reduced foetal movements with growth restriction: induction or caesarean section.
• Estimated weight over 4500 g (or 4000 g in diabetes) at term: counselling about delivery, with discussion of caesarean section and the risk of shoulder dystocia.
• Placenta praevia with bleeding, abruption or vasa praevia: urgent obstetric care.
• Twin-to-twin transfusion syndrome: urgent referral to a foetal medicine centre for laser therapy.

Limitations and artefacts.

• Estimated foetal weight has an error of 10 to 15% (up to 20% at extremes of weight), larger in obesity, oligohydramnios, engaged head, and macrosomia.
• It cannot be used alone to decide on induction or delivery mode. Repeated estimates need an interval of at least 2 weeks (a fall in growth velocity is more informative than a single value).
• Dating errors (wrong dates, late booking) produce false diagnoses of growth restriction or macrosomia. The date is fixed by the first trimester crown rump length when available.
• Measurement errors arise from incorrect planes, foetal position, engaged head (head circumference underestimated), and caliper placement.
• Different charts (population, customised, international) classify different babies as small, and the chart used should be stated.
• Doppler is angle dependent. Waveforms vary with foetal heart rate, breathing movements, maternal position and place of sampling on the cord. Flow changes along the cord, and the free loop is the usual site.
• Ultrasound cannot diagnose the cause of growth restriction. Investigation for infection, chromosomal anomalies, maternal disease, thrombophilia and placental pathology is needed.
• Reduced liquor can result from maternal dehydration; checks are repeated.
• Doppler of the umbilical artery is useful only in high risk pregnancy; in low risk pregnancy it is not beneficial and may cause unneeded intervention.
• Routine third trimester scanning in low risk women has not improved outcomes, and has raised false positives and induction rates.
• Prolonged Doppler exposure raises thermal concerns, so the lowest output and shortest time apply.

Special groups.

• Women with hypertension, pre-eclampsia, renal disease, lupus, antiphospholipid syndrome, or sickle cell disease: serial growth scans every 2 to 4 weeks with Doppler.
• Women with diabetes: growth scans at 28, 32 and 36 weeks for macrosomia and polyhydramnios.
• Multiple pregnancy: growth scans every 2 to 4 weeks (every 2 weeks in monochorionic twins).
• Women with previous stillbirth or growth restricted baby: serial scans from 26 to 28 weeks, with uterine artery Doppler at 20 to 24 weeks.
• Women with obesity: reduced accuracy; repeated measures.
• Post-term pregnancy: amniotic fluid and Doppler assessment before induction.
• Regions: growth restriction from malnutrition, malaria, anaemia, hypertension, HIV and infections is common, and stillbirth rates are high.
• Fundal height is the main tool in many clinics. Basic ultrasound at referral centres improves detection of growth restriction, multiple pregnancy and malpresentation.

Follow up tests. Serial growth scans every 2 weeks, repeat Doppler (twice a week in absent or reversed end diastolic flow), cardiotocography (computerised short term variability), biophysical profile, maternal investigations (blood pressure, urine protein, full blood count, renal and liver function, glucose tolerance test, thrombophilia and antiphospholipid antibodies),

infection screen (cytomegalovirus, toxoplasmosis, syphilis, malaria), foetal karyotype or microarray for early severe growth restriction and anomalies, foetal middle cerebral artery Doppler and blood sampling for anaemia, antenatal corticosteroids and magnesium sulphate, delivery planning with the neonatal team, and referral to a foetal medicine unit.