Slowing fetal growth velocity from the mid-trimester may signal increased risks of perinatal morbidity and mortality: a retrospective cohort study.

BACKGROUND

Undetected fetal growth restriction is a major risk factor for stillbirth. Detecting small babies is a cornerstone of obstetric care, but we fail to detect most uteroplacental insufficiency impairing fetal growth, and most small fetuses. Slowing fetal growth is thought to flag fetal growth restriction, but uncertainty about what constitutes poor growth has hindered clinical translation. We aim to validate slowing fetal growth velocity as a measurable risk factor for adverse pregnancy outcomes, and to better define growth velocity assessment to aid clinical interpretation.

METHODS

We performed a retrospective cohort study of ultrasound and birth outcome data. All patients with singleton pregnancies and at least two ultrasound fetal size assessments between 18 and 39 weeks, from January 2009 to May 2022, were included. Universal third trimester ultrasound is not performed at our institution; hence, all pregnancies were referred for at least one scan. Primary outcomes were perinatal mortality (stillbirth or neonatal death) and a composite of adverse perinatal outcomes. Fetal growth velocity was calculated between first and last scans, standardized as exact estimated fetal weight (EFW) z-score change per week.

RESULTS

Among 24,395 pregnancies, most first scans were routine mid-trimester ultrasounds (median 20 weeks), with a median 12 weeks between first and last scans. Each z-score/week reduction in EFW growth rate increased perinatal mortality 23-fold (odds ratio (OR) (95% confidence interval (CI)) = 23.25 (7.03-66.45), p < 10), and adverse perinatal outcome 17-fold (OR (95% CI) = 17.54 (12.93-23.84), p < 10). Slowing fetal growth as EFW z-score change/week was associated with adverse perinatal outcome even among those with fetal size considered normal (Hadlock EFW ≥ 10th centile) at last scan, and when confined to term births (OR (95% CI) = 2.35 (1.66-3.33), p < 10; OR (95% CI) = 3.17 (2.10-4.76), p < 10, respectively). A growth rate cut-off of - 0.13 EFW z-scores/week was identified as optimal for perinatal mortality by Youden Index. Growth slower than this was associated with sixfold increased odds of perinatal death (OR (95% CI) = 6.40 (3.91-10.30), p < 10).

CONCLUSIONS

Slowing fetal growth velocity identifies pregnancies at increased risk of poor outcomes. A slowing growth rate < - 0.13 z-scores/week may represent a pragmatic clinical threshold. Fetal growth rate between scans could be incorporated into ultrasound reporting to better identify fetuses at risk.