Birth defects, which are structural, functional, or metabolic problems occurring during development, affect approximately one in every 33 babies born in the United States. Prenatal detection focuses on early identification to allow for timely intervention, specialized planning, or informed decision-making. The timeline begins in the first trimester, targeting genetic anomalies, and extends through later pregnancy and immediately after birth to identify structural or metabolic issues.
Early First Trimester Screening and Diagnostic Options (Weeks 9–14)
The earliest methods for assessing a baby’s health begin around the ninth week, focusing on genetic screening. Non-Invasive Prenatal Testing (NIPT) is a screening test using a simple maternal blood draw to analyze fragments of the baby’s DNA circulating in the bloodstream. Performed as early as nine weeks, NIPT provides a risk assessment for chromosomal abnormalities, specifically Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome).
Between 11 and 14 weeks of gestation, a Nuchal Translucency (NT) scan uses ultrasound to measure the fluid-filled space at the back of the baby’s neck. An increased measurement, combined with specific maternal blood markers, forms the First Trimester Combined Screening. This screening assesses the risk for the same chromosomal conditions as NIPT, and a high NT measurement can also suggest an increased risk for congenital heart defects.
If screening indicates a high risk, Chorionic Villus Sampling (CVS) is the earliest diagnostic option, typically performed between 10 and 13 weeks. This invasive procedure involves taking a small tissue sample from the placenta to analyze the chromosomes directly. CVS provides a definitive diagnosis for chromosomal abnormalities and certain genetic conditions.
Mid-Pregnancy Structural Assessment (Weeks 18–22)
The middle of the second trimester provides the first opportunity for a comprehensive structural evaluation. This detailed assessment is the Anatomy Scan (Level II Ultrasound), typically performed between 18 and 22 weeks. At this stage, the baby is large enough for a sonographer to examine the major organs, limbs, brain, face, and spine in detail.
The scan identifies structural issues not apparent earlier, such as congenital heart defects, spina bifida, cleft lip, or kidney malformations. Identifying these issues allows providers and parents to begin planning for postnatal treatment or interventions. Image clarity depends on the baby’s position and the mother’s body type, meaning some structures may require a follow-up assessment.
If the anatomy scan reveals a structural concern or if earlier screening tests were positive, Amniocentesis is the second common diagnostic procedure. Performed between 15 and 20 weeks, this test involves extracting a small sample of the amniotic fluid. The fluid contains fetal cells that are tested for chromosomal conditions, genetic syndromes, and neural tube defects, confirming or ruling out the suspected issue.
Late-Term Monitoring and Post-Birth Confirmation
In the late second and third trimesters, monitoring detects defects that develop later or are difficult to visualize. A Fetal Echocardiogram, a specialized ultrasound of the heart, is often performed between 18 and 24 weeks if a congenital heart defect is suspected. This detailed imaging allows a pediatric cardiologist to assess the heart’s structure and function with greater precision than the anatomy scan.
Fetal Magnetic Resonance Imaging (MRI) may be used in the late second or third trimester for a more detailed view of the brain, lungs, or soft tissues. MRI is helpful when ultrasound images are inconclusive or when a complex condition, such as a severe brain anomaly, requires further characterization for surgical planning. It serves as a complementary tool to ultrasound for complex cases.
The detection process continues immediately after birth with standard Newborn Screening, which includes a physical examination, a hearing screen, and the heel stick blood test. The heel stick test, performed 24 to 48 hours after birth, screens for treatable metabolic, genetic, and endocrine conditions. These include Phenylketonuria (PKU) and congenital hypothyroidism, which require early detection to prevent severe developmental problems.
Interpreting Results and Next Steps
Understanding the distinction between screening and diagnostic tests is important. A screening test, such as NIPT or the Anatomy Scan, only indicates an increased risk or likelihood of a condition being present. A positive screening result requires follow-up with a diagnostic procedure like CVS or Amniocentesis for confirmation.
A confirmed diagnosis initiates consultation with specialists, such as genetic counselors or maternal-fetal medicine doctors. These professionals provide detailed information on the condition, its prognosis, and management options. Early diagnosis allows parents and the medical team to create a specialized birth plan, involving necessary surgical teams or specialized care units immediately after delivery.