First Trimester Screening (NT Scan and Double Marker Test): What It Tells You and What It Cannot

Key takeaways

• Screening, not diagnosis: first trimester screening produces a risk figure such as 1 in 1,200, not a yes or no answer.
• Timing matters: the NT measurement is valid only within a defined crown-rump length window in the late first trimester; outside it, the test cannot be performed.
• It is optional: declining first trimester screening is a reasonable choice, and it should follow a counselling discussion.
• It misses a great deal: neural tube defects, most single-gene disorders and many structural anomalies are not assessed by first trimester screening.
• Indian law applies: under the PC-PNDT Act 1994, foetal sex is never disclosed during any prenatal ultrasound in India.

At a glance

Also known as

• NT scan / NT-NB scan (nuchal translucency, nasal bone)
• Double marker test (the blood component)
• Combined first trimester screening
• Dual marker test, first trimester sonography, 11-13 week scan
• Commonly said in clinics as "11 to 13 week USG" or "first trimester sonography"

What first trimester screening is

First trimester screening has two parts that are interpreted together, along with maternal age, weight, gestational age and other factors entered into a validated risk calculation software.

• Nuchal translucency ultrasound: measurement of the fluid-filled space at the back of the foetal neck. The measurement is combined with crown-rump length. Many units also record the nasal bone, ductus venosus flow and tricuspid flow where the sonologist is trained and certified to do so.
• Maternal serum markers: pregnancy-associated plasma protein-A (PAPP-A) and free beta human chorionic gonadotropin (free beta-hCG), reported as multiples of the median (MoM) adjusted for gestational age and maternal characteristics.

Neither part alone is a complete screen. A blood test without an accurately dated and measured NT, or an NT without serum markers, gives a weaker risk estimate.

Why first trimester screening is done

The purpose of first trimester screening is to identify pregnancies with a higher chance of common chromosomal trisomies early enough for the woman and her family to consider further testing and to plan. Early information also allows time for counselling, for arranging a diagnostic test if wanted, and for organising care around delivery if a condition is confirmed. The scan may also detect some major structural anomalies and confirm gestational age, viability and the number of foetuses.

Who should consider this test

• Any pregnant woman: combined first trimester screening is offered irrespective of age or risk category; it is not restricted to older mothers.
• Women who want early information and are willing to consider follow-up testing if the risk comes back raised.
• Women aged 35 and above, who have a higher background chance of trisomy, though screening is equally offered below this age.
• A previous pregnancy affected by a chromosomal condition, where a genetic counsellor may advise going straight to diagnostic testing rather than screening.
• Twin pregnancies: NT-based screening can be used, but interpretation differs and serum markers perform less well; discuss this specifically.

Screening may be less useful if you would not act on the result, or if you have already decided on a diagnostic test or NIPT. Women who present after 14 weeks cannot have first trimester screening and are offered alternatives such as the quadruple marker test or NIPT.

What first trimester screening cannot detect or exclude

This is the part most often misunderstood. First trimester screening is a probability estimate for a narrow set of conditions.

• It does not diagnose anything. A screen-positive result does not mean the baby has a trisomy; a screen-negative result does not mean the baby does not.
• It does not screen for neural tube defects such as spina bifida or anencephaly. These are assessed at the mid-trimester anomaly scan, sometimes with maternal serum AFP.
• It does not detect most single-gene disorders: thalassaemia, sickle cell disease, cystic fibrosis, spinal muscular atrophy, Duchenne muscular dystrophy and similar conditions require separate carrier or targeted testing.
• It does not detect most microdeletion or microduplication syndromes, mosaicism, or balanced rearrangements.
• It is not a complete anomaly scan. Many structural problems of the heart, brain, kidneys, limbs, face and abdominal wall become visible only later. A normal NT scan does not replace the 18-22 week anomaly scan.
• It does not predict autism, intellectual disability of non-chromosomal cause, cerebral palsy, or developmental outcomes in general.
• It does not assess placental function or predict preterm birth, although a low PAPP-A may prompt extra growth monitoring in some units.
• A normal result is not an "all clear". If problems are found later in pregnancy, that does not mean the earlier screen was performed wrongly.

Related but different examinations

No single test is superior across the board. NIPT performs better than combined screening for trisomy 21 detection, but combined screening includes an ultrasound that can pick up early structural problems and pregnancy complications that NIPT cannot see. Neither is diagnostic. Which to choose depends on gestational age, what you would do with the result, twin status and cost.

How to prepare

Preparation instructions are set by the unit performing the scan and vary between hospitals. Your unit's written instructions take precedence over anything here.

• Fasting: usually not required for PAPP-A and free beta-hCG. Some laboratories bundle other tests that do require fasting, confirm when booking.
• Bladder: some units ask for a moderately full bladder for a transabdominal NT scan; others do not. Follow your unit's instruction.
• Dating information: bring your last menstrual period date, any earlier scan reports, and details of any fertility treatment, as the risk calculation depends on accurate dating.
• Clothing: loose two-piece clothing makes abdominal access easier.
• Medicines: do not stop or change any prescribed medicine for this test. Tell the sonologist and the laboratory what you take, including thyroid medication, insulin, heparin and progesterone support.
• Information to declare: weight, ethnicity, smoking status, diabetes, IVF or donor egg conception and twin pregnancy all alter the risk calculation and must be recorded accurately.

If you have diabetes on glucose-lowering treatment

If your unit does ask for fasting for any bundled test, discuss timing with your treating doctor first so that insulin or tablet doses and meals are not disrupted. Do not skip meals or doses without that advice.

If you have a fluid restriction, heart failure or advanced kidney disease

Do not load up on water for a "full bladder" instruction. Tell the department before the appointment so the scan can be planned safely, including using a transvaginal approach if appropriate.

Twin and higher-order pregnancies

Each foetus needs its own NT measurement, and chorionicity must be documented. Serum marker interpretation is less reliable in twins, and in triplets serum screening is generally not used. Allow extra appointment time.

After IVF or donor egg conception

Gestational age is calculated from the embryo transfer date, and maternal age used in the calculation is the egg donor's age where relevant. Tell the unit, an error here changes the reported risk.

What happens during first trimester screening

• Blood sample: a venous sample from the arm, taking a few minutes. Some units draw the blood before the scan, others after; a few offer same-visit reporting where the analyser is on site.
• Ultrasound: you lie on your back, gel is applied to the lower abdomen, and the sonologist obtains a specific mid-sagittal view of the foetus with the neck in a neutral position. The measurement can only be taken when the foetus is in the correct position, so the sonologist may wait, ask you to walk about, cough, or return later.
• Transvaginal scan: occasionally needed for a clearer view, particularly with a retroverted uterus or higher body weight. The sonologist will explain and seek consent.
• What is measured: crown-rump length, nuchal translucency, foetal heart rate, number of foetuses, and in many units nasal bone and early structural survey.
• Foetal sex is not looked for and is not told to you, and the department will display a legally required notice to that effect.

How long the process takes

The blood draw itself takes only a few minutes. The NT ultrasound commonly takes 20 to 40 minutes, and longer if the foetus is not lying favourably or if there is more than one foetus. Allow one to two hours in the department overall, including registration, PC-PNDT Form F documentation and waiting. Reporting time depends on whether the serum assay is run in-house or sent to a reference laboratory; ask your unit when booking.

Understanding your report

A combined first trimester screening report typically contains:

• Gestational age and crown-rump length used for the calculation.
• Nuchal translucency measurement in millimetres, often with a MoM value.
• PAPP-A and free beta-hCG, in concentration units and as MoM.
• Background (age-related) risk and adjusted risk for trisomy 21, and usually a combined risk for trisomies 18 and 13.
• A screen-positive or screen-negative classification against the cut-off used by that laboratory's software.

A result such as "1 in 900" means that out of 900 pregnancies with the same profile, about one would be expected to be affected. Cut-offs differ between laboratories and software versions, so the same numbers can be labelled differently at two centres. Some units report an intermediate-risk band and offer NIPT as a second step.

A raised NT with a low-risk trisomy calculation still matters, increased nuchal translucency is independently associated with congenital heart disease and some genetic syndromes, and usually prompts a foetal echocardiogram and detailed anomaly scan. Results vary depending on individual clinical circumstances, and the report must be interpreted by the obstetrician or foetal medicine specialist who ordered it, alongside your history.

When results can be misleading

Factors that can falsely raise the calculated risk

• Wrong gestational age entered into the software, the single commonest cause of a misleading result.
• Incorrect NT technique or foetal position: hyperextension of the neck, amnion mistaken for skin, or measurement by an uncertified operator.
• Twin pregnancy or a vanishing twin, which distorts serum marker levels.
• Maternal weight, smoking, diabetes and ethnicity not correctly entered.
• IVF conception, which alters PAPP-A levels.
• About 5 in 100 unaffected pregnancies screen positive with combined screening at conventional cut-offs, so most screen-positive results are in fact unaffected pregnancies.

Factors that can falsely lower the calculated risk

• A normal NT does not exclude trisomy: a proportion of affected pregnancies have a normal measurement. Combined screening does not detect every case of trisomy 21.
• Serum markers overlapping the normal range in an affected pregnancy.
• Overestimated gestational age, pushing the calculation towards a reassuring number.
• Trisomies 18 and 13, triploidy and sex chromosome abnormalities are detected less consistently than trisomy 21.

A negative result describes what the screening found, not what exists. If concerns arise later, abnormal growth, abnormal findings on the anomaly scan, or new symptoms, go back to your obstetrician regardless of an earlier reassuring screen.

Risks and safety

The ultrasound component

Diagnostic obstetric ultrasound at recommended output settings has been used for decades and no harm to the foetus has been demonstrated. Standard practice is to keep exposure as low as reasonably achievable and to perform scans only for a medical indication. Non-medical "keepsake" imaging is not appropriate. A transvaginal scan may be briefly uncomfortable; light spotting afterwards is uncommon but possible.

The blood test component

Venepuncture may cause brief pain, bruising, a small haematoma or, rarely, fainting or local infection.

Psychological and decision-related risks

The main risk of screening is anxiety from a screen-positive result in a pregnancy that turns out to be unaffected, and the possibility of being drawn into invasive diagnostic testing that carries its own small miscarriage risk. This is why counselling before screening matters, and why the decision to screen is yours.

Follow-up diagnostic procedures

CVS and amniocentesis are invasive and carry a small procedure-related risk of pregnancy loss, along with rare risks of infection, leakage of amniotic fluid and an inconclusive result. These risks belong to those procedures, not to the screening test itself.

Special situations

Advanced maternal age

The background chance of trisomy rises with maternal age, so an identical NT and marker profile produces a higher final risk at 40 than at 25. Many women aged 35 and above choose NIPT or go directly to diagnostic testing after counselling.

Obesity or higher body weight

Image quality may be reduced and a transvaginal approach or a repeat visit may be needed. Maternal weight is also corrected for in the serum marker calculation.

Pre-existing diabetes or thyroid disease

These conditions are recorded in the risk software where the program allows. Continue your prescribed treatment and keep your endocrine follow-up alongside antenatal care.

Known haemoglobinopathy or thalassaemia trait

First trimester screening does not test for thalassaemia. Couple screening for thalassaemia trait is a separate and important test in India, ideally done in the first trimester, with partner testing if the woman is a carrier.

Previous affected pregnancy or known family genetic condition

Genetic counselling before screening is advised. Targeted diagnostic testing may be more appropriate than population screening.

Recurrent miscarriage or bleeding in this pregnancy

Screening can usually still be performed, but viability and dating must be confirmed first, and a vanishing twin will affect serum markers.

First trimester screening in the Indian context

• PC-PNDT Act 1994: every diagnostic ultrasound in pregnancy must be performed in a registered facility by a qualified, registered person, with Form F completed. Communicating foetal sex, directly or indirectly, is a criminal offence. Do not ask the sonologist, and do not interpret their silence as a hint.
• Medical Termination of Pregnancy (Amendment) Act 2021 governs what may follow a confirmed diagnosis; decisions are made with your obstetrician and, where required, a medical board.
• Operator certification matters. NT measurement is technique-dependent. Ask whether the sonologist and the risk-calculation software are certified for NT audit; an uncertified measurement produces an unreliable risk.
• Late booking is common. Many women present after 14 weeks, by which point combined screening is no longer possible and the quadruple marker test or NIPT becomes the option.
• Screening priorities differ here. Thalassaemia carrier screening, anaemia correction, rubella status and gestational diabetes screening sit alongside chromosomal screening in Indian antenatal care.
• NIPT availability is widening in Indian cities but remains largely self-funded, and sample logistics can affect turnaround.
• Counselling gap: reports are sometimes handed over without explanation. You are entitled to have the numbers explained, and to a genetic counselling referral if the risk is raised.

Cost and insurance in India

Costs vary widely between cities, between government and private facilities, and between standalone laboratories and hospital-based foetal medicine units. Rather than quoting a figure that may not apply to you, it is more useful to know what drives the price.

• Whether the NT scan and double marker test are billed together or separately.
• Who performs the scan: a foetal medicine specialist with NT certification usually costs more than a general sonography clinic.
• Whether risk-calculation software is included, and whether nasal bone, ductus venosus and tricuspid assessment are added.
• Whether the serum assay is run in-house or outsourced, and the platform used.
• City and facility tier, and whether the visit is part of an antenatal package.
• Follow-on tests: NIPT, foetal echocardiography, CVS or amniocentesis are billed separately and cost substantially more.

Insurance: routine antenatal outpatient screening is frequently excluded from standard indemnity health insurance in India, though some maternity benefit plans and corporate policies cover part of antenatal investigation after a waiting period. Government schemes and public hospitals provide antenatal ultrasound under national maternal health programmes. Confirm coverage with your insurer before the appointment. All costs quoted anywhere are indicative only and should be confirmed with the facility at the time of booking.

Myths and facts

Frequently asked questions

Can first trimester screening tell me the baby's sex?

No. Foetal sex is not part of first trimester screening, and under the PC-PNDT Act 1994 no person in India may communicate the sex of a foetus by any means. Radiologists, sonologists and laboratories are legally bound by this, and asking places the clinician at legal risk.

What happens if my result is screen-positive?

A screen-positive result means your calculated chance is above the laboratory's cut-off, not that the baby is affected. Your obstetrician will usually refer you for genetic counselling and offer NIPT or a diagnostic test such as CVS or amniocentesis. You can also choose no further testing.

I am past 13 weeks. What are my options?

Combined first trimester screening cannot be performed once the crown-rump length is beyond the valid window. Alternatives include the quadruple marker test at around 15 to 20 weeks, NIPT, and a detailed anomaly scan at 18 to 22 weeks. Discuss which combination suits your situation with your obstetrician.

Is the NT scan painful or risky for the baby?

The abdominal scan is painless; gel may feel cold. A transvaginal scan can feel mildly uncomfortable. Diagnostic ultrasound performed for a medical reason at recommended output settings has not been shown to harm the foetus. The blood test may cause brief pain or bruising at the needle site.

Can first trimester screening detect twins?

Yes. The ultrasound reliably identifies the number of foetuses in the late first trimester and, importantly, determines chorionicity, whether the twins share a placenta. This is one of the most valuable pieces of information from the scan, as chorionicity determines how closely a twin pregnancy is monitored.

Should I choose NIPT instead of combined screening?

NIPT detects more cases of trisomy 21 with fewer false positives, but it is still screening, costs considerably more, can fail to return a result, and does not include an ultrasound. Combined screening includes an NT scan that assesses structure, dating and twin status. Many units use both in sequence.

Does a normal nuchal translucency mean the baby's heart is fine?

No. A normal NT lowers but does not remove the chance of congenital heart disease. Cardiac structure is assessed at the anomaly scan and, where indicated, by foetal echocardiography at around 18 to 24 weeks. Increased NT is one trigger for that referral, but not the only one.

What if I decide not to have first trimester screening?

That is a legitimate choice, and it should not affect the quality of your antenatal care. You will still be offered routine antenatal tests and the mid-trimester anomaly scan. You can change your mind, though the first trimester window is time-limited, so tell your obstetrician early.

Does a low PAPP-A level on its own mean something is wrong?

Not necessarily. A low PAPP-A is often an isolated finding in a pregnancy with a normal outcome. Some units use it as a reason for additional growth scans later in pregnancy, because it has been associated with growth restriction and pre-eclampsia. Interpretation belongs with your obstetrician.

Sources

• Pre-conception and Pre-natal Diagnostic Techniques (Prohibition of Sex Selection) Act, 1994, Government of India.
• Medical Termination of Pregnancy (Amendment) Act, 2021, Government of India.
• International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) practice guidelines on first trimester foetal ultrasound.
• Fetal Medicine Foundation standards for nuchal translucency measurement and operator certification.
• American College of Obstetricians and Gynecologists guidance on screening for foetal chromosomal abnormalities.
• Federation of Obstetric and Gynaecological Societies of India (FOGSI) good clinical practice recommendations on antenatal screening.