First Trimester Ultrasound Screening Protocol (11–13+6 weeks) by ISUOG: CRL, NT, Nasal Bone, FHR
Optimal Examination Window
According to the updated ISUOG guidelines (Bilardo CM, Chaoui R, Hyett JA, et al. ISUOG Practice Guidelines (updated): performance of 11–14-week ultrasound scan. Ultrasound Obstet Gynecol. 2023;61(1):127–143), first trimester screening is conducted between 11 weeks 0 days and 13 weeks 6 days. In ISUOG patient-oriented materials, this period is also described as the window between 11 and 14 weeks.
Protocol Components
| Parameter | Purpose |
|---|---|
| CRL (crown-rump length) | Pregnancy dating; used as the primary biometric marker of gestational age (Napolitano R, et al. Pregnancy dating by fetal crown-rump length. BJOG. 2014;121(5):556–65) |
| NT (nuchal translucency) | Risk assessment for chromosomal abnormalities; historically measured between 10–14 weeks (Snijders R, et al. Lancet. 1998;352:343–346) |
| Nasal Bone | Marker within the risk assessment for aneuploidies (as part of the 11–14-week scan) |
| FHR / Cardiac Activity | Confirmation of viability; cardiac screening is regulated by ISUOG (Carvalho JS, et al. ISUOG Practice Guidelines (updated): fetal cardiac screening. Ultrasound Obstet Gynecol. 2023;61(6):788–803) |
Threshold Values and Norms
Specific numerical thresholds for NT, criteria for nasal bone visualization, and reference ranges for FHR are not provided in the given excerpts — [to be clarified] from the full text of ISUOG Practice Guidelines (updated): performance of 11–14-week ultrasound scan (Ultrasound Obstet Gynecol. 2023;61(1):127–143).
Additional Screening Tasks for the 11–13+6 Window
In this window, ISUOG also recommends uterine artery Doppler for pre-eclampsia screening between 11 weeks 0 days and 13 weeks 6 days; the second screening window is approximately at 20–24 weeks. Early identification of increased risk allows for timely preventive measures and enhanced monitoring.
Detection of Structural Anomalies
First trimester screening also allows for the detection of some fetal structural anomalies; the effectiveness of such screening depends on several factors (Karim JN, et al. Systematic review of first-trimester ultrasound screening. Ultrasound Obstet Gynecol. 2017;50:429–441).
Frequently asked questions
At what gestational age is the first trimester screening performed according to ISUOG?
In the window of 11 weeks 0 days to 13 weeks 6 days according to ISUOG Practice Guidelines (updated): performance of 11–14-week ultrasound scan (Ultrasound Obstet Gynecol. 2023;61:127–143); ISUOG patient materials indicate the interval as 11–14 weeks.
Which parameter is used for pregnancy dating?
Crown-rump length (CRL); its use for determining gestational age is confirmed by a systematic review of normative curves (Napolitano R, et al. BJOG. 2014;121:556–65).
What are the threshold values for NT according to ISUOG?
Specific NT thresholds are not provided in the given excerpts — [to be clarified] from the full text of ISUOG Practice Guidelines 2023 (Ultrasound Obstet Gynecol. 2023;61:127–143).
Can pre-eclampsia screening be performed in this window?
Yes, ISUOG recommends uterine artery Doppler between 11 weeks 0 days and 13 weeks 6 days with repeat screening around 20–24 weeks.
Which document regulates fetal cardiac screening?
ISUOG Practice Guidelines (updated): fetal cardiac screening (Carvalho JS, et al. Ultrasound Obstet Gynecol. 2023;61(6):788–803).