Clinical Practice Guidelines Antenatal care - Module I

7.1 Gestational age

Page last updated: 02 April 2013

Ultrasound examination in the first trimester allows accurate assessment of gestational age, and identifies and allows for appropriate care of women with multiple pregnancies.

7.1.1 Assessing gestational age

Methods 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.

Summary of the evidence

Accuracy and effectiveness

The 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.
  • Accuracy of screening tests — Evidence suggests that ultrasound is a more accurate predictor of gestational age than LMP (Okonofua 1989; Rowlands & Roysten 1993; Alexander et al 1995; Crowther et al 1999; Taipale 2001; Savitz et al 2002; Olesen & Thomsen 2006). If only LMP is available the estimated date of birth should be calculated as the first day of the LMP plus 282 days (Nguyen 1999). The estimated date of birth based on LMP is subject to significant error and will be influenced by the woman’s age, number of previous pregnancies, BMI and whether she smokes (Savitz et al 2002; Morin 2005).
  • Measurements used — Crown–rump length (CRL) measurement should be used in the first trimester for estimating gestational age (Selbing 1983; Taipale 2001). CRL > 90 mm is unreliable in estimating gestational age in second trimester and head circumference (HC) measurement, which appears more reliable than the biparietal diameter (BPD) (Johnsen et al 2006), should be used instead when establishing an estimated date of birth in the second trimester.
These findings are largely supported by subsequent lower level studies as follows.
  • A small comparative study (n=30)(Martins et al 2008) suggested that fetal head and trunk volume (HT) could be more accurate than CRL for estimating gestational age, possibly due to flexion of the fetal head affecting CRL measurement.
  • An Australian prospective cohort study (n=396)(McLennan & Schluter 2008) found that CRL measurement predictions were superior to BPD measurement predictions.
  • A retrospective comparative study (n=165,908)(Dietz et al 2007) found that ultrasound-based estimates of gestational age were more accurate than LMP based-estimates of gestational age.
  • A prospective cohort study (Hoffman et al 2008) found that LMP classified more births as post term than ultrasound (4.0% vs. 0.7%), with a greater difference among young women, non-Hispanic Black and Hispanic women, women of non-optimal body weight and mothers of low birth weight babies.
  • A retrospective study (n=40,730)(Koster et al 2008) showed that the estimation of gestational age from CRL was not consistent, with reported age for a single CRL differing by up to 10 days. This highlights the need to ensure that reference curves and standards are consistently applied.
  • A prospective cross-sectional study (n=200) (Salpou et al 2008) concluded that significant ethnic differences between mothers were not reflected in fetal biometry at second trimester. The results support the recommendation that ultrasound in practical health care can be used to assess gestational age in various populations with little risk of error due to ethnic variation.
A recent 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.

Timing of assessment

The 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.

Recommendation - Grade B

2. Provide information and offer pregnant women who are unsure of their conception date an ultrasound scan between 8 weeks 0 days and 13 weeks 6 days to determine gestational age, detect multiple pregnancies and accurately time fetal anomaly screening.

Use crown–rump length (CRL) measurement to determine gestational age. If the CRL is above 84 mm, estimate the gestational age using head circumference.

Practice point

b. The timeframe for ultrasound assessment of gestational age overlaps with that for assessment of nuchal translucency thickness as part of screening for fetal chromosomal abnormalities (11 weeks to 13 weeks 6 days), which may enable some women to have both tests in a single scan. This should only occur if women have been provided with an explanation of both tests and have given their consent to them both.

Calculating the estimated date of birth

The 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):
  • if the LMP was certain and menstruation regular, compare the LMP estimate to the ultrasound estimate:
    • ultrasound performed between 6 and 13 weeks pregnancy — if the two dates differ by 5 days or less, use the LMP estimate; if the dates differ by more than 5 days, use the ultrasound estimate;
    • ultrasound performed between 13 and 24 weeks pregnancy — if the two dates differ by 10 days or less, use the LMP estimate; if the dates differ by more than 10 days, use the ultrasound estimate;
  • if the ultrasound was performed between 6 and 24 weeks pregnancy and the LMP was not certain or menstruation irregular, use the ultrasound estimate;
  • if the LMP was certain and menstruation regular and no ultrasound was performed between 6 and 24 weeks pregnancy (or none with a heartbeat), use the LMP estimate.
Practice point

c. The agreed due date should not be changed without advice from a health professional with considerable experience in antenatal care.

7.1.2 Other considerations in gestational age assessment


The 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.


An 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 Appendix E). The analysis aimed to balance the costs of additional scans undertaken against the savings resulting from:
  • optimising the timing and performance of maternal serum screening and thereby reducing the number of diagnostic tests (chorionic villus sampling and amniocentesis) undertaken; and
  • reducing rates of inductions (which in turn may reduce the rate of caesarean section).
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 2005; 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 screening 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.

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.

Practice point

d. Ultrasound assessment of gestational age should only be performed by a person who has had specific training.
e. Repeated ultrasound assessments should only be used when clinically indicated.

Additional considerations for Aboriginal and Torres Strait Islander women.

Accurately assessing gestational age is particularly important among Aboriginal and Torres Strait Islander women as:
  • many women live in rural and remote areas and move to a larger centre to give birth, requiring logistical arrangements to be made around the estimated date of birth (see below); and
  • the higher rates of preterm birth and intrauterine growth restriction.
Issues of access in rural and remote areas

In 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 particular attention to ongoing assessment of fetal growth and wellbeing.