The 12-week scan, often known as the nuchal translucency (NT) scan, is a routine prenatal check-up performed between 11 and 14 weeks of pregnancy. This ultrasound offers early insights into the baby’s development and overall well-being, providing information for expectant parents and their healthcare providers.
Purpose of the 12-Week Scan
The 12-week scan serves several important purposes beyond looking for potential abnormalities. A primary goal is to accurately determine the gestational age and calculate an estimated due date. The scan also confirms the baby’s viability by visualizing the fetal heartbeat, which beats between 110 to 160 beats per minute at this stage. Additionally, for multiple pregnancies, the scan identifies the number of babies and, if twins, determines chorionicity—whether they share a placenta, which influences pregnancy management due to differing risks.
Chromosomal Conditions Screened
A primary focus of the 12-week scan is screening for specific chromosomal conditions. This involves measuring the nuchal translucency (NT), the fluid-filled space at the back of the baby’s neck. While some fluid is normal, an increased NT measurement, over 3.5 mm, can be associated with a higher chance of chromosomal differences.
This NT measurement is combined with maternal blood tests that analyze levels of pregnancy-associated plasma protein-A (PAPP-A) and free beta-human chorionic gonadotropin (hCG). In pregnancies affected by Down syndrome (Trisomy 21), PAPP-A levels are lower and free beta-hCG levels are higher than in unaffected pregnancies. In Edwards syndrome (Trisomy 18) and Patau syndrome (Trisomy 13), both PAPP-A and free beta-hCG levels are decreased.
Additional markers can further refine the risk assessment for these conditions. Sonographers assess the presence or absence of a nasal bone, as its absence is linked to chromosomal differences. Blood flow through the ductus venosus and the presence of tricuspid valve regurgitation are also evaluated. These combined findings provide more comprehensive data for a precise individual risk assessment.
Structural Abnormalities Identified
While a more detailed structural assessment occurs around 20-22 weeks, the 12-week scan can identify or raise suspicion of major physical developmental abnormalities. Examples include brain abnormalities, such as anencephaly, or spinal defects like spina bifida. Heart defects and limb abnormalities may also be visible during this early scan.
Abdominal wall defects, such as gastroschisis or omphalocele, where internal organs are outside the body, might also be detectable. These structural findings are less common than chromosomal risks but can be visually assessed. Additionally, the scan can include a screening for pre-eclampsia risk by measuring blood flow in the uterine arteries using Doppler ultrasound and combining it with maternal blood pressure and biochemical markers.
Understanding Your Results
The 12-week scan provides a risk assessment for chromosomal conditions, categorizing the pregnancy as low, intermediate, or high risk, rather than a definitive diagnosis. If a high-risk result is indicated, healthcare providers will discuss options for further investigation. These options may include non-invasive prenatal testing (NIPT), which analyzes fetal DNA in the mother’s blood, or more invasive diagnostic procedures.
Diagnostic tests, such as chorionic villus sampling (CVS) or amniocentesis, provide a conclusive diagnosis. CVS is performed between 11 and 13 weeks of pregnancy, while amniocentesis is done after 15 or 16 weeks. If structural abnormalities are suspected, further specialized scans and consultations with specialists are recommended to confirm the findings.
The 12-week scan also offers an opportunity to diagnose a miscarriage if fetal viability cannot be confirmed. In such cases, the scan may reveal an embryo without a heartbeat, or an empty gestational sac. Most 12-week scans show normal findings, offering reassurance for expectant parents.