From the Guidelines
A short femur at 35 weeks of gestation can be caused by various factors, including constitutional smallness, intrauterine growth restriction (IUGR), chromosomal abnormalities like Down syndrome, or skeletal dysplasias, with the most recent evidence suggesting that a shortened femur is identified when the ratio of the observed femoral length to expected femoral length is <0.92 1. The causes of a short femur at 35 weeks of gestation can be categorized into several groups, including:
- Constitutional smallness, where the baby is genetically predisposed to be smaller
- Intrauterine growth restriction (IUGR), which can be caused by various factors such as placental insufficiency or maternal medical conditions
- Chromosomal abnormalities, such as Down syndrome, which can be associated with a shortened femur
- Skeletal dysplasias, which are a group of disorders that affect the development of the skeleton According to the most recent evidence, a shortened femur is identified when the ratio of the observed femoral length to expected femoral length is <0.92, with a positive likelihood ratio (LR) ranging from 1.5 to 2.7 1. It is essential to note that the significance of a short femur depends on how short the femur is compared to expected measurements and whether other abnormalities are present. The femur length is typically measured in percentiles, with measurements below the 5th percentile raising more concern. Management of a short femur at 35 weeks of gestation focuses on determining the underlying cause, monitoring fetal well-being, and planning appropriate delivery timing. If IUGR is suspected, more frequent monitoring of fetal heart rate, movement, and blood flow may be necessary. The prognosis varies widely depending on the cause, with constitutional smallness generally having excellent outcomes, while certain skeletal dysplasias may have more significant implications for the baby's health. In terms of evaluation and management, the American Journal of Obstetrics and Gynecology recommends that a thorough evaluation and measurement of all appendicular bones should be performed and compared with nomograms for bone length by gestational age 1. Additionally, a third-trimester ultrasound examination for reassessment and evaluation of growth is recommended for fetuses with isolated shortened humerus, femur, or both 1.
From the Research
Causes of Short Femur at 35 Weeks of Gestation
The causes of a short femur at 35 weeks of gestation can be attributed to various factors, including:
- Skeletal dysplasia, such as achondroplasia 2
- Chromosomal abnormalities, including Trisomy 21 (T21) and Turner's syndrome 3, 2
- Fetal growth restriction (FGR) 3, 4, 5
- Congenital infections, such as TORCH 3
- Normal variants during fetal development, with some fetuses having constitutionally short femurs 3
- Other genetic abnormalities, such as Ellis-van Creveld Syndrome and Pallister-Killian syndrome 2
Associated Risks
Fetuses with short femur are at an increased risk of:
- Fetal growth restriction 4, 5
- Preterm birth 4, 5
- Low birth weight 5
- Small for gestational age (SGA) 5
- Adverse perinatal outcomes, including preeclampsia and preterm birth 4
Diagnostic Considerations
Prenatal diagnosis of short femur can be made using ultrasonography, and genetic testing, such as karyotype analysis or single nucleotide polymorphism microarray, can be used to detect chromosomal abnormalities 2. Invasive testing, such as amniocentesis, may also be considered to detect genetic abnormalities 3. Serial growth assessment and follow-up ultrasound scans may be warranted in cases of isolated short femur to monitor fetal growth and well-being 3, 4.