Severe Early-Onset Fetal Growth Restriction: The Yield of Antenatal and Postnatal Genetic Testing.

We evaluated the diagnostic yield of karyotype (KT) and chromosomal microarray (CMA) with isolated severe FGR diagnosed before 32 weeks' gestation. Exome and genome sequencing (ES/GS) level data were available in a subset of this population. We performed a retrospective review of singleton pregnancies (delivered 2022-2025) with estimated fetal weight or abdominal circumference < 3rd percentile before 32 weeks' gestation, no sonographic structural anomalies, and diagnostic testing (KT, CMA, ES, or GS) via amniocentesis or cord blood. Cases with abnormal cell-free DNA were excluded. Forty cases were included (mean diagnosis 26.8 weeks). Testing was performed via amniocentesis in 42% and cord blood in 58%. All KT (27/40) were normal. CMA (39/40) identified one pathogenic CNV (2.5%) and three (7.5%) variants of uncertain significance (VUS). Four (10%) showed ≥ 1 region of absence of heterozygosity (AOH) > 10Mb; one revealed maternal uniparental disomy of chromosome 6. Sequencing (N = 10) detected one VUS in COL1A1. Acute viral infection was not observed in any of the cases. CMA was diagnostic in 2/39 (5.1%) cases, a pathogenic CNV implicating the SHOX gene, and maternal UPD 6, which were consistent with FGR. There was a remarkable rate (10%) of AOH. Further investigation, including placental studies and genome sequencing, may elucidate whether AOH is a contributing factor for FGR.