Abstract: Thirteen trials recruiting 34,980 women were included in the systematic review. Risk of bias was low for allocation concealment and selective reporting, unclear for random sequence generation and incomplete outcome data and high for blinding of both outcome assessment and participants and personnel. There was no difference in antenatal, obstetric and neonatal outcome or morbidity in screened versus control groups. Routine late pregnancy ultrasound was not associated with improvements in overall perinatal mortality. There is little information on long-term substantive outcomes such as neurodevelopment. There is a lack of data on maternal psychological effects. Overall, the evidence for the primary outcomes of perinatal mortality, preterm birth less than 37 weeks, induction of labour and caesarean section were assessed to be of moderate or high quality with GRADE software. There was no association between ultrasound in late pregnancy and perinatal mortality (risk ratio (RR) 1.01, 95% confidence interval (CI) 0.67 to 1.54; participants = 30,675; studies = eight; I² = 29%), preterm birth less than 37 weeks (RR 0.96, 95% CI 0.85 to 1.08; participants = 17,151; studies = two; I² = 0%), induction of labour (RR 0.93, 95% CI 0.81 to 1.07; participants = 22,663; studies = six; I² = 78%), or caesarean section (RR 1.03, 95% CI 0.92 to 1.15; participants = 27,461; studies = six; I² = 54%). Three additional primary outcomes chosen for the 'Summary of findings' table were preterm birth less than 34 weeks, maternal psychological effects and neurodevelopment at age two. Because none of the included studies reported these outcomes, they were not assessed for quality with GRADE software. Based on existing evidence, routine late pregnancy ultrasound in low-risk or unselected populations does not confer benefit on mother or baby. There was no difference in the primary outcomes of perinatal mortality, preterm birth less than 37 weeks, caesarean section rates, and induction of labour rates if ultrasound in late pregnancy was performed routinely versus not performed routinely. Meanwhile, data were lacking for the other primary outcomes: preterm birth less than 34 weeks, maternal psychological effects, and neurodevelopment at age two, reflecting a paucity of research covering these outcomes. These outcomes may warrant future research.

Plain-language summary: The ultrasound scan protocols in each trial varied, as did the reasons for ultrasound scans after 24 weeks' gestation. The influence of first and second trimester ultrasounds is difficult to disentangle, and assessment of most measures at late pregnancy is based on gestational reference data, which rely on accurate gestational dating in early pregnancy. Trials were undertaken over a period of time covering early introduction into clinical practice to widespread use, during which time how to assess fetal size and well being ultrasonographically were still being debated. As ultrasound technology continues to advance and become more accessible, it is important to maintain a clear idea of its relevance. Ultrasound, being a clinical investigation, may be used to detect abnormality without the impact of such detection on clinical outcomes being full assessed. Exposure of the expectant mother to uncertainty and possible anxiety about the health of her baby has implications that may be far reaching. In addition, little is known about how the baby that was compromised in the uterus develops after birth and in the first years of life.