Analysis of Fetal Short Femur: Characteristics That Influence Prenatal Diagnosis

The skeletal dysplasias pose a particular diagnostic challenge due to their rarity, phenotypic overlap, and the dynamic evolution of sonographic features across gestation.

To evaluate the relative prevalence of skeletal dysplasia (SD) and delineate antenatal ultrasound markers aiding diagnosis of SD in cases with short femur.

The records of targeted ultrasound of fetuses with suspected SD from 2021 to 2025 were reviewed. Femur length was expressed as standard deviations below mean and associated visceral and skeletal abnormalities were recorded among diagnosed and undiagnosed cases.

A total of 76 cases were fully followed; the mean gestational age was 28.2 ± 6.6 weeks. Majority of the cases were detected in the late second or early third trimester. More than two-third of them were suspected to have SD, whereas the rest were either FGR or constitutionally small fetus. The ultrasound findings guided us to distinguish FGR and constitutionally small fetuses from SD cases. It was a valuable tool in determining lethality. The molecular testing could be done in only one third of the suspected SD cases. Apart from ultrasound findings such as severely short femur, polydactyly, poor mineralization and fractures, the bending of femur, narrow thorax, micrognathia, and visceral anomalies helped in pinning the clinical diagnosis of SD in many cases. The FL/AC and TC/AC ratio individually had less sensitivity and high specificity for the prediction of lethality, but had 100% sensitivity for lethality prediction if both were abnormal. Fetal growth restriction was diagnosed in 17/76 (22.4%), familial short stature in 4/76 (5.3%). SD was suspected in 55/76 (72.3%). Among them, the genetically diagnosed, clinically suspected, and undiagnosed cases were 19/55 (34.5%), 24/55 (43.6%), and 12/55 (21.8%). Bent femur was most common anomaly (29/55, 52.7%), followed by narrow thorax (20/55, 36.4%). Renal anomaly was the commonest visceral anomaly (5/55, 9.1%). The diagnosis of SD was made in 40/76 (52.6%). Ciliopathies were the largest group (12/40, 40%), followed by osteogenesis imperfecta (11/40, 27.5%).

The study highlights the importance of structured ultrasound assessment and pattern recognition in skeletal dysplasia along with genetic testing for improved clinical decision-making and parental counseling.