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Down Syndrome Risk by Maternal Age Calculator

See the average chance of having a baby with Down syndrome (trisomy 21) by the mother’s age at delivery, at birth and at 16 weeks of pregnancy, from the classic Hook maternal-age table.

Mother’s age at delivery

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How it works

Chance = 1 ÷ N from the maternal-age table (Hook 1983) · percentage = 100 ÷ N · the chance at 16 weeks is higher than at birth because some affected pregnancies miscarry

The chance of trisomy 21 rises with the mother’s age because errors in egg cell division become more common. Hook’s 1983 analysis of live births (as summarized by Thompson & Thompson) gives about 1 in 1,100–1,400 in the twenties, 1 in 350 at 35, 1 in 100 at 40 and 1 in 25 at 45 or older. The chance at 16 weeks — when amniocentesis is done — is higher because about a quarter to a third of affected pregnancies end in miscarriage. Age alone is a poor screening tool: ACOG recommends offering every pregnant person screening (combined first-trimester test or cell-free DNA) and diagnostic testing regardless of age.

Worked example

At a maternal age of 35 at delivery, the table gives a chance at birth of 1 in 350, which is 100 ÷ 350 = 0.29% — so 99.71% of babies are born without Down syndrome. At 16 weeks of pregnancy the chance is higher, 1 in 250.

Sources: Hook EB (1983) Down syndrome rates and relaxed selection at older maternal ages. JAMA 249:2034–2038 — table as summarized by DS-Health · ACOG Practice Bulletin No. 226: Screening for fetal chromosomal abnormalities (2020)

Frequently asked questions

What is the risk of Down syndrome at 35?

About 1 in 350 at birth (0.29%), and about 1 in 250 at 16 weeks of pregnancy. Age 35 was historically used as the cut-off for offering amniocentesis, but today screening is offered to everyone.

What is the risk of Down syndrome at 40?

About 1 in 100 births (1%) at 40, rising to about 1 in 25 at 45 or older. Even at 40, 99 out of 100 babies are born without Down syndrome.

Does the father’s age matter?

Most cases of trisomy 21 come from the egg, so maternal age is by far the main factor. A father aged over 40–45 may add a small extra risk, but the effect is much weaker.

How can I know my individual risk?

Screening tests — the first-trimester combined test (nuchal translucency plus blood markers) or cell-free fetal DNA (NIPT) — give a personal risk. Only diagnostic tests such as chorionic villus sampling or amniocentesis can confirm the diagnosis.

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This calculator is for education only and does not replace your midwife, obstetrician or doctor. Contact your care team straight away about bleeding, fluid leaking, reduced baby movements, severe pain, fever or anything that worries you.