Since its introduction in the field of obstetrics in 1958, ultrasound has grown to become an invaluable tool in the assessment of the unborn fetus. Advances in technology have helped improve the resolution and the magnification of examined structures to levels previously thought to be impossible.
The advent of the Doppler ultrasound test and 3D/4D ultrasound has further enhanced our understanding of what happens to the unborn fetus in utero. With its non-invasive nature and high patient acceptability, ultrasound has become the primary tool for any medical practitioner who cares for pregnant mothers.
First trimester ultrasound
High-resolution ultrasound now permits the obstetrician to assess the fetus from the earliest stages of pregnancy. First trimester ultrasound was initially reserved for confirmation of intra-uterine pregnancy, dating with crown-rump length (CRL) and viability. Although these parameters remain the cornerstone of first trimester ultrasound, technological advances now allow the obstetrician to assess the fetus in extensive detail. This includes the determination of chorionicity in multiple pregnancies, detection of molar pregnancy, fetal anomalies and Doppler parameters.
Ultrasound from 11-13 weeks + 6 days allows assessment of the Nuchal translucency (NT) thickness of the fetus. An increased NT thickness is associated with an increase risk of not only Down’s syndrome, but also all major chromosomal defects. The assessment of NT thickness has become part of routine clinical practice and is used in combination with blood tests for first trimester screening of aneuploidy.
The absence of fetal nasal bone, short fetal maxilla length, short humerus and femur length and short ears during the first trimester ultrasound are associated with an increased risk of Down’s syndrome. The presence of single umbilical artery, megacystitis, exomphalos, and decreased placental volume at 11-14 weeks ultrasound are associated with Trisomy 18 (Edward’s syndrome). The presence of “soft markers” such as choroid plexus cyst, pylectasis and cardiac echogenic foci during first trimester ultrasound are also associated with chromosomal anomalies.
Fetal heart rate measured by Doppler ultrasound in the first trimester can be predictive of both fetal anomalies as well as miscarriage. Abnormal ductus venosus flow is associated with chromosomal defects, fetal cardiac anomalies and adverse pregnancy outcomes. Raised uterine artery Doppler study in the first trimester is a predictor for the onset of pre-eclampsia, pregnancy induced hypertension and intra-uterine growth restriction (IUGR).
Second trimester ultrasound
The role of second trimester ultrasound has long been established as a part of routine clinical practice. The ability to detect structural anomalies has been enhanced by high-resolution ultrasound so that even sub-centimeter defects can now be detected. A fetal anomaly scan is usually carried out between 18-23 weeks and a vast array of fetal structural anomalies from head to toe can be visualised.
Apart from the detection of structural anomalies of the fetus and placenta, middle cerebral and uterine artery Doppler can be used for the detection of early fetal growth restriction and even fetal anaemia. The presence of multiple soft markers or characteristic features of chromosomal anomalies warrant invasive confirmatory testing with amnioscentesis. Advances in fetal echocardiography now allow the obstetrician to assess not only structural anomalies of the heart but also arrhythmias and fetal heart block.
Pregnant mothers with pre-existing or underlying medical conditions, previous abnormal babies and advanced maternal age should have an anomaly scan at 18-23 weeks.
Third trimester ultrasound
Fetal growth assessment, amniotic fluid levels and placental localisation still represent the main parameters in third trimester ultrasound. Measurement of the Bi-parietal diameter (BPD), Head circumference (HC), abdominal circumference (AC) and Femur length (FL) remain as the most accurate indices in determining fetal growth and weight. These parameters are well established and are performed routinely even in the primary care setting.
Amniotic fluid levels can be determined by amniotic fluid volume, deepest vertical pool measurement, or the most commonly used amniotic fluid index (AFI). Umbilical artery and middle cerebral artery Doppler in the third trimester help in detecting utero-placental insufficiency and intra-uterine growth restriction early. This allows preparations to be made for a possible early delivery of a growth restricted fetus.
In obese mothers, ultrasound may be the only reliable way of determining the fetal presentation.
The role of ultrasound in obstetrics has long been established and continues to grow with improvements in technology. Adequate training along with high-resolution ultrasound machines improves diagnostic accuracy and early detection of anomalies, thereby ensuring prompt, appropriate care of the mother and unborn child.