Usefulness and timing of the third-trimester ultrasound scan: a review of guidelines and underlying evidence.

OBJECTIVES

Recommendations regarding the use of third-trimester ultrasound lack universal consensus. Yet, there is evidence which supports its value in assessing fetal growth, fetal well-being, and a number of pregnancy-related complications. This literature review evaluates the available scientific evidence regarding its applications, usefulness, and the timing of the third-trimester scan in a low-risk population.

METHODS

A literature search (PubMed, Embase, Scopus; until April 1st, 2025) identified English-language studies dealing with third-trimester ultrasound screening, its aims, and its best timing. International and national guidelines were reviewed to assess the current recommendations. Large, high-quality studies were then used to evaluate the completeness of the current recommendations and to analyze additional parameters that could be considered when assessing the usefulness of a third-trimester ultrasound screening.

RESULTS

Six international and national sets of guidelines regarding the third-trimester ultrasound were analyzed, showing wide variability in recommendations. While all six support its use, mainly for fetal growth, amniotic fluid, and placental assessment, only ISUOG and SIEOG specify the gestational age at which it should be performed (32-36 weeks). Fetal biometry and anatomy are universally suggested, while recommendations for Doppler studies, biophysical profile, and cervical length are inconsistent. Newer applications, such as pre-eclampsia screening and prediction of labor onset and intrapartum complications, are not yet included. Recent major studies in low-risk populations suggest 36-week gestation as the optimal screening time.

CONCLUSIONS

There is a broad recognition of third-trimester ultrasound being useful for fetal monitoring, but consensus is lacking regarding its routine use in low-risk populations, its outcomes and timing. Major evidence supports ultrasound screening at 36-week gestation in low-risk pregnancies to predict fetal growth restriction, macrosomia, structural anomalies, complications due to placental abnormalities, pre-eclampsia and onset of labor. When preceded by effective risk stratification done earlier in pregnancy, this strategy may enhance pregnancy management potentially improving maternal-fetal outcomes.