Obstetric Ultrasound

Obstetric ultrasound was first introduced in the late 1950s. It is now widely used and has become a useful tool in monitoring and diagnosis.

The first ultrasound scan is usually performed before 15 weeks.¹
The purpose is to:

• Diagnose pregnancy
• Accurately determine gestational age - this is essential for intervention of post-maturity, and for accurate serum screening for Down's syndrome
• Viability - to confirm presence of heartbeat and exclude ectopic pregnancy and hydatidiform mole
• Determine fetal number and, in multiple pregnancies, the chorionicity/amnionicity
• Detect gross fetal abnormalities

Procedure

• Measurement of crown rump length accurately measures gestational age if performed before 13 weeks. After 13 weeks, the fetus becomes increasingly flexed so results are inaccurate. Alternatives that can be used after this include bi-parietal diameter, and/or head circumference, or femur length.
• Usually performed abdominally, although occasionally a vaginal scan is necessary
• Nuchal translucency scans for risk of Down’s syndrome are best performed between 10-14 weeks

This scan is offered to pregnant women ideally between 18-20 weeks' gestation.¹ This scan can provide dating information and diagnosis of multiple pregnancy, in units where no booking scan is performed.
The main purpose is:

• To reassure the mother that her baby appears to have no gross structural abnormalities.
  • 50% significant abnormalities will be detected by the 20-week screening scan.
  • To provide the parents with options, e.g. termination, preparation, appropriate care throughout rest of pregnancy and delivery.
• To determine placental morphology and localisation.
  • Where the placenta extends across the internal cervical os, another scan at 36 weeks should be offered.
• To confirm that fetal growth is appropriate.

Procedure

• Assess growth by measurement methods below:
  • Bi-parietal diameter (most accurate for dating up to 20 weeks)
  • Head circumference
  • Femur length
  • Abdominal circumference
• Look at head shape and internal structures:
  • Cavum septum pellucidum
  • Cerebellum
  • Ventricular size at atrium (<10 mm)
• Minimum standards:
  • Spine: longitudinal and transverse
  • Abdominal shape and content at level of stomach
  • Abdominal shape and content at level of kidneys and umbilicus
  • Renal pelvis (<5 mm AP measurement)
  • Longitudinal axis – abdominal-thoracic appearance (diaphragm/bladder)
  • Thorax at level of 4-chamber cardiac view
  • Aortic arch
  • Arms – three bones and hand (not counting fingers)
  • Legs – three bones and orientation of feet (not counting toes)
• Optimal standards:
  • Cardiac outflow tracts
  • Face, nose and lips; 15% women may have to return for further checks

Aneuploidy scans are not routinely performed, as many normal pregnancies may have some of these features - i.e. there is a high false-positive rate.¹ Pregnancies affected by aneuploidy (abnormal chromosome number) will have sonographic markers. However, 50-80% of affected cases will already be identified by triple test, maternal age and nuchal translucency measurements.
Indications for 'marker' scan include:

• Family history of abnormalities, such as neural tube defect (NTD)
• Multiple pregnancies
• Maternal diabetes or epilepsy
• Recurrent miscarriage
• Polyhydramnios
• AFP abnormal/maternal age >35 years
• Oligohydramnios

These are set by the Royal College of Obstetricians and Gynaecologists to assure the quality of service provision. They include providing clear, written advice that includes detection rates for defined, common conditions. A trained counsellor in the area of diagnosis and screening should be available, as should a quiet room for breaking bad news about the baby. It should be possible to discuss the findings with an obstetrician within 24 hours or soon after detection of the anomaly.

Using standard 20-week scan checklist:

• Anencephaly - 100%
• Spina bifida - 92%
• Major cardiac anomalies (hypoplastic ventricle) - 61%
• Diaphragmatic hernia - 62%
• Gastroschisis - 100%
• Exomphalos - 92%
• Major renal tract problems (renal agenesis) - 85%

• Suspected fetal malpresentation, e.g. breech, should be confirmed by an ultrasound examination after 36 weeks.²
• A short cervix diagnosed by vaginal ultrasound has a good predictive value for preterm delivery, even in a low-risk population. Research has shown a relative risk of preterm delivery ( prior to 35 weeks) of 6.19 for lengths measured at 28 weeks which are at or below the 10th percentile of normal cervical lengths for that gestation (26 mm), or 9.49 for lengths at or below the 5th percentile (22 mm).³

This is a type of fetal assessment using specific criteria to reach a well-being score for high-risk pregnancies. It is based on the Apgar scoring system used to assess the condition of the newborn.
It was introduced in the 1980s and, despite minor refinements of the original test, the assessments still include five main features:

• Monitoring of fetal movements
• Fetal tone
• Fetal breathing
• Assessment of amniotic fluid volume
• Assessment of fetal heart rate by electronic monitoring

Biophysical profile fetal assessment is based on the association between low biophysical scores and poor pregnancy outcome.⁴ Hence the procedure aims to detect acute and chronic fetal compromise with changes in fetal heart patterns, decreased body and breathing movements, reduced amniotic fluid, oliguria, and redistribution of regional blood flow leading to a reduction in fetal renal blood flow.

Doppler ultrasound uses high-intensity sound waves to detect the blood circulation in the baby, uterus and placenta.

• The application has extended from the umbilical cord to fetal vessels (aorta, cerebral and renal arteries) as well as maternal vessels supplying the placental intervillous space.
• It is used for high-risk pregnancies where there is concern about baby's well-being, e.g. intrauterine growth retardation, hypertensive disorders of pregnancy, and to distinguish between the normal small fetus and the 'sick' small fetus.
• Despite its advances, it is not of use in routine antenatal screening because several studies have shown it is an unnecessary intervention and may cause possible adverse effects.⁵ Its current role in optimising management, particularly timing of delivery, remains unclear.