Ultrasound

Charting Foetal Growth through Ultrasound

Dr Suman Bijlani examines the role of sonography is assessing intrauterine foetal growth and assesses its importance

Sonography has become an essential tool in the assessment of foetal wellbeing and growth. Sonography is safe, easily accessible, fairly reliable and affordable. Let us see the different roles sonography can play in assessing foetal growth and monitoring a growth restricted pregnancy.

Dates are Important

"The role of sonography could be for screening for intrauterine growth restriction in normal or high risk pregnancies or in confirming its diagnosis and for foetal surveillance as also to help determine the timing and route of delivery" - Dr Suman Bijlani Director Gyneguide

Talks of foetal growth assessment follows that ‘dating’ the pregnancy is the basis on which all the further tests are interpreted. Hence, dating has to be as accurate as possible. ‘Good dates’ would mean good patient recall with regular pre-pregnancy cycles or a first trimester ultrasound report. In the absence of good dates, the diagnosis of growth restriction could be tricky and would rely heavily on serial measurements and supporting evidence like oligohydramnios. Growth restricted pregnancies are often complicated, with antepartum and intrapartum foetal distress and the need for caesarean delivery. The perinatal mortality rate is much higher and newborns are at a much higher risk for NICU transfer.

The clinician's challenge is to identify the IUGR foetus whose health is endangered in utero because of a hostile intrauterine environment and to monitor and intervene appropriately. It also includes identifying small but healthy foetuses and avoiding interventional harm to them or their mothers in the form of unnecessary inductions of labour, caesarean sections and iatrogenic prematurity. The role of sonography could be for screening for intrauterine growth restriction (IUGR) in normal or high risk pregnancies or in confirming its diagnosis and for foetal surveillance as also to help determine the timing and route of delivery. For screening purposes, an initial scan may be obtained in the middle of the second trimester (at 18-20 week) to confirm dates, evaluate for anomalies. A repeat scan may be scheduled at 28-32 weeks' gestation to assess foetal growth, evidence of asymmetry and adequacy of liquor.

Tailored to Individuals

It is important that the growth chart against which the sonographic measurements are interpreted be adjusted for maternal height, weight and ethnicity. The newer machines allow uploading of tailor-made charts. Several clinical and sonographic parameters should be used in combination to establish the diagnosis of intrauterine growth retardation (IUGR) with greater certainty. Most ultrasonographic machines report aggregate gestational age measurements and individual parameters. Assessing individual values is important to identify a foetus that is growing asymmetrically.

Four Parameters

The commonest parameters measured are the Biparietal Diameter (BPD), Head Circumference (HC), Femur Length (FL) and Abdominal Circumference (AC). Of these four individual parameters, the first three are bony measurements and are more accurate for assessing gestational age whereas the abdominal circumference is a soft tissue measurement taken at the level of the liver. Since the liver is sensitive to the effects of nutrition, this is the first measurement to be affected in asymmetric growth restriction. Of the first three, FL varies least with the foetal nutritional status and is most accurate for dating throughout the three trimesters. Expected foetal weight is calculated on the basis of all the above parameters combined and the value is plotted on charts (as mentioned earlier) to give a comparison with the expected weight for that age. Weights below the 10th percentile would qualify as growth restriction.

In this context, it is important to understand that symmetric growth restriction occurs in about 25 per cent of cases and is a result of early and prolonged foetal insult in the form of intrauterine infection, placental injury or limitation, severe and prolonged maternal malnutrition or a genetic or structural malformation. Because the insult occurs early in development, there is no brain sparing. These babies would grow slowly throughout pregnancy, the head and body both being smaller than expected. This would mean an ‘overall’ small baby with normal ratios. It is very difficult to distinguish it from ‘mistaken dates’ and serial measurements are often the way to go.

On the other hand, asymmetric growth restriction occurs in 75 per cent of cases and usually manifests between 28 to 32 weeks of gestation and is a result of diminished placental-foetal blood flow, hypoxia or malnutrition. Foetal brain sparing mechanisms are effective; hence there is head-body disproportion. Oligohydramnios is a common associated finding. A decrease in amniotic fluid index (AFI) correlates with declining placental function. Studies have shown an increased rate of IUGR among foetuses with decreasing maximum vertical pocket (MVP) values. In the presence of normal head and femur measurements, abdominal circumference (AC) measurements of less than 2 standard deviations below the mean appear to be a reasonable cut-off to consider a foetus asymmetric; hence the use of ratios such as FL/AC ratio or HC/AC ratio. A decrease in AC would result in an increase in these ratios, an indicator for asymmetric growth restriction. These ratios become important from the second trimester onwards when the foetal growth accelerates.

Doppler Effect

An important development in the last decade has been the evolving role of Doppler studies for foetal surveillance in IUGR and ‘at risk’ pregnancies. The rationale for performing a Doppler study in the diagnosis of IUGR is that many cases of growth restriction are thought to be associated with small vessel disease in the foetoplacental or uteroplacental circulation. Studies have shown that one stage uterine artery screening at 23 weeks gestational age is effective in identifying pregnancies at risk for developing poor pregnancy outcomes before 34 weeks due to uteroplacental insufficiency. An abnormal uterine artery study result could mean bilateral uterine artery notches in early diastole or a mean pulsatility index (PI) of greater than 1.45 in both arteries. But sensitivity and specificity for uterine artery measurements is low and it is not included in routine surveillance protocols.

Umbilical Artery Doppler Measurements:

In normal pregnancies, umbilical artery (UA) resistance shows a continuous decline; however, this may not occur in foetuses with uteroplacental insufficiency. The most commonly used measure of gestational age–specific UA resistance is the systolic-to-diastolic ratio of flow, which changes from a baseline value to an elevated value with worsening disease. A ratio of more than three in the third trimester is considered abnormal. As the insufficiency progresses, end-diastolic velocity is lost and, finally, reversed.

Middle Cerebral Artery Doppler:

The cerebral arterial waveform illustrates a very interesting point in foetal physiology. When the foetus is hypoxic, the cerebral arteries tend to become dilated in order to preserve the blood flow to the brain. The systolic to diastolic (A/B) ratio will decrease (due to an increase in diastolic flow) in the presence of chronic hypoxic insult to the foetus. This ratio should consistently be above four. The middle cerebral artery peak systolic velocity (PSV) is one of the single best measurements to predict IUGR related perinatal mortality.

Venous Doppler Waveforms:

Venous Doppler has been measured at the ductus venosus (DV), umbilical vein (UV), inferior vena cava (IVC), and seven other sites. This provides information about foetal cardiovascular and respiratory responses to its intrauterine environment. These measurements have been reported to become consistently abnormal when a foetus is severely compromised, thus providing additional information for the timing of delivery, especially in extremely premature (<32 wk) gestations.

In summary, the diagnosis of IUGR should be based on dating, serial measurements and evaluation of associated findings (like oligohydramnios or abnormal ratios), while the role of Doppler is usually foetal surveillance and identifying the compromised foetus who needs urgent delivery. Obstetricians, who are relying on these tools, should be aware of the limitations. In obese patients and patients with reduced liquor, biometric measurements may not be as accurate.