A Cochrane review included 11 studies with a total of 37505 subjects. Routine (revealed) vs selective (concealed) ultrasound for fetal assessment in early pregnancy reduces the failure to detect multiple pregnancy by 24 weeks' gestation (RR 0.07, 95% CI 0.03 to 0.17; 7 trials, n=295). Routine scan is associated with a reduction in inductions of labour for post-term pregnancy (RR 0.59, 95% CI 0.42 to 0.83; 8 trials, n= 25516). Routine scans do not seem to be associated with reductions in adverse outcomes for babies or in health service use by mothers and babies. Long-term follow up of children exposed to scan in utero does not indicate that scans have a detrimental effect on children's physical or cognitive development.
Comment: The quality of evidence is downgraded by study quality (lack of blinding and some other shortcomings) and by inconsistency (heterogeneity in both the timing and the number of scans women received) and upgraded by large magnitude of effect.