| Summary: | <strong>Background</strong>
Routine third-trimester ultrasound is frequently offered to pregnant women to identify fetuses with abnormal growth. Infrequently, a congenital anomaly is incidentally detected.
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<strong>Objective</strong>
To establish the prevalence and type of fetal anomalies detected during routine third-trimester scans using a systematic review and meta-analysis.
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<strong>Search strategy</strong>
Electronic databases (MEDLINE, Embase and the Cochrane library) from inception until August 2019.
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<strong>Selection criteria</strong>
Population-based studies (randomised control trials, prospective and retrospective cohorts) reporting abnormalities detected at the routine third-trimester ultrasound performed in unselected populations with prior screening. Case reports, case series, case-control studies and reviews without original data were excluded.
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<strong>Data collection and analysis</strong>
Prevalence and type of anomalies detected in the third trimester. We calculated pooled prevalence as the number of anomalies per 1000 scans with 95% confidence intervals. Publication bias was assessed.
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<strong>Main results</strong>
The literature search identified 9594 citations: 13 studies were eligible representing 141 717 women; 643 were diagnosed with an unexpected abnormality. The pooled prevalence of a new abnormality diagnosed was 3.68 per 1000 women scanned (95% CI 2.72–4.78). The largest groups of abnormalities were urogenital (55%), central nervous system abnormalities (18%) and cardiac abnormalities (14%).
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<strong>Conclusion</strong>
Combining data from 13 studies and over 140 000 women, we show that during routine third-trimester ultrasound, an incidental fetal anomaly will be found in about 1 in 300 scanned women. This information should be taken into account when taking consent from women for third-trimester ultrasound and when designing and assessing cost of third-trimester ultrasound screening programmes.
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<strong>Tweetable abstract</strong>
One in 300 women attending a third-trimester scan will have a finding of a fetal abnormality.
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