First Trimester NT Scan (Nuchal Translucency Scan)
Early Pregnancy Diagnosis: When to Test and When an Ultrasound Is Needed
Towards the end of the first trimester, your doctor will usually recommend a specialized ultrasound called the first trimester scan, often referred to as the nuchal translucency (NT) scan.
What started years ago as simply measuring the fluid at the back of the baby’s neck has now become a much more detailed early ultrasound. It checks the baby’s early development and looks for markers that may be linked to chromosomal conditions.
What Is the NT Scan?
The NT scan is typically performed between 11 weeks and 13 weeks + 6 days of pregnancy. During the ultrasound, the doctor carefully measures the nuchal translucency — the small, fluid-filled space at the back of your baby’s neck.
A thicker-than-normal NT measurement can be associated with chromosomal conditions, such as Down syndrome (trisomy 21), other chromosomal abnormalities, certain genetic syndromes or some structural problems or birth defects.
How Is the NT Measurement Interpreted?
The measurement is assessed according to your baby’s exact gestational age and crown-rump length (CRL). Generally, an NT measurement above the 95th percentile for gestational age, or 3.0 mm or greater is considered increased and may indicate a higher chance of chromosomal, genetic, or structural issues.
A normal (thin) NT measurement is reassuring and lowers the risk, but it’s important to remember that the NT scan is a screening test, not a diagnostic one. On its own, it detects around 70–80% of babies with Down syndrome. The detection rate improves significantly when the NT is combined with first-trimester blood tests (free β-hCG and PAPP-A) or with NIPT (cell-free DNA).
What Else Is Checked During the First Trimester Scan?
Beyond measuring the NT, your doctor will also look at several other important markers, including whether the nasal bone is present or absent, blood flow across the tricuspid valve (checking for backflow or regurgitation) and blood flow in the ductus venosus (a small vessel in the baby’s abdomen).
The scan also includes an early look at your baby’s basic anatomy, such as the skull and brain, arms and legs, abdominal wall, stomach, and bladder. In some cases, certain structural abnormalities can already be spotted at this stage.
Does the NT Scan Replace the Later Anatomy Scan?
No, it doesn’t.
The first trimester NT scan gives valuable early information, but it does not replace the detailed second-trimester anatomy scan (usually done between 18 and 22 weeks). That later scan remains the most thorough evaluation of your baby’s organs and structures.
Additional Assessments During the Scan
In certain cases, your doctor may also check the uterine artery Doppler flow. When combined with your medical history and blood markers, this can help estimate the risk of developing preeclampsia later in pregnancy.
What If the Results Show Increased Risk?
If the NT measurement or other markers suggest a higher risk, your doctor will sit down with you to discuss your options. This may include additional screening (such as NIPT), diagnostic testing (like CVS or amniocentesis), if appropriate, or referral for genetic counseling.
Every pregnancy is unique, and the decision is always yours after having a full, balanced discussion.
References
1. Salomon LJ, Alfirevic Z, Bilardo CM, et al.; International Society of Ultrasound in Obstetrics and Gynecology (ISUOG). ISUOG Practice Guidelines (updated): performance of 11–14-week ultrasound scan. Ultrasound Obstet Gynecol. 2023;61(1):127-143. doi:10.1002/uog.26106.
2. Nicolaides KH. Nuchal translucency and other first-trimester sonographic markers of chromosomal abnormalities. Am J Obstet Gynecol. 2004;191(1):45-67. doi:10.1016/j.ajog.2004.03.090.
Maternal–Fetal Medicine Specialist in Bangkok
Dr. Narisra Srikureja Firer (Dr. Niti)
Dr. Niti is a Bangkok-based OBGYN with subspecialty training in Maternal–Fetal Medicine (MFM). She provides comprehensive care for women across all stages of life, including pregnancy care, deliveries, and general gynecological conditions.
She dedicates her time to private practice at Ruamjairak Hospital and Mission Hospital, and serves as a Maternal–Fetal Medicine Consultant at a large government tertiary care center.
To read Dr. Niti’s full bio, click here.
