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
Unaffected Probability
The Down syndrome risk calculator is a free, browser-based tool that estimates the chance of a trisomy 21 pregnancy from maternal age at delivery and gestational timing, using published population data from Hook (1981) and Morris et al. (2002). It returns a risk as a fraction, a percentage, and a per-10,000 figure in under one second. Updated 2026-08-27.
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.
What to Do After You Get Your Down Syndrome Risk Result
An age-based number is a starting point, not a diagnosis, so the next step is the same whether your figure looks high or low: bring it to your OB-GYN or midwife at your next appointment and ask which screening you are eligible for. Per the American College of Obstetricians and Gynecologists, screening should be offered to every pregnant patient regardless of age, so you do not need an elevated number to request NIPT or a combined first-trimester screen. If a screening test then comes back high-risk, a diagnostic test (CVS or amniocentesis) is the only way to confirm. Ask for a referral to a genetic counselor before booking any invasive test.
How Common Is Down Syndrome in the United States?
Context helps interpret a personal risk figure. According to the U.S. Centers for Disease Control and Prevention, about 5,775 babies are born with Down syndrome in the United States each year, making it the most common chromosomal condition at birth. Because most births occur to people under 35, the majority of babies with Down syndrome are born to mothers in that younger group even though the per-pregnancy risk is far lower there. That is why a low age-based number never rules the condition out, and why screening is offered across all age bands rather than only after 35. Updated 2026-08-27.
Frequently Asked Questions
How accurate is this age-based risk calculator?
This calculator uses well-established population data from Hook (1981) and Morris et al. (2002), the same data used by genetic counselors. However, it provides only an age-based estimate. Your actual individual risk depends on screening test results (NT scan, blood markers, NIPT) which can significantly refine the estimate up or down.
Why does maternal age affect Down syndrome risk?
The risk increases with maternal age because older eggs are more likely to have errors in chromosome separation (nondisjunction) during cell division. This means the egg may end up with two copies of chromosome 21 instead of one, leading to trisomy 21 after fertilization.
What is the difference between screening and diagnostic tests?
Screening tests (NT scan, blood tests, NIPT) estimate probability and carry no procedural risk. Diagnostic tests (CVS, amniocentesis) analyze fetal chromosomes directly and provide definitive answers but carry a small risk (0.1-0.3%) of complications. Screening identifies who might benefit from diagnostic testing.
Why is the risk higher at 12 weeks than at birth?
Approximately 30% of pregnancies with trisomy 21 end in spontaneous loss (miscarriage or stillbirth) between the first trimester and delivery. So the prevalence at 12 weeks is higher than at birth, because some affected pregnancies will not continue to term.
Is my data saved or sent anywhere?
No. All calculations happen entirely in your browser using JavaScript. No data is sent to any server. We do not collect, store, or transmit your inputs. Your privacy is completely protected.
Should I be worried if my risk seems high?
Even a risk of 1 in 100 means there is a 99% chance the pregnancy is unaffected. These numbers are meant to help you and your healthcare provider make informed decisions about further testing, not to cause alarm. Speak with a genetic counselor or your OB-GYN for personalized guidance.
How common is Down syndrome overall?
The CDC reports that about 5,775 babies are born with Down syndrome in the United States each year, making it the most common chromosomal condition at birth. Because most births happen to people under 35, most babies with Down syndrome are born to mothers in that younger group even though the per-pregnancy risk is much lower there. Source: U.S. Centers for Disease Control and Prevention.
What should I do after getting my risk result?
Take the number to your OB-GYN or midwife and ask which screening you are eligible for. ACOG recommends offering prenatal screening to every pregnant patient regardless of age, so you do not need a high number to request NIPT or a combined first-trimester screen. If a screening test comes back high-risk, ask for a referral to a genetic counselor before booking a diagnostic test such as CVS or amniocentesis.