Down Syndrome Chance by Age
The chance of a baby being born with Down syndrome rises with the age of the woman giving birth. This is the starting point that every screening test then adjusts.[1]
Every age
Chance of a baby born with Down syndrome, by the woman's age at delivery. Your age is highlighted. Ages above 45 are shaded because the published estimates disagree there.[1][2]
What this number is, and is not
It is the chance at birth. It comes from the National Down Syndrome Cytogenetic Register for England and Wales, which recorded nearly all cases diagnosed before or after birth from 1989 to 1998. The researchers estimated how many babies with Down syndrome would be born at each age if there were no prenatal screening and no terminations.[1]
The chance is higher earlier in pregnancy. Pregnancies with Down syndrome are more likely to end in miscarriage. On average, 32% of them would miscarry between the time of chorionic villus sampling (CVS) in the first trimester and term, and 25% between amniocentesis and term. The loss rate rises with age: from the time of CVS, 23% at age 25 and 44% at age 45.[3] So the chance at the time of first-trimester screening is higher than the number shown here.
Above 45, the estimates differ. Published models agree closely up to age 35, are reasonably similar from 35 to 45, but differ above 45.[2] The register data also showed that the chance does not keep rising faster and faster after 45, as older models assumed.[1]
Age is not a test. ACOG and SMFM recommend that every patient, at any age, be offered both screening and diagnostic tests, including cell-free DNA screening, and that every patient has the right to accept or decline them.[4]
Other trisomies are less common. In England and Wales from 1997 to 2004, in the absence of screening and termination, trisomy 18 occurred in 2.3 and trisomy 13 in 1.4 per 10,000 births.[5]
The math behind it
The register data follow an S-shaped curve: a low, nearly flat chance in the twenties, a steep rise through the late thirties, and a slower rise after 45. The researchers fitted this formula to it:[1]
1 in N is simply 1 divided by the chance. A chance of 0.0028 is 1 in 354, or 0.28%.