The Nuchal Translucency (NT) Calculator is a simple tool designed to help you assess the expected size of the nuchal translucency based on the Crown Rump Length (CRL).
By entering the CRL and the measured NT value, you can quickly determine if the nt percentile is within the normal range, providing valuable insight during early pregnancy screenings.
This nt percentile calculator aids in the initial evaluation of potential chromosomal abnormalities, offering peace of mind or prompting further investigation.
Nuchal Translucency (NT) Calculator
Disclaimer:
This Nuchal Translucency (NT) Calculator is intended for informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. The results provided by this nt percentile calculator are based on general formulas and should not be relied upon for making any medical decisions. Always consult with a qualified healthcare provider to interpret NT measurements and for any concerns regarding pregnancy and fetal health.
This calculator is created using the research-backed results from here and here.
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Normal Expected NT and Percentile NT
Nuchal translucency (NT) is a measurement taken via ultrasound during the first trimester of pregnancy (usually between 11 and 14 weeks of gestation) to assess the risk of chromosomal abnormalities, such as Down syndrome. The measurement refers to the fluid-filled space at the back of the fetus's neck.
Below is a simplified table that provides approximate normal expected NT values and corresponding percentiles based on the gestational age (in weeks) and crown-rump length (CRL) of the fetus.
Table of Normal Expected NT and Percentile NT
| Gestational Age (Weeks) | CRL (mm) | 5th Percentile (mm) | 50th Percentile (mm) | 95th Percentile (mm) |
|---|---|---|---|---|
| 11 | 45-54 | 0.8 | 1.5 | 2.2 |
| 12 | 55-64 | 0.9 | 1.6 | 2.4 |
| 13 | 65-74 | 1.0 | 1.7 | 2.6 |
| 14 | 75-84 | 1.1 | 1.8 | 2.7 |
How to Interpret the Table:
- Gestational Age (Weeks): Indicates the number of weeks since the last menstrual period (LMP).
- CRL (mm): Crown-rump length, the measurement of the fetus from the top of the head to the bottom of the rump.
- 5th Percentile: The NT measurement below which 5% of the normal population falls.
- 50th Percentile: The NT measurement below which 50% of the normal population falls (median value).
- 95th Percentile: The NT measurement below which 95% of the normal population falls. Values above this may warrant further investigation.
Notes:
- NT values can vary slightly based on the equipment and methods used during the ultrasound.
- The 95th percentile is often used as a threshold for determining if an NT measurement is abnormally high.
- NT measurements are just one part of the first-trimester screening process and should be interpreted along with other factors like maternal age, blood test results, and family history.
This table provides a general guide, but clinicians should always use population-specific charts and adjust for factors such as maternal age, race, and specific gestational age to make accurate assessments.
CRL to Gestational Age
Below is a standard table that estimates gestational age based on the crown-rump length (CRL) measurement. This table is commonly used in clinical practice to determine gestational age during the first trimester of pregnancy.
CRL to Gestational Age Table
Know that the below table is an estimate. For more detailed and accurate data visit here and here.
| Crown-Rump Length (CRL) (mm) | Gestational Age (Weeks + Days) | Gestational Age (Days) |
|---|---|---|
| 2.0 | 6 + 1 | 43 |
| 3.0 | 6 + 3 | 45 |
| 4.0 | 6 + 5 | 47 |
| 5.0 | 6 + 6 | 48 |
| 6.0 | 7 + 0 | 49 |
| 7.0 | 7 + 1 | 50 |
| 8.0 | 7 + 2 | 51 |
| 9.0 | 7 + 3 | 52 |
| 10.0 | 7 + 4 | 53 |
| 11.0 | 7 + 5 | 54 |
| 12.0 | 7 + 6 | 55 |
| 13.0 | 8 + 0 | 56 |
| 14.0 | 8 + 1 | 57 |
| 15.0 | 8 + 2 | 58 |
| 16.0 | 8 + 3 | 59 |
| 17.0 | 8 + 4 | 60 |
| 18.0 | 8 + 5 | 61 |
| 19.0 | 8 + 6 | 62 |
| 20.0 | 9 + 0 | 63 |
| 21.0 | 9 + 1 | 64 |
| 22.0 | 9 + 2 | 65 |
| 23.0 | 9 + 3 | 66 |
| 24.0 | 9 + 4 | 67 |
| 25.0 | 9 + 5 | 68 |
| 26.0 | 9 + 6 | 69 |
| 27.0 | 10 + 0 | 70 |
| 28.0 | 10 + 1 | 71 |
| 29.0 | 10 + 2 | 72 |
| 30.0 | 10 + 3 | 73 |
| 31.0 | 10 + 4 | 74 |
| 32.0 | 10 + 5 | 75 |
| 33.0 | 10 + 6 | 76 |
| 34.0 | 11 + 0 | 77 |
| 35.0 | 11 + 1 | 78 |
| 36.0 | 11 + 2 | 79 |
| 37.0 | 11 + 3 | 80 |
| 38.0 | 11 + 4 | 81 |
| 39.0 | 11 + 5 | 82 |
| 40.0 | 11 + 6 | 83 |
| 41.0 | 12 + 0 | 84 |
| 42.0 | 12 + 1 | 85 |
| 43.0 | 12 + 2 | 86 |
| 44.0 | 12 + 3 | 87 |
| 45.0 | 12 + 4 | 88 |
| 46.0 | 12 + 5 | 89 |
| 47.0 | 12 + 6 | 90 |
| 48.0 | 13 + 0 | 91 |
| 49.0 | 13 + 1 | 92 |
| 50.0 | 13 + 2 | 93 |
| 51.0 | 13 + 3 | 94 |
| 52.0 | 13 + 4 | 95 |
| 53.0 | 13 + 5 | 96 |
| 54.0 | 13 + 6 | 97 |
| 55.0 | 14 + 0 | 98 |
| 56.0 | 14 + 1 | 99 |
| 57.0 | 14 + 2 | 100 |
How to Use the Table:
- CRL: Measure the crown-rump length (CRL) in millimeters using an ultrasound.
- Gestational Age (Weeks + Days): Find the corresponding gestational age in weeks and days.
- Gestational Age (Days): Alternatively, use the gestational age in days for more precise calculations.
This table provides standard values, but actual measurements might vary slightly. Always consult with healthcare professionals to confirm the gestational age, especially when using it for medical decisions.
What is a normal nuchal translucency level at 12 weeks?
A normal nuchal translucency (NT) measurement at 12 weeks is typically between 1.0 mm and 2.5 mm. Most measurements below 3.0 mm are considered within the normal range. The NT measurement tends to increase with gestational age and crown-rump length (CRL).
How is NT calculated?
Nuchal translucency (NT) is measured using an ultrasound during the first trimester, usually between 11 and 14 weeks of pregnancy. The sonographer measures the clear (translucent) space in the tissue at the back of the fetus's neck. The NT measurement is taken as the thickness of this fluid-filled space, and it is done with the fetus in a neutral position to ensure accuracy.
What if nuchal translucency is 1.5 mm?
An NT measurement of 1.5 mm at 12 weeks is considered within the normal range. It indicates a low risk for chromosomal abnormalities, such as Down syndrome, and is generally not a cause for concern. However, the NT measurement is just one part of a screening process that may include blood tests and other assessments.
How to calculate NT scan date?
The NT scan is usually performed between 11 weeks and 14 weeks of gestation. To calculate the date for your NT scan, you can determine it by adding 11 weeks to the first day of your last menstrual period (LMP). Alternatively, if you know your expected delivery date, you can count backward to find the appropriate range for the NT scan.
What is a good nuchal result?
A good nuchal translucency result is one where the NT measurement falls within the normal range, typically between 1.0 mm and 2.5 mm at 12 weeks. This result, particularly when combined with normal blood test results, suggests a low risk for chromosomal abnormalities like Down syndrome.
What is a bad nuchal translucency?
A bad nuchal translucency result generally refers to an NT measurement that is higher than normal, typically above 3.0 mm. An increased NT measurement could indicate a higher risk of chromosomal abnormalities, such as Down syndrome, or other potential issues like congenital heart defects. Further testing, such as amniocentesis or chorionic villus sampling (CVS), may be recommended in such cases.
What is the NT value for a boy?
NT measurements are not dependent on the gender of the fetus. The NT value for a boy is assessed in the same way as for a girl. NT is a measure of fluid behind the neck of the fetus and is not influenced by whether the fetus is male or female.
Can fluid behind the fetus's neck go away?
Yes, the fluid behind the fetus's neck, as measured by the NT scan, can decrease or resolve as the pregnancy progresses. NT measurements are typically most relevant between 11 and 14 weeks of gestation, and the fluid may not be as prominent or measurable after this period.
Is a normal baby with a thick nuchal fold?
A thickened nuchal fold, which typically refers to an increased NT measurement, can sometimes be seen in fetuses that are otherwise normal. However, a thicker NT can also be associated with an increased risk of chromosomal abnormalities, congenital heart defects, or other conditions. Follow-up testing and monitoring are usually recommended.
Can a baby with high NT be normal?
Yes, it is possible for a baby with a high NT measurement to be completely normal. While an increased NT can indicate a higher risk of chromosomal abnormalities or other issues, many babies with elevated NT measurements are born healthy. Additional diagnostic tests can help determine the underlying cause.
What abnormalities can be seen at a 12-week scan?
At a 12-week scan, several potential abnormalities can be identified, including:
- Increased Nuchal Translucency: May indicate a risk of chromosomal abnormalities like Down syndrome.
- Absent Nasal Bone: May be associated with Down syndrome.
- Structural Abnormalities: Such as congenital heart defects or neural tube defects.
- Growth Delays: Indicating potential issues with the pregnancy.
- Multiple Pregnancy: The scan can also confirm if there are twins or more.
This scan is often combined with other tests to assess the overall health and development of the fetus.
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Reference
- Hadlock FP, Shah YP, Kanon DJ, Lindsey JV. Fetal crown-rump length: reevaluation of relation to menstrual age (5-18 weeks) with high-resolution real-time US. Radiology. 1992 Feb;182(2):501-5. doi: 10.1148/radiology.182.2.1732970. PMID: 1732970.
- Chung JH, Yang JH, Song MJ, Cho JY, Lee YH, Park SY, Moon MJ, Lim HJ, Choi JS, Kim JO, Shin JS, Ahn HK, Han JY, Kim MY, Choi KH, Ryu HM. The distribution of fetal nuchal translucency thickness in normal Korean fetuses. J Korean Med Sci. 2004 Feb;19(1):32-6. doi: 10.3346/jkms.2004.19.1.32. PMID: 14966338; PMCID: PMC2822260.
- Salman Guraya S. The associations of nuchal translucency and fetal abnormalities; significance and implications. J Clin Diagn Res. 2013 May;7(5):936-41. doi: 10.7860/JCDR/2013/5888.2989. Epub 2013 Mar 20. PMID: 23814750; PMCID: PMC3681077.
- Xu Y, Ni M, Zhang Q, Zhao J, Tang Z, Liu Z. Correlation between crown-rump length in the first trimester of pregnancy and neonatal outcomes. BMC Pediatr. 2022 Jul 1;22(1):386. doi: 10.1186/s12887-022-03426-8. PMID: 35778680; PMCID: PMC9248167.
- Self A, Daher L, Schlussel M, Roberts N, Ioannou C, Papageorghiou AT. Second and third trimester estimation of gestational age using ultrasound or maternal symphysis-fundal height measurements: A systematic review. BJOG. 2022 Aug;129(9):1447-1458. doi: 10.1111/1471-0528.17123. Epub 2022 Mar 10. PMID: 35157348; PMCID: PMC9545821.