Prenatal screening refers to the use of simple and non-invasive testing methods during pregnancy to identify fetal risks for severe genetic disorders and structural abnormalities, aiming to detect high-risk populations. The implementation of prenatal screening requires certain conditions to be fulfilled: (1) there must be clearly defined target disorders for the screening; (2) the target disorders should have a relatively high incidence in the population and serious consequences; (3) high-risk pregnancies identified through screening should have access to further prenatal diagnostic testing and effective interventions; (4) the screening methods should be simple, non-invasive, and acceptable to patients.
Prenatal screening tests are not diagnostic tests. A positive screening result indicates an increased risk of disease, but it does not confirm a diagnosis; similarly, a negative result suggests a lower risk but does not guarantee absence of disease. Patients with positive screening results should undergo further diagnostic testing, as screening results alone should not be the basis for decisions such as pregnancy termination. Additionally, prenatal screening and diagnostic testing must adhere to the principle of informed consent. Currently, prenatal screening is widely used for conditions such as chromosomal aneuploidies, neural tube defects, and structural abnormalities. In recent years, international guidelines have started recommending carrier screening for women during preconception or early pregnancy, to identify high-risk carriers of common and severe recessive genetic diseases.
Prenatal Screening for Chromosomal Aneuploidies
Approximately 8% of fertilized eggs exhibit chromosomal aneuploidies, with about 50% ending in early pregnancy loss. Chromosomal aneuploidies in live-borns, often accompanied by defects, occur in 0.64% of newborns. Chromosomal aneuploidies, exemplified by Down syndrome, are a primary focus of prenatal screening. Maternal age is closely related to the risk of fetal chromosomal aneuploidies; however, age-based screening has a high false-positive rate. Over the past two decades, advances in serum biochemical markers, fetal ultrasound genetic markers, and cell-free fetal DNA (cfDNA) in maternal plasma have improved the accuracy of early screening for chromosomal aneuploidies and reduced the need for unnecessary invasive diagnostic procedures. Common prenatal screening strategies for trisomy 21, trisomy 18, and trisomy 13 include:
First-Trimester Combined Screening
This approach includes assessment of nuchal translucency (NT) thickness via ultrasound and maternal serum markers such as pregnancy-associated plasma protein-A (PAPP-A) and free beta-human chorionic gonadotropin (β-hCG). Testing typically occurs between 11 and 13+6 weeks of gestation. Combined use of serum markers and NT screening achieves an 85% detection rate for Down syndrome with a 5% false-positive rate. NT measurement requires specialized technical training and robust quality control systems.
Second-Trimester Screening
Second-trimester screening uses serum biochemical markers such as alpha-fetoprotein (AFP), total or free β-hCG, and unconjugated estriol (uE3) in triple marker screening. Alternatively, inhibin-A may be added to create a quadruple screening panel. Risk calculation incorporates maternal age, gestational age, weight, and other factors. Testing typically takes place between 15 and 20 weeks of gestation. The detection rate for Down syndrome ranges from 60% to 75%, with a 5% false-positive rate. This method is also used for screening trisomy 18 and neural tube defects.
Integrated First- and Second-Trimester Screening
The integration of indicators from both first- and second-trimester screenings enhances detection rates and reduces false-positive results. Although integrated screening spans a longer timeframe, it may impose psychological burdens on pregnant women. There are three types of integrated screening approaches:
- Fully Integrated Screening: This method combines first-trimester measurements (PAPP-A, β-hCG, and NT between 10 and 13+6 weeks) with second-trimester quadruple testing (15–20 weeks). Six indicators are analyzed together to calculate the risk of Down syndrome. Compared to first-trimester screening alone at the same detection rate, fully integrated screening reduces false-positive rates.
- Serum Sequential Screening: This approach excludes NT and relies on serum markers, producing detection rates similar to those of first-trimester combined screening.
- Contingent Screening: Risk is assessed in stages, beginning with early screening. For those with an extremely high fetal risk for Down syndrome (≥1/50), chorionic villus sampling (CVS) is offered. Other individuals proceed to second-trimester quadruple testing for a comprehensive risk assessment.
Ultrasound-Based Genetic Marker Screening
Some ultrasound findings during routine scans at 11–13+6 weeks and 20–24 weeks are associated with chromosomal abnormalities and are referred to as ultrasound genetic markers or soft markers. These include increased NT thickness, absent nasal bone (first trimester), thickened nuchal fold, echogenic bowel, renal pelvis dilation, femoral or humeral shortening, intracardiac echogenic focus, and choroid plexus cyst (second trimester). A single soft marker suggests an increased risk for chromosomal abnormalities and should prompt consideration of fetal structural developmental issues or the presence of other markers. Risk assessment may combine ultrasound findings with biochemical or non-invasive screening results, and individuals at high risk for chromosomal abnormalities may consider further invasive diagnostic testing.
Non-Invasive Prenatal Testing (NIPT)
Non-invasive prenatal testing leverages cell-free fetal DNA in maternal plasma, analyzed using next-generation sequencing and bioinformatic techniques, to screen for common chromosomal aneuploidies. Testing is typically recommended from 12 weeks of gestation. NIPT is currently the most sensitive and specific technique for detecting trisomy 21, trisomy 18, and trisomy 13 in both singleton and twin pregnancies. It can serve as secondary screening for high-risk groups or as primary screening for low-risk populations. However, NIPT results may be influenced by maternal and placental factors. Positive high-risk results require genetic counseling and invasive prenatal diagnostic procedures rather than being used to directly guide decisions such as pregnancy termination.
Prenatal Screening for Neural Tube Defects
Neural tube defects (NTDs) are congenital structural abnormalities caused by failures in neural tube closure and are associated with severe morbidity, mortality, or disability. Approximately 95% of cases have no family history, but about 90% of affected pregnancies present with elevated levels of alpha-fetoprotein (AFP) in the maternal serum and amniotic fluid. Screening is typically conducted between 15 and 20 weeks of gestation, with results expressed in multiples of the median (MoM). The upper limit for normal AFP levels is commonly set between 2.0 and 2.5 MoM, achieving a sensitivity of over 90%. However, maternal serum AFP levels can be influenced by various factors, including gestational age, maternal weight, ethnicity, diabetes, fetal demise, multiple pregnancies, fetal abnormalities, and placental abnormalities, resulting in a positive predictive value of only 2%–6%. Additionally, 3%–5% of cases are due to closed NTDs, in which AFP levels remain within the normal range in amniotic fluid.
Advances in ultrasound technology have enabled the diagnosis of 99% of NTDs during mid-pregnancy ultrasounds. Consequently, when maternal serum AFP levels are elevated but ultrasound findings appear normal, the measurement of AFP through amniocentesis is no longer recommended.
Prenatal Screening for Fetal Structural Abnormalities
Prenatal ultrasound examination is currently the primary method of screening for fetal structural abnormalities. All pregnant individuals are encouraged to undergo mid-trimester ultrasound screening for fetal structural anomalies. This type of screening is typically performed between 20 and 24 weeks of gestation. Structural abnormalities that can be detected through this approach include anencephaly, severe encephalocele, severe open spina bifida, significant thoracoabdominal wall defects with organ protrusion, single-chamber heart, lethal skeletal dysplasias, and others. Detection rates for major fetal structural abnormalities during mid-trimester ultrasound range from approximately 50% to 70%. The main reasons for missed diagnoses include:
- Maternal Factors: These include gestational age, amniotic fluid volume, fetal position, and the thickness of the maternal abdominal wall.
- Low Detection Rates for Certain Structural Anomalies: Prenatal ultrasound has limited diagnostic sensitivity for some anomalies, such as atrial septal defects, ventricular septal defects, ear abnormalities, digital anomalies (e.g., finger or toe malformations), anal atresia, esophageal atresia, external genital abnormalities, and closed forms of spina bifida.
- Undetectable Structural Anomalies: Some fetal structural abnormalities, such as thyroid agenesis or Hirschsprung disease (congenital megacolon), are currently beyond the capabilities of ultrasound imaging to detect.
Additionally, certain structural anomalies emerge dynamically over time and may not be detectable during the screening process, only becoming evident with the progression of gestation or even postnatally after birth.