Perinatal & Pediatric
Updated reference ranges for normal second trimester growth velocity Elizabeth Williams* Elizabeth Williams Maddy St. Ville Zhen Chen Jessica Gleason Dian He John Owen Roger Newman Edward Chien William Grobman Daniel Skupski Angela Ranzini Anthony Sciscione Jagteshwar Grewal Cuilin Zhang Fasil Tekola-Ayele Katherine Grantz
Monitoring fetal growth is essential, as deviations from normal growth are associated with adverse outcomes. However, clinical practice currently lacks a universally effective process for identifying pathologic fetal growth. Individualized growth assessment (IGA) has been proposed as an improvement, using second trimester (2T) ultrasound measurements to model individual third trimester (3T) growth potential. A criterion for using IGA is for a fetus to have a “normal” 2T growth trajectory, defined using reference ranges that were established in a small, racially homogeneous cohort (n=119, 88% White). However, these ranges have never been formally re-assessed.
To address this gap, we calculated updated 95% reference ranges for normal 2T growth velocity for head circumference (HC), abdominal circumference (AC), and femur length (FL) using the NICHD Fetal Growth Studies Standard Population (n=1,668). We compared the prior and updated ranges using t-tests and by comparing their 3T IGA classifications.
Compared to prior IGA reference ranges, we observed higher mean velocities and narrower ranges for all three parameters, with statistically significant differences for HC and FL. Applying the updated ranges to the full NICHD Fetal Growth Studies cohort (n= 2,447), 2-3% of fetuses previously identified as having normal 2T growth velocity were reclassified as abnormal. Among these 2-3%, fetuses with slower 2T growth were more likely to be flagged as overgrowing their 3T potential, and fetuses with faster 2T growth were more likely to be flagged as undergrowing their 3T potential using IGA (Table).
This apparent growth disparity between trimesters raises concerns about IGA methods pathologizing potentially healthy “catch-up” growth, calling into question the assumption that 2T growth velocity accurately predicts 3T growth potential. This finding underscores the need for a critical reappraisal of IGA to ensure its validity across diverse populations and growth patterns.
