Down Syndrome Risk Calculator

Estimate your age-based risk of Down syndrome (trisomy 21) using published population data. Enter your age at delivery and gestational timing. Everything runs in your browser — your data stays private.

Per 10,000 Pregnancies

at this age would be affected

Unaffected Probability

chance of no Down syndrome
Lower risk Higher risk
Important: This calculator provides age-based risk estimates only, using published population data (Hook 1981, Morris et al. 2002). Your actual risk depends on individual screening results including nuchal translucency (NT) scan, maternal serum markers, and NIPT/cfDNA testing. This is not a diagnosis — consult your healthcare provider for personalized risk assessment.
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Understanding Down Syndrome Risk by Age

Down syndrome (trisomy 21) is the most common chromosomal condition, occurring when an extra copy of chromosome 21 is present. Maternal age is the single strongest known risk factor, with the probability rising significantly after age 35. At age 25, the risk is approximately 1 in 1,383 live births, while at age 40 it increases to about 1 in 100. This exponential increase is why prenatal screening is routinely offered to all pregnant individuals, with particular attention for those over 35.

The data used in this calculator comes from large population-based studies published by Hook (1981), Morris, Mutton, and Alberman (2002), and data compiled by the National Down Syndrome Society (NDSS). These figures represent well-established epidemiological estimates used by genetic counselors worldwide.

How This Calculator Works

You enter your age at the expected date of delivery and select the gestational timing. The calculator looks up the published risk for your age at birth, then adjusts for gestational age if needed. At 12 weeks, the estimated risk is approximately 43% higher than the live birth risk because about 30% of trisomy 21 pregnancies result in spontaneous loss between the first trimester and birth. At 16 weeks, the adjustment is about 25% higher, reflecting approximately 20% spontaneous loss between the second trimester and delivery.

For ages between published data points, the calculator uses linear interpolation to provide a smooth estimate. The result is displayed as a fraction (e.g., "1 in 356"), a percentage, and a per-10,000 context figure to help you understand the number in practical terms.

Screening Tests vs. Diagnostic Tests

This age-based estimate is just the starting point. Modern prenatal screening combines maternal age with ultrasound findings (nuchal translucency measurement) and blood markers to produce a much more personalized risk figure. Combined first-trimester screening detects about 85% of cases with a 5% false-positive rate. Non-invasive prenatal testing (NIPT), which analyzes cell-free fetal DNA in maternal blood, has detection rates above 99% with very low false-positive rates.

Diagnostic tests like chorionic villus sampling (CVS) at 10-13 weeks or amniocentesis at 15-20 weeks provide definitive answers by analyzing fetal chromosomes directly. These carry a small procedural risk (about 0.1-0.3%) and are typically offered when screening suggests elevated risk. Your healthcare provider can help you decide which tests are right for your situation.

What the Numbers Mean

A risk of "1 in 356" means that out of 356 pregnancies in people of that age, statistically one would be expected to have Down syndrome and 355 would not. Even at age 45, where the risk is about 1 in 30, the vast majority of pregnancies (97%) are unaffected. These numbers help inform decisions about further testing but should always be discussed with a qualified healthcare professional who can consider your complete medical picture.