This is a plain English summary of an original research article. The views expressed are those of the author(s) and reviewer(s) at the time of publication.
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).
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