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Offering all women third trimester ultrasounds did not reduce the rate of serious illness or death in babies in the first week of life.

Monitoring fetal growth is part of routine antenatal care, using regular tape measurements from the pubic bone to the top of the uterus. To date, it has been unclear whether also monitoring babies’ growth using ultrasound late in pregnancy can reduce the risks for babies.

The large Dutch IRIS trial investigated whether a policy of offering third trimester ultrasound reduced the risk of serious illness or perinatal death for the babies of women with uncomplicated pregnancies. The comparison was against the previous policy of measuring the uterus by hand and doing ultrasound when needed. Routine scanning did not improve the 1.8% rate of severe problems seen with usual care.

The finding supports existing guidance that not all women with low-risk pregnancies need ultrasound scanning after 24 weeks.

Why was this study needed?

Fetal growth restriction, where a baby fails to reach its growth potential, is currently detected in around 3 in 100 UK pregnancies, though most pregnant women are at lower risk. It is associated with an increased risk of stillbirth and death during the first week of life, as well as important morbidity. It can be detected by checking for fetal size, although some small babies will just be naturally small, rather than growth-restricted.

The standard approach is to take regular measurements of fundal height – the distance from the pubic bone to the top of the uterus – referring women for ultrasounds if there are concerns. Another approach is offering routine ultrasounds to all women in their third trimester as an additional means of detecting it, but it is uncertain whether doing so reduces the incidence of illness or death.

This study aimed to assess the impact of this policy on adverse birth outcomes and obstetric interventions in women with low-risk pregnancies (who might therefore not normally be scanned).

What did this study do?

The IRIS study was a stepped wedge cluster randomised trial conducted in 60 primary care midwifery practices in the Netherlands. It included 13,046 women with low-risk singleton pregnancies. The study compared the effect of usual care with provision of additional ultrasounds at 28 to 30 and 34 to 36 weeks’ gestation, using the tenth centile of fetal abdominal circumference, or a drop of 20 centiles, as the cut-off measure.

The composite endpoint was perinatal death and other serious adverse outcomes (Apgar score less than four, impaired consciousness, asphyxia, seizures, assisted ventilation, septicaemia, meningitis, bronchopulmonary dysplasia, intraventricular haemorrhage, periventricular leukomalacia, or necrotising enterocolitis).

The structure of antenatal care and the diagnosis and management of suspected fetal growth restriction is similar between the Netherlands and UK, so the results are relevant to the UK population. Over half of women have scans anyway so this was a pragmatic trial of a policy rather than a trial of scanning.

What did it find?

  • The intervention did not significantly reduce the incidence of severe adverse perinatal outcomes, occurring in 1.7% of the ultrasound group versus 1.8% of the usual care group (adjusted odds ratio [aOR] 0.88, 95% confidence interval [CI] 0.70 to 1.20).
  • In low-risk pregnancies, routine ultrasonography in the third trimester detected more babies who were small for gestational age compared with usual care alone: 32% (179/556) versus 19% (78/407), though still fewer than a third of those found to be small for gestational age.
  • Routine ultrasonography was associated with a moderately increased risk of labour induction (aOR 1.16, (95% CI 1.04 to 1.30) and a lower incidence of augmentation of labour (aOR 0.78, 95% CI 0.71 to 0.85). However, maternal morbidity and mortality and other obstetric interventions did not differ between the two strategies.

What does current guidance say on this issue?

The NICE guideline on routine antenatal care (CG62) updated in 2019 states that routine ultrasound scanning after 24 weeks should not be offered to women with low-risk pregnancies and is not supported by the evidence. However, symphysis to fundal height (pubic bone to the top of the uterus) should be measured and recorded at each antenatal appointment after 24 weeks.

The Royal College of Obstetrics and Gynaecology in a 2014 guideline recommends serial measurement and plotting of fundal height for the fetus’ size using customised charts which take maternal height, weight, parity and ethnic origin into consideration. There are directions for detecting and managing at-risk populations.

What are the implications?

These study findings do not support the use of routine third trimester ultrasonography for all low-risk pregnancies. Many women have scans anyway under the existing policy. Providing scans to all women appears to offer no benefit.

The currently used measurements appear not useful enough at predicting serious outcomes to justify scanning all women in late pregnancy. Future research to develop better ways of predicting problems in the baby could be worthwhile.

Citation and Funding

Henrichs J, Verfaille V, Jellema P et al. Effectiveness of routine third trimester ultrasonography to reduce adverse perinatal outcomes in low risk pregnancy (the IRIS study): nationwide, pragmatic, multicentre, stepped wedge cluster randomised trial. BMJ. 2019;367:l5517.

This study was supported by a grant from the Netherlands Organisation for Health Research and Development.



NICE. Antenatal care for uncomplicated pregnancies. CG62. London: National Institute for Health and Care Excellence; 2008 (updated 2019).

RCOG. Small-for-gestational-age fetus, investigation and management (green-top guideline no. 31). London: Royal College of Obstetricians and Gynaecologists; 2013 (minor revisions 2014).

Produced by the University of Southampton and Bazian on behalf of NIHR through the NIHR Dissemination Centre


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Expert commentary

Both restricted and excessive fetal growth are associated with an increased risk of perinatal death. Ultrasonic estimation of fetal weight was first described more than 40 years ago, but we still do not know whether its routine use to detect fetal growth disorders results in improved outcomes.

This stepped wedge cluster randomised controlled trial failed to show that universal third trimester ultrasound prevented adverse outcomes for the baby. However, the main outcome of interest is stillbirth, and much larger sample sizes are required to achieve statistical power for this outcome. Hence, the key question of interest remains unanswered.

However, usefully, the study demonstrated that universal ultrasound in low-risk women did not appear to cause harm to the mother or baby.

Professor Gordon Smith, Department of Obstetrics and Gynaecology, University of Cambridge

The commentator declares research support for studies of screening in pregnancy from GE and Roche Diagnostics. He also declares being one of three named co-inventors of a biomarker test for poor fetal growth.


Expert commentary

From the Dutch IRIS study, it seems that routine third trimester ultrasound in so-called ‘low-risk’ pregnant women should not be used or it can cause interventions such as induction of labour. This study should not, however, mean the end of routine third trimester ultrasound. More small babies were identified but it was not reported what happened to them in each hospital, where unnecessary induction of labour could have happened due to local policies.

The NIHR is funding more research to help clarify the impact of routine third trimester scanning in the UK.

Dr Raffaele Napolitano, Consultant in Obstetrics and Fetal Medicine, Subspecialist in Maternal and Fetal Medicine, Fetal Medicine Unit, Elizabeth Garrett Anderson Wing, University College London Hospitals NHS Foundation Trust; Honorary Senior Lecturer, University College London

The commentator declares no conflicting interests


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