Presentation: Fetus with an isolated short femur

A primigravid (first-time pregnant) woman is referred to fetal medicine because she has just had her fetal anomaly ultrasound at 20 weeks’ gestation and, although the head circumference and the abdominal circumference plot onto the 50th centile, the femur length is on the first centile.

• When the femur length in the second or third trimester is less than the first centile for gestational age.
• When the femur length is disproportionately small compared with the head circumference and abdominal circumference.
• When there is no evidence of placental insufficiency.
• When there is a higher-chance screening result for trisomies, or when the parents have previously declined further information but now wish to receive it.
• When there are other anomalies noted on ultrasound other than short femur length.
• When there are other bony anomalies seen on a scan (unusual head shape, small chest size, signs of bowing or fractures in the long bones – see our In the Clinic scenario: Pregnancy with suspicion of a skeletal dysplasia).

• Look carefully at both parents for height and disproportion.
• Ask about family history of short stature.
• Refer to local guidance regarding fetal medicine referral. Further review in a fetal medicine unit is usually warranted.
• A fetal medicine review will determine whether genomic testing is appropriate and referral to clinical genetics will be considered.
• The relevant team will refer to the NHS England National Genomic Test Directory eligibility criteria to determine which tests are available to your patient. The directory itself provides a list of all available tests.
  • For those working within NHS Wales, please consult the All Wales Medical Genomics Service website for information on how to arrange testing.
• Depending on the clinical scenario, a range of different genomic tests may be considered:
  • R22 Fetus with a likely chromosomal abnormality. This will process both:
    • R22.1 Genome-wide common aneuploidy testing; and
    • R22.2 Chromosomal microarray.
  • Where there are multiple or complex anomalies and/or above testing is non-diagnostic, fetal exome sequencing may be considered:
    • R21 Fetal anomalies with a likely genetic cause: fetal exome sequencing.
      • Referral to clinical genetics and/or multi-disciplinary discussion is required.
      • A record of discussion (RoD) form is required (one for each parent).

• For all DNA-based tests (all the above tests), an EDTA sample is required. For many of the tests (particularly whole genome and exome sequencing), parental samples are also needed or are helpful. Please refer to your local Genomic Laboratory Hub for details of test request forms and where to send samples.
• Information about patient eligibility and test indications was correct at the time of writing. When requesting a test, please refer to the National Genomic Test Directory to confirm the right test for your patient.

For clinicians

• NHS England: National Genomic Test Directory

References:

• Benacerraf BR, Gelman R and Frigoletto Jr FD. ‘Sonographic identification of second-trimester fetuses with Down’s syndrome.’ The New England Journal of Medicine 1987: volume 317, issue 22, pages 1,371–1,376. DOI: 10.1056/NEJM198711263172203
• Nyberg DA, Resta RG, Hickok DE and others. ‘Femur length shortening in the detection of Down syndrome: Is prenatal screening feasible?‘ The American Journal of Obstetrics and Gynecology 1990: volume 162, issue 5, pages 1,247–1,252. DOI: 1016/0002-9378(90)90028-6
• Nyberg DA, Souter VL, El-Bastawissi A and others. ‘Isolated sonographic markers for detection of fetal Down syndrome in the second trimester of pregnancy.’ Journal of Ultrasound in Medicine 2001: volume 20, issue 10, pages 1,053–1,063. DOI: 10.7863/jum.2001.20.10.1053