1st Trimester Screening for Trisomy 21 (T21), Trisomy 18 (T18), Trisomy 13 (T13) in Singleton pregnancies: Nuchal Translucency (NT) Scan (499)

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Appointments for a dating scan including 1st trimester screening for Down’s, Edwards’ and Patau’s Syndrome (NT measurements) should be no less than 25 minutes. This should include time to get “on and off the couch”, time to perform the ultrasound examination and time to complete the report.

All Sonographers performing NT measurements must be appropriately trained and accredited and their results subjected to rigorous audit and performance management. To assure continuing satisfactory performance each Sonographer must perform a minimum of 25 nuchal translucency measurements every 6 months and have DQASS ‘Green’ or ‘Amber’ flag status.

The ultrasound equipment used must meet NSC FASP specifications. It should have a cineloop function and calliper precision to one decimal point i.e. 0.1mm. Operators should be aware of and adhere to BMUS guidelines for safe use of ultrasound including exposure times.

The screening period is between 11+2 to 14+1 weeks gestation. The scan will be targeted at a gestation of approx 12 weeks. The scan can be performed by the transabdominal or transvaginal route.

Consent

Sonographers must ensure formal consent has been obtained. Check the details of the booking on the ‘antenatal assessment’ tab on Badger net under ‘screening and scans offered/accepted or declined'.

Women can chose to have screening for T21 syndrome only or to have screening for T21, T18 & T13. It is not possible to have screening for T18 and/or T13 without screening for T21.

Prior to beginning scan, give a brief explanation of the scan, including limitations and obtain verbal consent to continue.

If consent for 1st trimester screening is not obtained a “dating” scan only should be performed. The Sonographer should also advise that if an increased NT ≥ 3.5mm is detected, this can be indicative of a structural abnormality and would require referral to a Medical Sonographer.

The Ultrasound Examination

Where both dating and 1st trimester screening are requested and the CRL is between 45.0 and 84.0mm, the pregnancy should be dated using the CRL measurement.

Criteria for measurement of the fetal crown rump length (CRL) as part of the combined 1st trimester screening programme

The CRL range should be between 45.0 and 84.0 mm.

The magnification of the fetus should be as large as possible clearly demonstrating the entire crown-rump length.

A midline sagittal section of the whole fetus should be obtained with the fetus horizontal on the screen, either supine or prone. The fetus should be in a neutral position with fluid visible between the fetal chin and chest, neither hyper extended nor flexed.

The best of three measurements should be taken. Linear callipers should be used to measure the maximum un-flexed length. Intersection of the callipers (+) should be placed on the outer margin of the skin borders of the CRL. Two images of the measured CRL must be retained, one for the patient record and one for audit purposes.

If the CRL is < 45.0mm re-appoint the patient within the 11+2 – 14+1 weeks screening window.

If the CRL is > 84.0mm arrange appointment for 2nd trimester biochemistry screening and date pregnancy using Head Circumference (HC).

The NT Measurement

The NT measurement should only be performed if a CRL measurement, which meets the recommended NHS FASP criteria for CRL has been obtained.

Criteria for measurement of the fetal nuchal translucency (NT) measurement

A midline sagittal section of the fetus should be obtained. The fetus should be horizontal on the screen, either supine or prone.

Care must be taken to distinguish between fetal skin and amnion. The fetus should be in a neutral position.

The image should be magnified, such that only the fetal head and upper thorax occupy the whole screen. In magnifying the image (pre- or post-zoom) it is important to turn down the gain.

The widest part of the translucency must always be measured. Measurements should be taken with the horizontal lines of the callipers placed ON the lines that define the NT thickness.

During the scan more than one measurement must be taken and the maximum one which meets all the criteria should be recorded. Two images of this measured NT should be retained, one for the patient record, one for audit purposes.

If the NT measurement is ≥ 3.5mm, perform combined screening test and follow pathway for raised nuchal translucency (NT) ≥ 3.5mm

Too early/late/unable to obtain measurements

Too Early: CRL measurement <45.0mm – re-appoint at appropriate gestation.

Too late: CRL measurement >84.0mm – arrange appointment for 2nd trimester biochemistry (≥ 15+0 weeks) screening and date pregnancy using Head Circumference (HC).

Unable to obtain measurements – offer a 2nd attempt. This second attempt at screening should be on the same day.

If unable to obtain measurements after two attempts, explain limitations of scan and record on report. Arrange 2nd trimester dating scan to coincide with biochemistry appointment.

Ultrasound Images

One set of paired CRL and NT images to be inserted into brown image envelope in patient notes, one set to be kept aside for audit purposes.

Ultrasound Report

The Ultrasound report should be documented on Badgernet under ‘Key Notes – New Ultrasound Note’. Ensure all appropriate fields are filled including the authorization box (your digital signature) for the report to be valid.

If unable to obtain NT measurements indicate reason for failed attempt i.e. poor views due to fetal position. If patient to be re-appointed for 2nd trimester screening enter suggested date.  

Medical Genetics Form (Appendix A)

Attach a patient label, with name, address, DOB and CHI number to the Medical Genetics First Trimester Combined Ultrasound and Biochemical (CUB) Screening form.

Enter the following data:

  • Hospital
  • Consultant
  • Maternal Weight
  • Number of Fetuses
  • Chorionicity if multiple pregnancy
  • Maternal Family Origin

Indicate YES or NO for the following categories:

  • Screening required – Down’s Syndrome T21
  • Screening required – Edwards’ Syndrome T18 and Patau’s Syndrome T13
  • Current Smoker
  • Previous Trisomy Pregnancy
  • IDDM

Complete the Ultrasound Details section;

  • Date of scan
  • Estimated date of delivery
  • CRL (mm)
  • NT (mm)
  • Ultrasonographer code

If assisted conception pregnancy, record all relevant details in the Assisted Conception section

“Date of sample” and “Sample taken by” fields to be entered by the Midwife/HCSW who performs the venipuncture.

Medical Genetics form to be passed to the Midwife/HCSW for completion and sent together with the biochemistry sample to Medical Genetics Labs.

Appendix A - Medical Genetics Form

Medical Genetics, First Trimester Combined Ultrasound and Biochemical (CUB) Screening Form.

 

Editorial Information

Last reviewed: 03/05/2024

Next review date: 04/05/2028

Author(s): Donna Bean.

Version: 3

Co-Author(s): Alan Mathers.

Approved By: Maternity Governance Group

Document Id: 499