Clinical Practice Guidelines Antenatal care - Module I

Gestational age assessment

Page last updated: 02 April 2013

Accurate assessment of gestational age through ultrasound measurement of CRL will impose a cost associated with ultrasound screening for chromosomal abnormalities in the first trimester. Due to time, resource and data constraints, this analysis focussed on a relatively narrow set of costs and benefits, namely the financial cost of first trimester screening against cost-savings from a reduction in diagnostic testing and labour inductions at birth. The analysis is based on a whole-of-cohort model, and
results will not be affected unless the age profile of pregnant woman in Australia significantly changes.

1 Potential costs and benefits of the proposed recommendation

Potential costs

A recommendation that pregnant women are offered an ultrasound scan in the first trimester is likely to increase the volume of scans — more health professionals may offer the scan and more women accept the offer.

Introducing the proposed recommendation would have a range of effects on health expenditure. In particular, it would be likely to:
  • increase Medicare expenditure;
  • increase public hospital expenditure where women receive ultrasounds as hospital outpatients; and
  • potentially impose costs on individuals (eg where Medicare, or private health insurance, does not cover ultrasounds, and travel costs for women in rural and remote areas).

For reasons discussed later, this analysis focuses on the impact of introducing the recommendation on Medicare expenditure. This is also consistent with How to Compare the Costs and Benefits: Evaluation of the Economic Evidence (NHMRC 2001), which indicates that cost-benefit analyses should focus on health sector budget effects.

Potential benefits

Broadly, the proposed recommendation would, for the additional women receiving first trimester ultrasound scans, enable an accurate determination of gestational age in the first trimester. Potential benefits from this include:
  • reducing the number of pregnancies that are incorrectly diagnosed as having gone beyond term, thereby reducing the number and the costs associated with unnecessary labour inductions;
    • reducing the number of unnecessary labour inductions has in turn been found to reduce the risk of caesarean section (see Maslow & Sweeney 2000); and
  • optimising the timing and performance of maternal serum screening usually undertaken later in pregnancy to detect chromosomal abnormalities (eg trisomy 21);
    • improving the performance of maternal serum screening would potentially generate a cost saving by reducing the number of women referred for follow-up tests where no abnormality is present.

Ideal approach to cost-benefit analysis

Ideally, this cost-benefit analysis would:
  • estimate the annual increase in first trimester ultrasound scans as a result of introducing the recommendation;
    • estimate the cost to Medicare of this annual increase; and
  • compare this cost with estimated cost savings from:
    • the estimated annual reduction in labour inductions and the consequential estimated annual reduction in the number of caesarean sections; and
    • the estimated annual reduction in follow-up tests for chromosomal abnormalities.
However, the data limitations discussed in Section 2 require a modified approach to be undertaken, as set out in Section 4.
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2 Data and related limitations

Potential costs

Data are available for Medicare-funded ultrasound scans. Relevant Medicare-funded ultrasound scans include:
  • ultrasounds in the first 12 weeks of pregnancy (which are eligible for Medicare benefits under certain maternal or gestational conditions);
  • nuchal translucency thickness scans between 11 and 13 weeks; and
  • ultrasound scans between 17 and 22 weeks.

No data are available on pregnant women receiving ultrasounds as hospital outpatients or ultrasounds that are self-funded. Consequently, as noted above, the analysis is limited to the impact of introducing the proposed recommendation on Medicare expenditure.

Potential benefits

Reducing inductions
In 2008, labour was induced in 25% of pregnancies, a figure that remained broadly constant between 1999 and 2008 (Laws et al 2010). There are a number of reasons why health professionals decide to induce labour,
including because the pregnancy is believed to have gone beyond term – typically beyond 41 or 42 weeks. Laws et al (2010) report on the reasons given for inducing labour. However, the proportion of inductions for post-term pregnancy was not published because of data problems.

In any case, as labour inductions predominately occur in hospital, it would only be possible to investigate correlations between Medicare-funded ultrasound scans and inductions incurring in hospitals by linking the relevant data sets. Investigating the potential for such a data linkage project is beyond the scope of the current study.

For these reasons, the current study was unable to use the available Australian data to estimate the likely reduction in the number of inductions from introducing the recommendation. Instead, the analysis relied on findings in the literature (see Section 3) to provide a broad indication of the potential reduction in inductions and caesarean sections from introducing the proposed recommendation.

Optimising maternal serum screening
Medicare data are available on the number of maternal serum screening, amniocentesis and chorionic villus sampling procedures undertaken and their associated cost. Between 2006 and 2009:
  • around 550,000 women received a Medicare benefit for maternal serum screening; and
  • of those, around 5.3 per cent received either an amniocentesis or chorionic villus sampling.
The Admitted Patient Data Collection held by the Department potentially could have provided data on amniocentesis and chorionic villus sampling received by admitted hospital patients. However, these data are not reported for privacy reasons.

3 Literature summary

A literature review of previous studies on this topic was undertaken. For the review, the Cochrane, Embase and Medline databases were searched for English-language articles published since 2006. Combinations of the following key words were used for the search — cost effectiveness, utility, benefit; economic; ultrasound; gestational age; pregnancy; antenatal; and prenatal. The search did not return any cost-benefit analyses of gestational age screening since 2006. The search was subsequently broadened to include
literature from any year, as well as clinical studies. The terms crown to rump length (CRL), last menstrual period, inductions, induce, maternal serum screening, first trimester and screening were added to the above list of key words. Google, including Google Scholar, searches were conducted and the journals British Journal of Obstetrics and Gynaecology and Obstetrics & Gynaecology were searched directly. This identified:
  • five studies on the clinical outcomes of first trimester ultrasound scans (Gardosi et al 1997; Crowther et al 1999; Maslow & Sweeny 2000; Bennett et al 2004; Whitworth et al 2010), including one comparative review (Whitworth et al 2010) and one study on the clinical outcomes of induction (Maslow & Sweeny 2000); and
  • five economic studies relating to ultrasound screening (Roberts et al 1998; Benn et al 1999; MSAC 2002; Harris 2003; Ritchie et al 2005), including one Medical Services Advisory Committee evaluation (MSAC 2002).

Clinical studies

Whitworth et al (2010) conducted a comprehensive international review of studies assessing whether routine early pregnancy ultrasound scanning influences 33 antenatal, neonatal or postnatal outcomes. ‘Early pregnancy’ was defined as ‘prior to 24 weeks’; that is, not just in the first trimester. The authors reported significant differences between women receiving routine and selective ultrasound screening in eight of the 33 outcomes, including:
  • induction of labour for any reason and specifically for “post-term” pregnancy;
  • detection of fetal abnormalities before 24 weeks gestation;
  • detection of major anomaly before birth; and
  • termination of pregnancy for fetal abnormality.
Assessing the implications of these results for the current study is complicated by the fact that women received scans at different points during the first 24 weeks of pregnancy in the studies reviewed by Whitworth et al (2010). For example, the general conclusion that early pregnancy ultrasound screening ‘may result in fewer inductions for post-maturity’ (Whitworth et al 2010) is not directly relevant to the current study as it covers scans undertaken in the first 24 weeks, not specifically the first trimester
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Whitworth et al (2010) reviewed two studies that analysed whether ultrasound screening in the first 14 weeks impacted upon the rate of induction for post-term pregnancy — Harrington et al (2006) and Ewigman et al (1990). Whitworth et al (2010) collated the results from these two studies and found no significant impact (RR: 0.99; CI: 0.67–1.46).

In the Harrington et al (2006) study, the intervention group received a first trimester scan in addition to a routine 20 week anomaly scan, while the control group received just the routine 20 week anomaly scan. The antenatal care strategy for the control group was not made clear in Ewigman et al (1990).

In an Australian study, Crowther et al (1999) found no significant difference in the rates of induction (for any reason). Similar to Harrington et al (2006), both the control and intervention groups received routine screening in the second trimester.

In a United States study, Bennett et al (2004) assigned pregnant women to either a first trimester scan group or a second trimester scan group. A significantly higher number of women in the second trimester group had their labour induced (12 women), compared with those in the first trimester group (5 women) (P=0.04; RR: 0.37; CI: 0.14–0.96).

Maslow and Sweeny (2000) conducted a historical review to analyse the risk of caesarean section associated with induction of labour. Of the 1,135 women reviewed, 263 (23.2%) elected to have their labour induced. Of these women, 11.0 per cent underwent caesarean section, compared with 4.2 per cent of those who did not have their labour induced (P < 0.001).

Benn et al (1999)1 analysed the effect of ultrasound dating scans on improving the screening power of maternal serum screening. The authors found that ultrasound dating scans considerably increase the number of positive Down syndrome cases detected through maternal serum screening while reducing the number of false positive results.

Economic studies

The literature review did not reveal any Australian economic evaluations of ultrasound dating screening. Although there were some international economic studies, none were wholly relevant to this study.

Roberts et al (1998) compared different ultrasound strategies in pregnancy in the United Kingdom. The performance of each screening strategy was based on the detection rate of fetal anomalies, the false positive rate and the population prevalence of the anomaly. The results showed that first and second trimester screening together detected a higher number of fetal anomalies and missed a lower number of anomalies than second trimester screening alone. However, the cost per case detected was considerably higher for first and second trimester screening.

Ritchie et al (2005) performed an economic evaluation for a United Kingdom Health Technology Assessment on antenatal screening for identification of fetal abnormalities. The authors assessed the cost effectiveness of six screening strategies, including two involving a scan at the first antenatal visit and four involving NT scans. However, none of these strategies were compared against a strategy that did not involve a first trimester scan. The evaluation was not therefore able to inform the current analysis.

MSAC released its assessment of nuchal translucency thickness screening in the first trimester of pregnancy in 2002. While it did not specifically relate to ultrasound dating, a number of inputs and variables were taken from the assessment and applied to this study.

4 Cost benefit analysis

Estimating the increase in ultrasound scans

Preferably, data on, or studies of, changes in the patterns of first trimester screening where policies or guidelines similar to the proposed recommendation have been introduced would be used to estimate the increase in the number of ultrasound scans from introducing the recommendation. However, the study was not able to identify any relevant studies. Medicare data between 2006 and 2009 were also examined but did not reveal any changes in trends that might have assisted the analysis.

Ultimately, in the time available, the current study was unable to identify any reliable indicator of how the proposed recommendation might affect rates of first trimester ultrasound screening. Consequently, the study estimated the maximum number of additional scans that would be likely to result from introducing the recommendation.

Broadly, women may receive Medicare-funded ultrasound scans, scans not funded by Medicare or they may not receive a scan. This can be represented as follows:
mfsT1 + nmfsT1 + nsT1 = pT1
mfsT2 + nmfsT2 + nsT2 = pT2 ≈ b

mfs: annual number of women receiving Medicare-funded ultrasound scans
nmfs: annual number of women receiving non-Medicare-funded ultrasound scans
ns: annual number of women not receiving scans
p: annual number of pregnancies
b: annual number of births
T1: first trimester
T2: 17–22 weeks
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The maximum increase in first trimester scans is nsT1; that is, the number of women who would not receive a scan without the recommendation. As discussed above, data are only available about Medicare-funded scans. Consequently, to estimate nsT1, the following assumptions were made:
  • the rate of miscarriage by the start of the 17–22 period – μ – is 15%; that is, 15% of first trimester pregnancies end in miscarriage:2
μ * pT1 = pT2 where μ = 0.85
  • pT2 is approximately equal to the number of births in the year;
  • total annual births is 292,000;3
  • all pregnant women have an ultrasound scan between 17 and 22 weeks;4 that is: nsT2 = 0
  • the number of women who have Medicare-funded first trimester scans (including nuchal translucency thickness scans) is equal to the number of women who have Medicare-funded scans both in the first trimester and between 17 and 22 weeks; that is: mfsT2, T1 = mfsT1
where mfsT2, T1 is the number of women who have Medicare-funded scans both in the first trimester and between 17 and 22 weeks
  • the number of women who have non-Medicare-funded first trimester scans as a proportion of the number of women who have non-Medicare-funded scans between 17 and 22 weeks is equal to the number of women who have Medicare-funded scans in the first trimester and between 17 and 22 weeks as a proportion of women who have Medicare-funded scans between 17 and 22 weeks; that is:
nmfsT1/ nmfsT2 ≈ mfsT2, T1/mfsT2
Broadly, the assumption is that the proportion of first trimester to 17–22 week scans is the same in the Medicare and non-Medicare sectors.

Ultimately, under these assumptions, the maximum additional number of first trimester ultrasounds is estimated by the following equation: nsT1 = b(1 – mfsT1, T2/mfsT2)/ μ
This produces an estimate of approximately 75,500 for the maximum number of additional scans annually arising from the introduction of the recommendation. This is likely to be more than the actual number of additional scans resulting from the introduction of the recommendation as not all women may elect or be able to have an additional scan in the first trimester due to personal choice or circumstances (noting that eligibility for Medicare benefits is restricted).

Estimating the cost of an increase in ultrasound scans

The total cost of 75,500 first trimester scans was estimated to be $A4,530,000. It has been assumed that the cost of a first trimester ultrasound scan is equal to the Medicare Benefit Schedule fee for an ‘ultrasound scan prior to 12 weeks gestation where a specific condition is present – referred by a medical practitioner’. This fee is $A60.

The estimate of the maximum number of additional scans may include an unknown number of 12 to 16 week scans brought forward because of the introduction of the recommendation. Consequently, the total cost ($A4.53m) of the maximum additional first trimester scans may be offset by reductions in the number and cost of 12 to 16 week scans. As the level of this offset is unknown, it is not factored into the analysis at this point. However, the sensitivity of the analysis to this issue is tested below by assuming that all women who receive scans between 12 and 16 weeks bring forward their scan to the first 12 weeks and so offset the cost of their additional scan.

Change in post-term inductions

Estimating the decrease in inductions
As discussed in Section 2, it is not possible to estimate the impact of an increase in first trimester ultrasounds on the rate of post-term inductions from available data. Consequently, this study relies on key findings in the literature (see Section 3). In particular, Whitworth et al (2010) — relying on Harrington et al (2006) and Ewigman et al (1990) — and Crowther et al (1999) found no significant difference in the rate of induction between women who had a first trimester scan and women who had both a first and second trimester scan. These studies are particularly relevant to the current study, given that most women in Australia have a scan between 17 and 22 weeks. Consequently, this cost benefit analysis is effectively assessing whether women having an additional first trimester scan would materially reduce the rate of induction. The two studies cited indicate that this is unlikely. It then follows that introducing the proposed recommendation would be unlikely to reduce caesarean sections. The sensitivity of these conclusions to underlying assumptions is tested in Section 5 below.

Impact on the effectiveness of maternal serum screening

Estimating the increase in screening power of maternal serum screening
Increasing the quality, or screening power, of maternal serum screening potentially reduces the number of women referred for amniocentesis or chorionic villus sampling and may thereby generate cost savings. The screening power of maternal serum screening can by measured by:
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  • the detection rate — that is, the proportion of cases where chromosomal abnormalities exist which are detected by maternal serum screening (higher detection rates are better); and
  • the false positive rate — that is, the number of cases which incorrectly screen positive for chromosomal abnormalities as a proportion of cases where chromosomal abnormalities are not present (lower false positive rates are better).
The literature includes the following findings relevant to assessing the impact of introducing the recommendation on the screening power of maternal serum screening:
  • Laws et al (2010) reported the mean age of Australian women who gave birth in 2008 as 29.9 years;
  • according to Benn et al (1999), the maternal serum screening detection and false positive rates for women aged 30 are 61.9% and 5.9% respectively where gestational age is determined through ultrasound (CRL); and
  • for a similar detection rate, Benn (1999) report the false positive rate of women who calculate gestational age through last menstrual period alone as 10.0%.
The current study therefore assumes that the maternal serum screening false positive rate of those women predicted to receive a first trimester scan as a result of the guideline would fall from 10.0% to 5.9%. The sensitivity of this result to the assumptions above is tested in Section 5.

Estimating the likely cost impact
The number of women potentially affected by an increase in the screening power of MSS from introducing the recommendation was estimated as follows.
  • Of those women who received a Medicare benefit for a 17–22 week scan in 2009, around 50% also received maternal serum screening.
  • Of that 50%, around 26% did not receive a Medicare benefit for a first trimester scan (or any other ultrasound) prior to receiving maternal serum screening.
  • If these proportions are assumed to hold across the population of pregnant women as a whole, this equates to around 13% of pregnant women — that is, around 38,200 women — in the second trimester who receive maternal serum screening, but no first trimester scan.
In summary, for a maximum of 38,200 women, introducing the recommendation would lead to an increase in maternal serum screening power as measured by a fall in the false positive rate from 10% to 5.9%.

The maximum cost saving associated with the improvement in screening power was estimated by modelling the likely fall in the number of women referred for diagnostic tests where no abnormality is present, and multiplying this by the per unit cost of the relevant tests.

The modelling used a series of simulations where inputs, such as State/Territory of the mother’s residence, the prevalence of chromosomal abnormalities (particularly Down syndrome), rate of spontaneous miscarriage, the false positive rates of maternal serum screening and the acceptance of screening and diagnostic tests, were assigned probabilities and randomly applied to a population of 38,200 pregnant women.

The relevant Medicare Benefits Schedule fees were used as proxies for the cost of the relevant tests.

The results of this modelling are given in Table E1.

Table E1: Results of modelling of effect of ultrasound dating scan

Effect of ultrasound dating scan
Mean
95% confidence interval
(Lower bound)
95% confidence interval
(Upper bound)
Reduction in the number of false positive results for Down syndrome 1,560 1,483 1,637
Reduction in the number of amniocentesis/CVS tests where pregnancy is Trisomy 21 negative 1,327 1,225 1,430
Total cost saving of reduction in amniocentesis/ CVS tests $A233,372 $A214,915 $A251,828

Overall cost-benefit

The analysis above indicates that:
  • the maximum additional cost arising from introducing the recommendation (from an increase in first trimester ultrasounds) is around $A4.5m; and
  • the maximum additional benefit arising from introducing the recommendation (from an improvement in the power of maternal serum screening) is around $A230,000.
Consequently, the costs of introducing the proposed recommendation appear likely to substantially outweigh the benefits.
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5 Sensitivity analysis

Due to a lack of relevant, reliable and up-to-date data, a number of assumptions were made in the process of developing this analysis. Key assumptions include that:
  • findings from Whitworth et al (2010) and Crowther et al (1999) can be used to estimate the impact of ultrasound screening on the rate of induction; and
  • pregnant women do not bring forward scans obtained between 12 and 16 weeks to the first trimester as a result of the recommendation.
These assumptions might significantly influence the final conclusion of this study. In particular, as inductions are relatively costly (see below), if the assumption that first trimester ultrasound screening has no material impact on the level of inductions is significantly inaccurate, then the outcome of the costbenefit analysis may change. To test the sensitivity of the cost-benefit analysis to these assumptions:
  • data from a United States study — Bennett et al (2004), which found that ultrasound scans do reduce inductions — were used to estimate the impact of the maximum additional number of ultrasound scans from introducing the recommendation (adjusted for miscarriages) on the rate of induction;
    • the potential for reduced inductions to reduce caesarean sections then arises. Data from Maslow and Sweeny (2000) were used to estimate the increased risk of caesarean section where pregnancies are induced; and
  • it was assumed that all pregnant women who under current practice receive a scan between 12 and 16 weeks substitute this scan for a scan in the first 12 weeks as a result of the recommendation.

Utilising the risk ratio reported by Bennett et al 2004

Bennett et al (2004) report a risk ratio of 0.37 (95% CI 0.14–0.96). Data on the risk of caesarean section associated with the induction of labour were taken from Maslow and Sweeny (2000). The current study interprets the findings of the Maslow and Sweeny study to mean that every 100 inductions averted was estimated to avert 6.8 caesarean sections.

The average costs of an induction and caesarean section were estimated from admitted patient hospital data and the National Hospital Cost Data Collection and applied to the assumed fall in the number of inductions and caesarean sections.

The average cost of performing an induction of labour was estimated at $A860.48. For every caesarean section averted, an estimated $A3,851.39 was realised in cost savings.

Results are set out in Table E2.

Table E2: Maximum cost savings if inductions are reduced by first trimester screening

Bennett et al 2004 Value 95% confidence interval
(Lower bound)
95% confidence interval
(Upper bound)
Risk ratio
0.37
0.14
0.96
Maximum number of inductions averted (64,200 first trimester scans)5
19,893
41,955
663
Cost saving (A$)
17,117,901
36,101,475
570,574
Maximum number of caesarean sections averted
1352
2,853
45
Cost saving (A$)
5,209,981
10.987,796
173659
Total maximum cost saving (A$)
22,327,882
47,089,271
744,234

Adopting the findings in Bennett et al (2004) therefore gives an entirely different outcome — that is, the benefits appear to substantially outweigh the costs of introducing the recommendation (around $A4.5 million). It is also noted that, even if the link between inductions and caesarean sections is dropped, the benefits would still substantially outweigh the costs.

This dilemma is discussed further in Section 6.
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Substitute 12 and 16 week scan with scans before 12 weeks

In this study, if all women who received ultrasound screening between 12 and 16 weeks under current practice brought forward their scan to the first 12 weeks as a result of the introduction of the recommendation, a cost saving of $A2,007,125.65 would be realised — that is, the cost of the maximum increase in first trimester ultrasounds from introducing the proposed recommendation would be reduced from around $A4.5 million to around $A2.5 million. This reduction makes little difference to the outcome of this cost-benefit analysis.

6 Conclusion

Ultimately, this study has not been able to conclusively determine whether the benefits of introducing a recommendation that pregnant women have an ultrasound scan in the first trimester would be likely to outweigh the costs.

Initially, the study was unable to estimate the actual increase in first trimester ultrasound scans from introducing the recommendation, and so proceeded by using an estimate of the maximum additional number of scans (which was used to estimate both the dollar-value of the costs and the benefits of introducing the recommendation). This estimate itself relied on a range of assumptions that were needed because of data limitations.

Most importantly, data limitations meant that, at critical points, the study had to rely on findings in the literature. However, the literature — particularly on the impact of first trimester ultrasound scans on the rate of inductions — yielded starkly different findings. Whitworth et al (2010) suggested that a first trimester ultrasound had no significant impact on inductions, while Bennett et al (2004) suggested that first trimester ultrasounds would significant reduce the risk of induction. As inductions are relatively expensive, choosing one study over the other changed the outcome of the cost-benefit analysis.

References

ABS (2010) Births, Australia, 2009. ABS Cat No 3301. Canberra: Australian Bureau of Statistics.

Benn P, Rodis J, Beazoglou T (1999) Cost-Effectiveness of Estimating Gestational Age by Ultrasonography in Down Syndrome Screening. Obstet Gynaecol 94(1): 29–33.

Bennett K, Crane J, O’Shea P et al (2004) First trimester ultrasound screening is effective in reducing postterm induction rates: A randomized controlled trial. Am J Obstet Gynecol 190(4): 1077–81.

Crowther C, Kornman L, O’Callaghan S et al (1999) Is an ultrasound assessment of gestational age at the first antenatal visit of value? A randomised clinical trial. Brit J Obstet Gynaecol 106: 1273–79.

Ewigman B, Lefevre M, Hesser J (1990) A Randomized Trial of Routine Prenatal Ultrasound. Obstet Gynaecol 76(2): 189–94.

Gardosi J, Vanner T, Francis A (1997) Gestational age and induction of labour for prolonged pregnancy. Brit J Obstet Gynaecol 104: 792–97.

Harris A (2003) The Cost Effectiveness of Prenatal Ultrasound Screening for Trisomy 21. Working Paper 146. Melbourne: Centre for Health Economics Monash University.

Harrington D, Mackenzie I, Thompson K et al (2006) Does a first trimester dating scan using crown rump length measurement reduce the rate of induction for prolonged pregnancy? An uncompleted randomised controlled trial of 463 women. Brit J Obstet Gynaecol 113: 171–76.

Kaufman K, Bailit J, Grobman W (2002) Elective induction: An analysis of economic and health consequences. Am J Obstet Gynecol 187(4): 858–63.

Laws PJ, Li Z, Sullivan EA (2010) Australia’s Mothers and Babies 2008. Perinatal statistics series no 24. Cat no PER 50. Canberra: Australian Institute of Health and Welfare.

Maslow A & Sweeny A (2000) Elective induction of labour as a risk factor for caesarean delivery among low-risk women at term. Obstet Gynaecol 95(6; Pt1): 917–22.

MSAC (2002) Nuchal Translucency Measurement in the First Trimester of Pregnancy for Screening of Trisomy 21 and Other Autosomal Trisomies. MSAC Reference 04; Assessment Report. Canberra: Medical Services Advisory Committee.

NHMRC (2001) How to Compare the Costs and Benefits: Evaluation of the Economic Evidence. Canberra: National Health and Medical Research Council,.

Ritchie K, Bradbury I, Slattery J et al (2005) Economic modelling of antenatal screening and ultrasound scanning programmes for identification of fetal abnormalities. Brit J Obstet Gynaecol 112(7): 866–74.

Ritchie K, Boynton J, Bradbury I et al (2004) Routine ultrasound scanning before 24 weeks of pregnancy. Health Technology Assessment Report 5 2004; Glasgow: national Health Service Quality Improvement Scotland.

Roberts T, Mugford M, Piercy J (1998) Choosing options for ultrasound screening in pregnancy and comparing cost effectiveness: a decision analysis approach. Brit J Obstet Gynaecol 105: 960–70.
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Whitworth M, Bricker L, Neilson JP et al (2010) Ultrasound for fetal assessment in early pregnancy. Cochrane Database of Systematic Reviews 2010, Issue 4. Art. No.: CD007058. DOI: 10.1002/14651858.CD007058.pub2.

1 In addition to a clinical study, the authors performed an economic evaluation to estimate the cost effectiveness of ultrasound dating scans.
2 Advice from Expert Advisory Committee.
3 This is total births for 2008 (Laws et al 2010)
4 Between 2006 and 2009, around 916,000 women (or around 84 per cent of births) received one or more Medicare Benefits for a 17 to 22 week routine scan.
5 This is equal to maximum additional scans from introducing the recommendation (as calculated in Section 4) less scans received by women who subsequently miscarry (that is, it is 85% of maximum additional scans).