How Common Is Trisomy 18? Pregnancy and Birth Rates

Families researching trisomy 18 (also known as Edwards syndrome) often find conflicting numbers because studies measure its frequency at different points in time. Overall, when looking at early pregnancies (around the first trimester), trisomy 18 is found in roughly 15 out of every 10,000 pregnancies reaching this stage (about 1 in 650) ^[1]. However, when counting only live births, the number drops significantly to an average of about 1 in 10,000 ^[2], though some secondary estimates range between 1 in 3,000 and 1 in 10,000 ^[3][4]. When researchers look at “total prevalence”—a measurement that combines live births, stillbirths, and pregnancies ended by medical termination—the rate is typically around 4 to 5 per 10,000 ^[5][2].

Screening vs. Confirmed Diagnosis

If you have recently received abnormal test results, it is crucial to understand the difference between screening and diagnostic tests:

• Prenatal screening (such as cell-free DNA/NIPT or ultrasounds) estimates the chance that a fetus might have trisomy 18 ^[1].
• Diagnostic testing (such as chorionic villus sampling (CVS) or amniocentesis) analyzes cells from the placenta or amniotic fluid to confirm whether the extra chromosome is actually present.

The statistics below describe confirmed diagnoses. You should not use these population statistics to make decisions based on a screening result alone.

Why Fewer Cases Are Recorded at Live Birth

The significant difference between early pregnancy statistics and live birth statistics is primarily due to two factors: spontaneous fetal loss (miscarriage or stillbirth) and pregnancy termination.

Trisomy 18 is associated with severe chromosomal and congenital abnormalities that affect fetal growth and multiple organ systems. Because of these complex medical challenges, a large percentage of affected pregnancies naturally result in miscarriage or intrauterine fetal death (stillbirth) ^[6]. In one specific study of pregnancies with confirmed full trisomy 18 in a region where termination was not an option, 9% ended in miscarriage, 52% ended in stillbirth, and 37% resulted in a live birth (totaling 98% due to rounding) ^[6]. Another large registry in England and Wales found that for pregnancies with a confirmed diagnosis that were alive with a heartbeat at 12 weeks, there was an estimated 30% chance of the pregnancy continuing to a live birth ^[7].

Additionally, because of advanced prenatal screening, many cases are identified early in the first or second trimester ^[8][9]. Following a confirmed diagnosis, some families choose to end the pregnancy. Across multiple European registries between 2000 and 2011, about 73% of pregnancies diagnosed with either trisomy 13 or trisomy 18 ended in termination ^[5]. This combined historical figure reflects recorded decisions in specific regions, not a universal expectation or a medical recommendation.

Why Sources Report Different Statistics

If you are reading different medical websites or studies, you might see varying statistics. This happens because of how different health organizations track and report their data:

• What is Being Counted: A registry that only counts live births will report a much lower rate (about 1.07 per 10,000) than a registry that includes stillbirths and pregnancy terminations (about 4.08 to 5.0 per 10,000) ^[5][2].
• Maternal Age: The chance of a pregnancy having trisomy 18 gradually increases as the mother gets older ^[10][11]. Age 35 is often used as a reference point in studies, but the risk changes gradually and is not caused by anything the parents did.
• Registry Methods: Some health systems use “active case-finding” to track down every possible diagnosis by reviewing hospital records, which generally leads to higher reported numbers than passive systems that wait for doctors to submit reports ^[12].
• Regional Differences: Access to prenatal screening and local laws regarding pregnancy termination vary widely by country. These factors drastically change how many pregnancies with trisomy 18 result in a live birth in a specific region ^[2].

It is important to remember that these statistics describe broad population trends. They do not dictate the exact outcome for an individual pregnancy, nor do they account for differences between full, mosaic, or partial trisomy 18.