This guide outlines the prenatal diagnostic techniques available today for detecting fetal abnormalities. But before any test, parents need clear, honest information so they can make a genuine choice—including the choice not to test at all. That means understanding not just how the tests work, but what the results mean and what options follow.
Non-invasive techniques
Biochemical screening: A blood test that estimates the risk of genetic abnormalities. It gives a more accurate individual risk assessment for Down syndrome and spina bifida than maternal age alone. Remember: this test estimates risk. It does not tell you whether your baby has or does not have these conditions.
Fetal ultrasound: Uses sound waves to create images of the developing fetus.
In pregnancy, three main ultrasounds are typically performed:
- The first, in the first trimester, confirms that the pregnancy is in the right place in the uterus.
- The second, called the anatomy scan, happens in the second trimester (between 20 and 22 weeks). This is a detailed fetal examination that checks normal development and identifies the most common abnormalities (about 90%).
- The third, performed in the third trimester, confirms the absence of new abnormalities, re-evaluates fetal size, checks amniotic fluid volume and placental position, and helps determine whether vaginal delivery is possible.
The nuchal translucency measurement (between 11 and 14 weeks) deserves special mention. By measuring the thickness of the fold at the back of the fetal neck, it identifies additional risk factors for chromosomal abnormalities and other conditions like heart defects.
Invasive techniques
These are offered to women at higher risk of chromosomal or genetic abnormalities—those over 35, those with a family history, or those with positive screening results. It's crucial to understand that these procedures carry real risks, which you should discuss carefully with your doctor. The expertise of the specialists performing these tests and interpreting their results is essential for getting accurate results with the lowest possible complication rate.
Amniocentesis: Performed in the second trimester, this involves inserting a needle through the abdomen to draw a sample of amniotic fluid. It can diagnose chromosomal abnormalities, sex-linked genetic disorders (like fragile X syndrome), metabolic disorders, single-gene inherited conditions (like muscular dystrophies), and definitively diagnose neural tube defects like spina bifida.
CVS (chorionic villus sampling): Performed in the first trimester (between 10 and 12 weeks), this involves taking a tissue sample from the placenta. Like amniocentesis, it can diagnose chromosomal abnormalities and metabolic disorders, but it's especially useful for single-gene disorders such as thalassemia, Becker and Duchenne muscular dystrophy, cystic fibrosis, and hemophilia A and B. Compared to amniocentesis, CVS has a higher rate of sampling failures, more false positives and negatives, and a higher risk of limb malformations, but a lower miscarriage risk than amniocentesis performed before 15 weeks.
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