From the Guidelines
Management of a short femur detected at 35 weeks of gestation involves close monitoring and evaluation, with a third-trimester ultrasound examination recommended for reassessment and evaluation of growth, as stated in the Society for Maternal-Fetal Medicine consult series #57 1. The obstetrician should measure the femur length and calculate the femur length to abdominal circumference ratio to determine the severity. If the femur length is below the 5th percentile but the ratio is normal, this may represent constitutional smallness rather than pathology. Weekly ultrasounds should be scheduled to monitor fetal growth and well-being, including Doppler studies of umbilical and middle cerebral arteries to assess placental function. Some key points to consider in the management of a short femur include:
- The finding of a shortened humerus or femur has been associated with trisomy 21, with higher specificity for shortened humeri, as noted in the Society for Maternal-Fetal Medicine consult series #57 1.
- For pregnant people with no previous aneuploidy screening and isolated shortened humerus, femur, or both, counseling to estimate the probability of trisomy 21 and discussion of options for noninvasive aneuploidy screening with cell-free DNA or quad screen is recommended, as stated in the Society for Maternal-Fetal Medicine consult series #57 1.
- A thorough evaluation and measurement of all appendicular bones should be performed and compared with nomograms for bone length by gestational age, as recommended in the best practice guidelines regarding prenatal evaluation and delivery of patients with skeletal dysplasia 1.
- Low-dose CT scanning can be helpful in refining the diagnosis of suspected skeletal dysplasias, but it is not considered a primary screening imaging modality due to its use of ionizing radiation, as noted in the best practice guidelines regarding prenatal evaluation and delivery of patients with skeletal dysplasia 1.
- Determining predictive factors of lethality is important in the assessment of prenatal suspected skeletal dysplasia, with key predictors including chest to abdomen ratio <0.6, femur length to abdominal circumference ratio <0.16, and severely decreased mineralization of axial skeleton, as recommended in the best practice guidelines regarding prenatal evaluation and delivery of patients with skeletal dysplasia 1. Antenatal corticosteroids (betamethasone 12 mg IM, two doses 24 hours apart) should be considered if delivery before 37 weeks is anticipated. Genetic counseling and amniocentesis may be offered to rule out chromosomal abnormalities, though at 35 weeks the risks may outweigh benefits. Delivery planning should be based on overall fetal status, with consideration for early delivery by 37-38 weeks if growth restriction is present or worsening. Short femur length can be associated with skeletal dysplasias, chromosomal abnormalities like Down syndrome, or fetal growth restriction, so comprehensive evaluation is essential to determine the underlying cause and appropriate management, as noted in the Society for Maternal-Fetal Medicine consult series #57 1 and the best practice guidelines regarding prenatal evaluation and delivery of patients with skeletal dysplasia 1.
From the Research
Management of Short Femur Detected at 35 Weeks of Gestation
- The management of a short femur detected at 35 weeks of gestation involves a step-wise imaging approach to determine the etiology and possible outcomes of the fetus 2.
- Serial follow-up scans are important for cases of isolated short femur, as a majority of them are normal and do not require further intervention 2.
- Cases of non-isolated short femur require a step-wise approach to differentiate between dysplasia or aneuploidy 2.
- Isolated short femur can be a predictor of fetal growth restriction (FGR) and small-for-gestational-age (SGA) newborns 3, 4, 5.
- The risk of adverse pregnancy outcomes, such as low birth weight, preterm birth, and preeclampsia, is increased in cases of isolated short femur 5, 6.
- Uterine artery Doppler evaluation can be a useful clinical tool in identifying pregnancies at high risk for complications, such as severe intrauterine growth restriction and pre-eclampsia 6.
Key Considerations
- Isolated short femur is unlikely to be due to aneuploidy in a population previously screened by first-trimester fetal nuchal translucency measurement 6.
- Detailed ultrasound examination is necessary to identify associated fetal abnormalities and determine the appropriate management plan 6.
- Close monitoring of fetal growth and well-being is essential in cases of short femur, regardless of whether it is isolated or non-isolated 2, 3, 4, 5, 6.