Routine third trimester ultrasound: what is the evidence?

Policies for routine third trimester obstetrical ultrasound examinations differ among countries. In Canada, a routine third trimester ultrasound scan is not offered in the low-risk pregnancy population. This practice is based mainly on results of a meta-analysis published in 2001 that concluded "routine late pregnancy ultrasound in low-risk or unselected populations does not confer benefit on mother or baby." We reviewed in detail each study included in this meta-analysis in order to re-evaluate the Canadian practice regarding routine third trimester ultrasound in the low-risk pregnant population. The meta-analysis included outdated techniques and ultrasound examinations performed in the late 1970s and early 1980s. To assess the effect of routine third trimester ultrasound on perinatal outcome, the interventions prompted by an abnormal diagnostic test result must be considered. None of the trials included in the meta-analysis evaluated the effect of routine third trimester ultrasound on perinatal outcomes in a low-risk population when ultrasound assessment was followed by an altered perinatal management plan. Our assessment of the published evidence regarding routine third trimester ultrasound puts in question the contemporary validity of the conclusion of the 2001 meta-analysis. In fact, the 2001 meta-analysis has recently been withdrawn by the authors.