Ultrasound examination in the first trimester allows accurate assessment of gestational age, and identifies and allows for appropriate care of women with multiple pregnancies. Show
20.1 Assessing gestational ageMethods used to assess gestational age include known date of ovulation, date of the last menstrual period (LMP) and diagnostic ultrasound. Diagnostic ultrasound is a sophisticated electronic technology, which uses pulses of high frequency sound to produce an image. This imaging enables measurement of the fetus and estimation of the gestational age. 20.1.1 Accuracy and effectivenessThe NICE guidelines reviewed the diagnostic value and effectiveness of screening methods in determining gestational age. Studies identified included a Cochrane review, four RCTs and a number of observational studies. Findings were as follows.
These findings are largely supported by subsequent lower level studies as follows.
A Cochrane review (Whitworth et al 2010), which compared selective versus routine use of ultrasound in pregnancy, concluded that ultrasound improves the early detection of multiple pregnancies. 20.1.2 Timing of assessmentThe systematic review conducted to inform the development of these Guidelines identified one prospective cohort study (n=8,313) (Verberg et al 2008) that investigated the best time to conduct gestational age assessment. The study found that the earlier the ultrasound assessment in pregnancy (preferably between 10 and 12 weeks), the more accurate the prediction of date of birth. The results indicate that after 24 weeks of pregnancy, a reliable LMP provides better estimates. 20.1.3 Calculating the estimated date of birthThe ability to estimate the range of dates during which birth may occur is influenced by the regularity and length of a woman’s menstrual cycle, whether the date of ovulation (rather than that of intercourse) is known and the timing of any ultrasound assessment. Selection of the better estimate of the date of birth is based on the following criteria Altman & Chitty 1997, Campbell Westerway 2000, Callen 2008:
20.2 Other considerations in gestational age assessment20.2.1 SafetyThe NICE guidelines do not discuss the safety of ultrasound and the literature review conducted to inform these Guidelines identified only a single prospective observational study (n=52) (Sheiner et al 2007). The study found a negligible rise in temperature at the ultrasound beam’s focal point. No studies were identified that assessed psychological harms to the mother, risk of overdiagnosis of placenta praevia or its contribution to anxiety. 20.2.2 Cost-effectivenessAn analysis of the cost implications of recommending routine ultrasound for gestational age assessment in the first trimester was undertaken to inform the development of these Guidelines (see separate document on economic analyses). The analysis aimed to balance the costs of additional scans undertaken against the savings resulting from:
The analysis was limited by a lack of data on privately funded ultrasounds and those carried out in hospitals and therefore could only identify implications for Medicare expenditure. Data limitations also meant that the analysis had to rely on a range of assumptions and on the literature, which is inconsistent in some areas. While some studies have found no significant difference in the rate of induction between women who have a first trimester scan and women who have both a first and second trimester scan Crowther et al 1999, Ewigman et al 1990, Harrington et al 2006, Whitworth et al 2010, others have found decreased rates of induction associated with first trimester screening (Bennett et al 2004). The analysis was therefore unable to conclusively determine whether the benefits of the recommendation would be likely to outweigh the costs. While a maximum number of additional scans (75,500) and associated costs ($A4.53 million) was estimated, the benefits vary considerably depending on whether a decrease in inductions is assumed, from $A230,000 if only improved power of maternal serum testing is included, to around $A17 million if a decrease in inductions is assumed, with an additional saving of around $A5 million if the link between induction and caesarean section is included. 20.2.3 Who should conduct the assessment?A range of health professionals may be trained to carry out ultrasounds, including midwives, Aboriginal health workers and GPs. In addition to having appropriate training and accreditation, it is important that caseload is sufficient to maintain skills. Minimum standards for health professionals conducting ultrasound assessments are disseminated by the Australian Society for Ultrasound in Medicine, the Australasian Sonographer Accreditation Registry, the Royal Australian and New Zealand College of Radiologists, and the Royal Australian and New Zealand College of Obstetricians and Gynaecologists. 20.2.4 Additional considerations for Aboriginal and Torres Strait Islander womenAccurately assessing gestational age is particularly important among Aboriginal and Torres Strait Islander women as:
20.2.5 Issues of access in rural and remote areasIn remote regions, it may be difficult for women to access ultrasound examination early in pregnancy due to limited availability of adequate equipment, health professionals not offering ultrasound, a lack of accredited and trained professionals in some areas and the costs involved in travelling for the assessment (this is not consistently funded under State/Territory schemes to support travel and accommodation for women from rural and remote areas to access care and services). Health professionals should ensure that history taking is comprehensive and detailed, paying attention to ongoing assessment of fetal growth and wellbeing. 20.3 Practice summary: gestational ageWhenAt the first antenatal visit. Who
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