NT Scan and Dating Scan - Detailed Explanation

Dating Scan

What is it?

The dating scan is a first-trimester ultrasound performed primarily to accurately establish gestational age and the expected due date (EDD). It is often the first formal scan a pregnant woman receives.

When is it done?

Typically between 8 and 13+6 weeks of gestation
The optimum timing is in the first trimester when the crown-rump length (CRL) measurement is most accurate

Purpose

Confirm gestational age - The LMP (last menstrual period) can be unreliable. Ultrasound provides a better estimate of gestational age before 20 weeks than LMP alone.
Establish the estimated due date (EDD)
Confirm fetal viability - detect fetal heart activity
Detect multiple pregnancies (twins, triplets) and determine chorionicity
Identify the location of pregnancy - rule out ectopic pregnancy
Assess the uterus and ovaries

Key Measurement: Crown-Rump Length (CRL)

The CRL is the most accurate biometric parameter for establishing gestational age in the first trimester
Accurate to within ±5-7 days up to 13 weeks 6 days
The earlier the scan is done in the first trimester, the more accurate the dating
If CRL dating differs from LMP dating by >7 days (at 14-15 weeks) or >10 days (at 16-21 weeks), the EDD is changed to match the ultrasound date

Second-trimester dating (if missed)

If no first-trimester scan was done, dating in the second trimester uses a combination of:

BPD - Biparietal diameter
HC - Head circumference
AC - Abdominal circumference
FL - Femur length

These are less accurate than CRL - reliable to approximately ±10-14 days in the second trimester.

Source: Creasy & Resnik's Maternal-Fetal Medicine

NT Scan (Nuchal Translucency Scan)

What is it?

The NT scan is an ultrasound measurement of the subcutaneous fluid-filled space (nuchal translucency) at the back of the fetal neck, between the skin and the cervical spine. An increased NT is associated with chromosomal abnormalities and structural defects.

When is it done?

Between 11 weeks and 13 weeks 6 days of gestation (ideally)
The fetal CRL must be between 45 mm and 84 mm

What does NT measure?

NT measures the clear space at the back of the baby's neck. In normal fetuses this is small; in fetuses with chromosomal or structural problems, fluid accumulates and this space is enlarged.

Normal NT: depends on gestational age, but less than 3 mm is generally considered normal (or below the 95th percentile)
NT is typically reported as a Multiples of the Median (MoM) value
An NT of 2.0 MoM is the median value seen in fetuses with Down syndrome

NT as a standalone screening test

NT alone detects 60% or more of Down syndrome cases at a 5% false-positive rate - making it the best single screening test described to date for aneuploidy
However, neither NT alone nor serum tests alone have sufficient predictive power

The Combined First-Trimester Test (NT + Blood Tests)

The NT scan is almost always interpreted together with maternal blood tests to form the "combined test":

Marker	In Down Syndrome (Trisomy 21)
PAPP-A (Pregnancy-Associated Plasma Protein-A)	LOW (median 0.4-0.5 MoM)
Free beta-hCG	HIGH (median 1.7-2.2 MoM)
NT (ultrasound)	INCREASED (2.0 MoM)

Combined with maternal age and gestational age (CRL), this combined test:

Detects approximately 85% of Down syndrome cases at a 5% false-positive rate
Can also screen for Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome)

Source: Tietz Textbook of Laboratory Medicine, 7th Edition; Emery's Elements of Medical Genetics and Genomics

What can an increased NT indicate?

An elevated NT (especially ≥3.5 mm or above the 95th percentile) can be associated with:

Chromosomal abnormalities:
- Trisomy 21 (Down syndrome)
- Trisomy 18 (Edwards syndrome)
- Trisomy 13 (Patau syndrome)
- Monosomy X (Turner syndrome) - often presents as cystic hygroma
Structural abnormalities in chromosomally normal fetuses:
- Congenital heart defects (prevalence increases dramatically with NT thickness - from 0.8/1000 at normal NT to 195/1000 at NT ≥5.5 mm)
- Diaphragmatic hernias
- Skeletal abnormalities
- Neurologic abnormalities
Subchromosomal copy number variants and single-gene disorders

Risk Thresholds and Follow-up

A combined test risk of >1 in 150 is considered screen-positive and warrants further testing
Further testing options:
- NIPT (Non-Invasive Prenatal Testing) - tests cell-free fetal DNA in maternal blood; not diagnostic but high sensitivity
- CVS (Chorionic Villus Sampling) - invasive, diagnostic, done at 11-13 weeks
- Amniocentesis - invasive, diagnostic, done at 15-20 weeks

Key Differences: Dating Scan vs NT Scan

Feature	Dating Scan	NT Scan
Timing	8-13+6 weeks	11-13+6 weeks
Primary purpose	Confirm dates & viability	Screen for chromosomal/structural abnormalities
Key measurement	Crown-Rump Length (CRL)	Nuchal Translucency thickness
Blood tests needed?	No	Yes (combined with PAPP-A & beta-hCG)
Routine?	Yes, universal	Yes, offered to all; part of combined screening
Result type	Gestational age ± 5-7 days	Risk probability (e.g., 1 in 500 for Down syndrome)

In practice, the NT scan is often performed at the same appointment as the late dating scan (around 11-13 weeks), so many women have a combined appointment where both the CRL (for dating) and NT (for screening) are measured simultaneously. The blood sample for PAPP-A and beta-hCG is taken either just before or after.

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