The 12-week scan, formally known as the First Trimester Screening or Nuchal Translucency (NT) scan, is a routine ultrasound performed between 11 and 14 weeks of gestation. This non-invasive procedure confirms the viability of the pregnancy, establishes the gestational age, and determines if there is a single fetus or a multiple gestation. Its primary function is to calculate a statistical risk for certain genetic conditions, serving as a screening tool rather than a definitive diagnostic test.
Assessing Risk for Chromosomal Conditions
The primary assessment at this stage is the risk for chromosomal conditions, calculated using a combination of ultrasound and blood test markers. The ultrasound focuses on the Nuchal Translucency (NT) measurement, which is the thickness of the fluid-filled space at the back of the fetal neck. An increased NT measurement is a soft marker indicating a higher chance of a chromosomal abnormality, structural defect, or genetic syndrome.
The NT measurement is combined with biochemical analysis of the mother’s blood, measuring levels of Pregnancy-Associated Plasma Protein-A (PAPP-A) and free beta-human Chorionic Gonadotropin (hCG). Maternal age is also factored into a mathematical calculation to produce an individualized risk score. This combined screening approach has a high detection rate for the most common trisomies.
The pattern of these markers suggests the specific condition for which the fetus is at risk. For Trisomy 21 (Down Syndrome), the pattern typically shows an increased NT measurement, a higher level of free beta-hCG, and a lower level of PAPP-A. For Trisomy 18 (Edwards Syndrome) and Trisomy 13 (Patau Syndrome), both the PAPP-A and free beta-hCG levels tend to be lower than expected, alongside an increased NT measurement. The resulting risk score provides a ratio, such as 1 in 500, representing the probability of the condition being present.
Identifying Major Physical Abnormalities
Beyond the risk assessment, the high-resolution ultrasound allows for the direct visual identification of several major physical abnormalities. This early anatomical survey focuses on structures sufficiently developed to be visualized. One area of focus is the fetal head, where severe neural tube defects like anencephaly can be detected, characterized by the absence of a major portion of the brain and skull.
The scan also checks the development of the abdomen, where serious abdominal wall defects may be seen. Omphalocele and gastroschisis are two conditions that can be identified, where abdominal organs protrude outside the body. Omphalocele is distinguished by the organs being contained within a membrane sac at the umbilical cord insertion, while in gastroschisis, the organs are typically free-floating in the amniotic fluid.
Although a detailed heart scan is performed later, major heart abnormalities can sometimes be suspected, especially since an increased NT measurement is associated with congenital heart conditions. The presence of the nasal bone is another soft marker evaluated, as its absence or underdevelopment is associated with an elevated risk for Trisomy 21. Major limb reduction defects are also possible to visualize at this early stage.
Interpreting High-Risk Screening Results
A “high risk” result from the combined screening does not constitute a diagnosis; it means the calculated probability of a chromosomal condition is above a predetermined cutoff. A woman with a high-risk result is then offered further testing options to determine if the fetus is affected. The first step often involves Non-Invasive Prenatal Testing (NIPT), which analyzes cell-free fetal DNA circulating in the mother’s bloodstream.
NIPT is a highly accurate screening tool, offering a higher detection rate and lower false-positive rate than the combined screen, specifically for Trisomy 21, 18, and 13. If the NIPT result also returns a high probability, or if a definitive answer is sought immediately, diagnostic procedures are the next step. These invasive procedures, Chorionic Villus Sampling (CVS) and Amniocentesis, provide a definitive diagnosis with greater than 99% accuracy.
CVS involves taking a small sample of placental tissue, and it is usually performed between 11 and 13 weeks of gestation, offering an earlier diagnosis. Amniocentesis, which samples the amniotic fluid containing fetal cells, is typically performed after 16 weeks. A genetic counselor will discuss the accuracy, risks, and implications of each option, helping the parents make an informed decision about proceeding with diagnostic testing.