First Trimester Prenatal Screening Protocol

The NT is routinely measured in gestational week 11 to 13 + 6, as part of the first trimester prenatal-screening. The Danish sonographers adhere to the protocol of the Fetal Medicine Foundation (3 ) for scanning the NT. The first-trimester prenatal screening for syndromes and congenital anomalies include; Double-test with blood tests for PAPP-A and beta-hCG in gestational week 8–14, NT-measurement in gestational week 11–14. A risk-score is calculated based on the values from the double-test and the nuchal translucency and the maternal age. If the risk is above 1:300 for trisomy 21 and above 1:150 for trisomy 18 and 13, further diagnostics are offered. These further diagnostics include chorionic villus sampling with chromosomal microarray/array-CGH or amniocentesis. Non-invasive prenatal testing can be offered as an alternative to the further diagnostics, but this is not implemented as a routine or stand-alone tool by the Danish Fetal Medicine Society. In second trimester, gestational week 20–22, pregnant women are also offered a free fetal ultrasound scan to detect any fetal malformations. Prenatal screening is offered to all pregnant women. The screening is free-of-charge as part of access to tax-funded public free healthcare and >90% of all pregnant women attend this.
The NT was divided into NT < 95th centile or NT ≥ 95th centile. This cut-off was chosen as the 95th centile denotes an “increased” nuchal translucency, and a NT above the 95th warrants further prenatal testing. The 99th centile (3.5 mm) was included in the NT ≥ 95th centile, as this contained too few patients to analyze. The NT centiles were calculated based on the crown-rump-length (CRL) at the first trimester scan using the method and model as by the Fetal Medicine Foundation (39 (link)).