What Is Prenatal Diagnostic Testing?
Prenatal diagnostic testing uses invasive procedures during pregnancy to provide a definitive diagnosis of certain chromosomal, genetic, or structural conditions in the fetus. These tests are usually recommended when:
– A screening test (such as NIPT/cell-free DNA) shows a high-risk result
– Ultrasound detects structural abnormalities
– One or both parents are known carriers of a genetic condition (for example, thalassemia)
– The pregnancy has other high-risk features
Unlike screening tests, which only estimate risk, diagnostic tests analyze actual fetal or placental cells and give clear, confirmed results. This information helps families understand the condition, its prognosis, and options for pregnancy management or care after birth.
Screening Tests vs. Diagnostic Tests
Screening tests (such as NIPT or first-trimester combined screening) estimate the chance that the fetus has a condition like Down syndrome. A “low-risk” result greatly reduces concern but does not completely rule out the condition. A “high-risk” result means a higher chance but is not a diagnosis — false positives and false negatives can occur.
Diagnostic tests provide definitive answers and are offered when screening results are abnormal, ultrasound findings are concerning, or parents carry a known genetic condition.
Main Types of Prenatal Diagnostic Procedures
The choice of procedure depends on gestational age, the reason for testing, and your individual situation. All are performed under continuous ultrasound guidance by experienced specialists.
Amniocentesis
This is the most common diagnostic test. It is usually performed between 15 and 20 weeks of pregnancy (and can be done later if needed).
A thin needle is inserted through the mother’s abdomen into the amniotic sac to withdraw a small amount of fluid containing fetal cells. The fluid is quickly replaced by the body.
When performed by experienced operators, the additional procedure-related risk of miscarriage is very low — approximately 0.3%. Minor cramping or spotting may occur but usually resolves quickly.
Chorionic Villus Sampling (CVS)
CVS samples placental tissue (chorionic villi). It is performed earlier, typically between 11 and 13+6 weeks, either through the abdomen or cervix depending on placental position
The major advantage is that it provides results earlier in the pregnancy, which allows more time for decision-making.
However, the additional miscarriage risk is slightly higher than amniocentesis (around 0.5-1% in experienced centers). There is a small chance (1–2%) of confined placental mosaicism, which may require further testing for clarification. In some settings, including parts of Thailand, CVS availability is limited to specialized centers.
Cordocentesis (Percutaneous Umbilical Blood Sampling)
This procedure samples fetal blood directly from the umbilical cord and is usually performed after 18 weeks. It is technically more demanding and reserved for specific situations, such as diagnosing certain infections, blood disorders (e.g., severe anemia), or when other tests are insufficient.
Cordocentesis carries a higher additional miscarriage risk (approximately 1% or more depending on the indication and fetal condition) along with small risks of preterm labor or temporary changes in fetal heart rate. It is used only when necessary.
Counseling and Preparation
Before any procedure, you will receive detailed counseling about the reason for testing, benefits, risks, limitations, and possible outcomes. The decision is always personal and based on shared decision-making that respects your values and goals.
Your doctor will review your medical history, ultrasound findings, screening results, blood type, and Rhesus status, and may screen for infections that could affect the procedure.
Key Takeaway
Prenatal diagnostic testing can provide clear answers when needed, but it is always optional. Discuss your questions thoroughly with your Maternal–Fetal Medicine specialist so you can make the choice that feels right for your family.
References
1. American College of Obstetricians and Gynecologists. Prenatal genetic diagnostic tests. ACOG FAQ. Washington, DC: ACOG; 2025.
2. American College of Obstetricians and Gynecologists. Prenatal diagnostic testing for genetic disorders. ACOG Practice Bulletin No. 162. Obstet Gynecol. 2016;127(5):e108-22. (Reaffirmed with updates).
3. Ghi T, Sotiriadis A, Calda P, et al.; ISUOG Clinical Standards Committee. ISUOG Practice Guidelines: invasive procedures for prenatal diagnosis. Ultrasound Obstet Gynecol. 2016;48(2):256-268. doi:10.1002/uog.15945.
Maternal–Fetal Medicine Specialist in Bangkok
Dr. Narisra Srikureja Firer (Dr. Niti)
Dr. Niti is a Bangkok-based OBGYN with subspecialty training in Maternal–Fetal Medicine (MFM). She provides comprehensive care for women across all stages of life, including pregnancy care, deliveries, and general gynecological conditions.
She dedicates her time to private practice at Ruamjairak Hospital and Mission Hospital, and serves as a Maternal–Fetal Medicine Consultant at a large government tertiary care center.
To read Dr. Niti’s full bio, click here.
