First Trimester Ultrasound (12-Week Scan)

A first trimester ultrasound, performed at around 12 weeks of pregnancy, does far more than measure the nuchal translucency. Performed properly, it screens the fetus for chromosomal problems, diagnoses many major structural abnormalities, estimates the mother's risk of preeclampsia and fetal growth restriction, measures the cervix for preterm labour risk and establishes chorionicity in twins.

The scan is still widely known as the "NT scan", a name that ties it to that single measurement: the nuchal translucency (NT), the fluid collection just behind the fetal neck. Dr. Aanchal Sablok, Senior Consultant - Fetal Medicine, Artemis Hospitals, Gurugram, India, argues that this view now undersells it. Her summary is blunt: the 12-week scan is a one-stop assessment of risk, and clinicians should make the most of it.

What a first trimester ultrasound confirms first

Before any screening begins, a first trimester ultrasound confirms that the gestation is intrauterine, establishes the number of fetuses, confirms cardiac activity (the fetal heartbeat) and dates the pregnancy accurately. These are the foundational questions of the pregnancy, and every later risk calculation depends on them.

The route of the scan follows gestational age. Below 10 weeks, Dr. Sablok's practice is to scan transvaginally. Beyond 12 weeks, the examination starts transabdominally and then proceeds to a transvaginal scan as well, so that both views are available.

When to do the 12-week scan

The fetus is examined when the crown-rump length (CRL), the length of the fetus from the top of the head to the bottom, lies between 45 and 84 millimetres. That window corresponds to a gestation of 11 weeks 0 days to 13 weeks 6 days. In practical terms, Dr. Sablok finds it convenient to call women in at around 12 weeks.

The reason for not scanning at the very start of the window is that the first trimester is, in her words, an "evolving scan". At the smaller end of the range, structures such as the nasal bone may not yet be visible because they are still forming. Waiting until the fetus is somewhat larger allows the anatomy to be seen more completely and lets an evolving nasal bone show itself.

The inverted pyramid of prenatal care

The inverted pyramid of care places the heaviest screening effort at the first visit, at 12 weeks, rather than towards the end of pregnancy. The Fetal Medicine Foundation (FMF) proposed this reversal about sixteen years ago, and Dr. Sablok places the 12-week scan inside it.

In the traditional model, a woman was seen once at around 12 weeks, and the frequency of visits rose steadily as the pregnancy approached term. Under the inverted pyramid:

• At 12 weeks: every woman is screened for chromosomal abnormalities, structural abnormalities, preeclampsia and preterm labour.
• At 20 weeks: the anomaly scan follows.
• At 37 to 41 weeks: delivery.

If the first-trimester screen is done well, the number of subsequent visits can fall, because the women who need closer surveillance have already been identified. Dr. Sablok is careful to add that the inverted pyramid does not mean leaving the pregnancy alone. It means seeing the woman more often, to diagnose and to treat. Her own formulation is that the more you do and the earlier you do it, the better.

What a 12-week scan can detect

In Dr. Sablok's framing, a single 12-week examination answers five distinct questions:

1. Fetal aneuploidy, an abnormal number of chromosomes. The combined first-trimester screen for trisomy 21 and the other common aneuploidies is built from maternal age, four ultrasound soft markers and maternal biochemistry.
2. Fetal structural abnormalities. A systematic anatomical survey of the head, brain, heart, stomach, abdominal wall, limbs and spine.
3. Preeclampsia and fetal growth restriction. A risk estimate combining maternal history, blood pressure, uterine artery Doppler and placental biochemistry, with aspirin for women at high risk.
4. Preterm labour. Cervical length measured at the first-trimester scan rather than deferred to 20 weeks.
5. Twin pregnancy. Chorionicity assessment (whether the twins share a placenta) and prediction of twin complications.

The first of these, aneuploidy screening and the interpretation of nuchal translucency, is covered in depth in the companion article on Down syndrome screening detection rates. Preeclampsia, growth restriction and preterm labour are covered in preeclampsia screening at 12 weeks and preterm birth risk. The rest of this guide focuses on the case for looking beyond chromosomes and on the structural abnormalities the scan can already detect.

Why the 12-week scan must look beyond aneuploidy

Dr. Sablok's argument is that aneuploidy is only one part of the risk a pregnancy carries, and sometimes not the largest part. The "NT scan" label reflects the scan's origins in Down syndrome screening, but a chromosomally normal baby born too soon can face a long neonatal intensive care stay, severe morbidity or neonatal death. A chromosomally normal baby whose mother develops severe hypertension or an eclamptic fit faces a pregnancy that, in her words, is "taken for a toss". Deletion-duplication syndromes and other karyotype abnormalities also sit outside the classic trisomies of Down, Edwards and Patau syndromes.

A scan that reports only an NT measurement and an aneuploidy risk therefore leaves most of the 12-week opportunity unused.

Structural abnormalities a first trimester scan can detect

Many major structural abnormalities can be identified in the first trimester, provided the examiner looks for them deliberately. Dr. Sablok groups them into three categories: those that should always be detected, those that should be looked for carefully and those that cannot yet be seen.

Her working principle is that every scan is an anomaly scan. The practice guidelines of ISUOG, the international society for ultrasound in obstetrics and gynaecology, likewise include a basic assessment of fetal anatomy in the 11 to 14 week scan. The era of waiting until the fetus reaches about 20 weeks before looking for structural problems, in her view, is over.

Abnormalities that should always be detected

These are large, unambiguous defects that are visible even when the fetus is very small:

• Head and brain: anencephaly (in which the head has not formed), acrania, encephalocele and alobar holoprosencephaly.
• Abdominal wall: exomphalos and gastroschisis, in which abdominal contents lie outside the abdomen. She describes both as very easily diagnosed at this stage.
• Urinary tract: megacystis, an abnormally large bladder.
• Limbs: absent limbs and limb reduction defects, such as an absent radius.
• Body stalk anomaly, which can be diagnosed when the fetus measures only 7 to 8 centimetres.

Abnormalities that should be looked for carefully

A second group can be identified in the first trimester but demands a careful, targeted examination:

• open spina bifida and other neural tube defects
• hypoplastic left heart syndrome
• atrioventricular septal defects
• renal agenesis

Abnormalities that cannot yet be seen at 12 weeks

The third group is made up of evolving abnormalities, which cannot be diagnosed at 12 weeks because the relevant structure has not yet formed. Microcephaly, cerebellar hypoplasia and agenesis of the corpus callosum fall here. As Dr. Sablok puts it, when the corpus callosum has not yet formed, there is no way to assess problems with it. These conditions become detectable only once the brain has developed further, at approximately 20 to 22 weeks. This is one reason the first-trimester scan complements, rather than replaces, the 20-week anomaly scan.

Case examples of first-trimester diagnosis

Dr. Sablok illustrated these categories with cases from her own practice:

• Anencephaly. A very small fetus in which the head had not formed. Anencephaly is reliably detectable at the 11 to 13 week scan.
• Iniencephaly. A very rare case: a major neural tube defect she put at an incidence of about 1 in 50,000, recognised by the "star-gazing" posture of the fetus and demonstrated on a 3D image.
• Open spina bifida at 13 weeks and 5 days. A defect visible at the lower end of the spine, with protruding contents.
• Abdominal wall defect. Abdominal contents seen herniating outside the abdominal wall.
• Skeletal dysplasia at 13 weeks and 3 days. On first impression the fetus looked entirely normal. Closer examination showed bowing of the femur, a finding that only a deliberate look at the limbs would catch.
• Monosomy X in a twin pregnancy. In the second twin, the head and neck suggested a raised NT. Closer inspection revealed septations just behind the head, and the diagnosis was monosomy X (Turner syndrome).
• Cystic hygroma in a further case, and conjoined twins diagnosed at the first-trimester scan.

The skeletal dysplasia and the twin with Turner syndrome make the same point. In both, a cursory look would have produced a reassuring report. The diagnosis depended on an examiner who looked closer.

How findings are recorded and risks calculated

At Artemis Hospitals, Dr. Sablok's team follows the FMF protocol and records findings in dedicated fetal medicine software of the kind used by fetal medicine units around the world. The CRL, the NT and the other soft markers (ductus venosus Doppler, nasal bone and tricuspid flow) are entered alongside a structural checklist of the brain, head, heart, stomach, hands and spine. The software then produces the risk assessment. Separate calculators generate the risks of preeclampsia and of spontaneous delivery before 34 weeks, so a single visit produces a structured risk profile.

Key takeaways for clinicians

Dr. Sablok closes on three points:

1. The 12-week scan is a window of opportunity that should be used in full, not reduced to a single neck measurement.
2. Early prediction enables risk stratification, timely referral to a specialist clinic and time for early intervention, whether that is invasive testing, aspirin, progesterone or a cervical stitch.
3. If the first scan is done properly from a fetal medicine standpoint, examining both the baby and the mother, most of the problems that evolve later in pregnancy can be anticipated and managed.