Late Third-Trimester FGR represents a significant challenge in modern obstetrics because it often remains undetected until delivery. Traditionally, clinicians relied on symphysis-fundal height measurements, which lack precision. However, universal ultrasound screening at 35-36 weeks offers a systematic way to identify fetuses at risk. Specifically, this approach allows for the differentiation of growth phenotypes based on standardized consensus criteria.
Identifying Risks with Late Third-Trimester FGR Phenotypes
The study at John Radcliffe Hospital evaluated five distinct phenotypes using a hierarchical approach. Researchers primarily focused on the ISUOG fetal growth restriction (FGR) definition, which utilizes Delphi criteria. These criteria include an estimated fetal weight (EFW) below the 3rd centile or an EFW below the 10th centile with abnormal Doppler markers. In contrast, constitutional small-for-gestational-age (SGA) fetuses show an EFW below the 10th centile but maintain normal blood flow patterns. Consequently, this classification helps doctors separate healthy small infants from those experiencing placental insufficiency. Furthermore, the researchers observed that Delphi-defined FGR carries the highest risk of adverse perinatal outcomes. Specifically, these pregnancies showed a stronger correlation with severe morbidity and stillbirth compared to constitutional SGA. Therefore, the distinction between these phenotypes is clinically vital for management decisions.
Universal Screening and Birth Outcome Correlation
Implementing a universal scan late in the third trimester significantly improves the detection rate of pathologically small fetuses. Moreover, this study demonstrates that most adverse outcomes occur in fetuses that meet the FGR criteria rather than just the SGA definition. Consequently, relying solely on weight centiles may lead to unnecessary interventions for healthy babies while missing those in distress. Additionally, the 35-36 week window is optimal for detecting late-onset growth issues that might be missed at earlier scans. However, clinicians must integrate these ultrasound findings with other clinical risk factors. Ultimately, this evidence supports the transition toward phenotype-based risk assessment in late pregnancy.
Frequently Asked Questions
Q1: What is the main difference between Delphi-defined FGR and constitutional SGA?
Delphi-defined FGR includes fetuses with very low weight or those with moderate low weight combined with abnormal Doppler findings. In contrast, constitutional SGA refers to fetuses that are small but have normal Doppler studies and no signs of growth failure.
Q2: Why is the 35-36 week ultrasound scan preferred over earlier third-trimester scans?
Late-onset fetal growth restriction often only becomes apparent in the final weeks of pregnancy. Therefore, a scan at 35-36 weeks has a higher sensitivity for detecting growth abnormalities that could lead to complications at term.
References
- D’Alberti E et al. Fetal Growth Restriction at a Universal Late Third-Trimester Scan and Relationship With Adverse Outcome: Retrospective Cohort Study. BJOG. 2026 Mar 08. doi: 10.1111/1471-0528.70207. PMID: 41796020.
- International Society of Ultrasound in Obstetrics and Gynecology. ISUOG Practice Guidelines: performance of third‐trimester obstetric ultrasound scan. Ultrasound Obstet Gynecol. 2024;63:131-147.
- American College of Obstetricians and Gynecologists. Fetal Growth Restriction: ACOG Practice Bulletin, Number 227. Obstet Gynecol. 2021;137(2):e16-e44.