Ultrasound examination between 18 and 20 weeks gestation allows assessment of fetal development and anatomy. It is also used to estimate gestational age when this has not been assessed in the first trimester.
Diagnostic ultrasound is a sophisticated electronic technology that uses pulses of high frequency sound to produce an image. This imaging can enable measurement of the baby, estimation of the gestational age and identification of structural anomalies. Gestational age assessment and testing for chromosomal anomalies in the first trimester are discussed in Chapter 20 and Part H. This section discusses the second trimester scan to assess the development and anatomy of the baby and the position of the placenta. This assessment is also known as the morphology scan.
21.1.1 Congenital anomalies in Australia
In Australia in 2010, congenital anomalies (including chromosomal and structural anomalies) was the leading cause of perinatal death in single pregnancies (29%) and accounted for 76.1% of neonatal deaths of babies born at 32–36 weeks gestation and 44.1% of deaths of babies born after 37 weeks gestation. Available data on neural tube defects among babies born to Aboriginal and Torres Strait Islander women show a higher overall prevalence than among non-Indigenous women (16.6 vs 7.3 per 10,000 total births in 2006–2008) .
21.2 Offering assessment of fetal development and anatomy
21.2.1 Accuracy and effectiveness of ultrasound assessment of fetal development and anatomy
While gestational age assessment using ultrasound is more accurate in the first trimester, some women may not have access to ultrasound until later in pregnancy. Gestational age has been successfully estimated in the second trimester .
Ultrasound has been used in the second trimester to detect anomalies of the heart, renal tract and umbilical artery , neural tube defects and anomalies resulting from exposure to alcohol . The rate of detection of structural anomalies is generally higher in the second than in the first trimester .
While the combination of nuchal thickness and biochemical markers in the first trimester is more effective in identifying chromosomal anomalies (see Part H), some markers (eg echogenic bowel, short femur, short humerus, thickened nuchal fold, absent nasal bone) identified in the second trimester ultrasound occur more frequently in babies with chromosomal anomalies . A combination of markers is more accurate than a single marker alone, for example, only 5% of babies with identified chromosomal anomalies had echogenic bowel as the only finding .
Second trimester ultrasound has effectively identified placental location, overlap of the cervical os , placental length (which may assist in identifying risk of having a small-for-gestational age baby) and placenta praevia (which may resolve in women with [61%] and without [90%] a previous caesarean section) .
Type of ultrasonography
Accurate assessment can be performed using standard 2D ultrasonography. Assessment may be performed more rapidly using 3D ultrasonography.
21.2.2 Timing of ultrasound assessment of fetal development and anatomy
Recommended timing of the ultrasound scan varies in international guidelines but is generally in the range of 18–20 weeks as:
- sensitivity in detecting structural anomalies increases after 18 weeks gestation
- detection of structural anomalies before 20 weeks gestation gives women the choice of ending the pregnancy, where this is permitted under jurisdictional legislation.
Ultrasound can be used to assess gestational age up to 24 weeks gestation and to detect anomalies throughout the pregnancy.
Offer pregnant women ultrasound screening to assess fetal development and anatomy between 18 and 20 weeks gestation.
Approved by NHMRC in June 2014; expires June 2019
Timing of the ultrasound will be guided by the individual situation (eg for women who are obese, visualisation may improve with gestational age).
Approved by NHMRC in June 2014; expires June 2019
There is no benefit from repeated diagnostic ultrasound assessments unless clinically indicated. Repeated tests may increase costs for women, be inconvenient and have the potential to increase anxiety (eg through false positives). As well, access for some women is limited as this technology is not available in all settings.
Repeated ultrasound assessment may be appropriate for specific indications but should not be used for routine monitoring.
Approved by NHMRC in June 2014; expires June 2019
21.3 Other considerations
21.3.1 Benefits and harms
A Cochrane reviewfound a reduced number of inductions for ‘prolonged pregnancy’ and no significant differences in birth weight, size for gestational age, Apgar scores and rates of admission to neonatal intensive care between babies exposed to ultrasound in early pregnancy (before 24 weeks) and those not exposed. There were no significant differences in growth and development, visual acuity or hearing for children aged 8–9 years . Follow-up at 15–16 years (n=4,458) found no significant effect on overall school performance .
No studies were identified that assessed psychological benefits or harms to the mother. Women may not be fully informed about the purpose of routine ultrasound and may be made anxious, or be inappropriately reassured by scans. A small systematic review found insufficient evidence to support either high or low levels of feedback during ultrasound to reduce maternal anxiety and change maternal health behaviour (smoking, alcohol use) .
21.3.2 Who should conduct the assessment?
Minimum standards for health professionals conducting ultrasound assessments are disseminated by the Australian Society for Ultrasound in Medicine, the Australasian Sonographer Accreditation Registry, the Australian Sonographers Association, the Royal Australian and New Zealand College of Radiologists, and the Royal Australian and New Zealand College of Obstetricians and Gynaecologists.
Ultrasound assessment should only be performed by healthcare professionals with appropriate training and qualifications, within the appropriate scope (eg diagnostic or point of care).
Approved by NHMRC in June 2014; expires June 2019
21.3.3 Access to ultrasound
The costs associated with ultrasound may limit access for some women, particularly if bulk-billed services are not available in their area.
In remote regions, it may be difficult for women to access ultrasound examination due to limited availability of appropriate equipment, a lack of accredited and trained professionals in some areas and the costs involved in travelling for the assessment. It is noted that there is a lack of consistency in funding across the States and Territories to support travel and accommodation for women from rural and remote areas to access care and services.
21.3.4 Cost effectiveness
An economic analysis carried out to inform the development of these Guidelines (see separate document on economic analyses) found that screening for congenital anomalies at 18–20 weeks is moderately cost-effective, without generating significant risks, although without driving substantive benefits. This excludes the positive psychological value of the information obtained from the ultrasound (which may be associated with improvements in fetal wellbeing) and benefits from the detection of placental problems and confirmation of gestational age, making these estimates fairly conservative.
21.4 Discussing assessment of fetal development and anatomy
Not all women will want an ultrasound and some may not understand the purpose of the assessment or think that it is being offered because there is something wrong with the pregnancy.
In discussing the ultrasound scan, it is important to explain:
- that it is the woman’s decision whether the ultrasound takes place
- where ultrasound services are available if the woman chooses to have one
- that ultrasound does not detect all fetal and maternal anomalies
- any costs involved for the woman and the timeframe for receiving results
- choices if any anomalies are detected (some parents may not want an ultrasound if there is no change in birth outcomes).
21.5 Practice summary: fetal development and anatomy
Between 18 and 20 weeks.
- Aboriginal and Torres Strait Islander Health Practitioner
- Aboriginal and Torres Strait Islander Health Worker
- multicultural health worker.
- Discuss the purpose of the ultrasound
Explain that ultrasound assessment is offered to all women to check the anatomy and growth of the baby and can also be used to estimate gestational age if this has not already been done.
- If a woman chooses to have an ultrasound, arrange an appointment or referral
When arranging referral, ensure that the ultrasound takes place before 20 weeks of pregnancy.
- Take a holistic approach
Provide advice to assist women in accessing services (eg availability of bulk-billed services and interpreters). For women who need to travel for assessment, explain the need to plan early and organise travel and accommodation. Provide information on available funding to assist with these costs.
- Arrange follow-up
Routinely make sure that women are informed of the results of the scan and document these in her antenatal record. If an anomaly is suspected or identified, offer women access to appropriate counselling and ongoing support by trained health professionals.
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