Background Routine third‐trimester ultrasound is frequently offered to pregnant women to identify fetuses with abnormal growth. Infrequently, a congenital anomaly is incidentally detected. Objective To establish the prevalence and type of fetal anomalies detected during routine third‐trimester scans using a systematic review and meta‐analysis. Search strategy Electronic databases (MEDLINE, Embase and the Cochrane library) from inception until August 2019. Selection criteria 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. Data collection and analysis 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. Main results 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%). Conclusion 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. Tweetable One in 300 women attending a third‐trimester scan will have a finding of a fetal abnormality. Tweetable One in 300 women attending a third‐trimester scan will have a finding of a fetal abnormality.