Chapter 6. Chapter 6 Graphic Content

The probability that a 35-year-old woman giving birth will have a baby with Down syndrome is approximately 1 in 333 (i.e., 3/1,000).

A woman of age 45 giving birth is unlikely to have a baby with Down syndrome. In fact, according to the data in the graph, a woman of age 45 giving birth has a 98.1% probability of having a baby that does not have Down syndrome. This is because the incidence, according to the figure, is approximately 1.9% (19 in 1,000).

You cannot conclude from the figure that most babies with Down syndrome are born to mothers over 35. The figure shows only, for mothers of a given age, the proportion of babies born with Down syndrome. But because relatively few of the babies born in any year are born to mothers older than 35, the total number of babies with Down syndrome in this group of mothers is small. In fact, since so many more babies are born to women under 35, it turns out that 80% of babies with Down syndrome are born to mothers under 35.

From the graph it is not possible to determine the chances of having a baby with Down syndrome after age 45. This may be due to lack of data, since a much smaller number of women have children past the age of 45. Additionally, a pregnant female may have a greater chance of having a miscarriage if a fetus has Down syndrome, making it difficult to determine the actual rate. Note: Some research indicates that the risks are slightly reduced after age 47 but more studies are needed.

Based on doctors’ recommendations, women aged 35 and higher are likely to have amniocentesis, which will reveal whether a fetus has trisomy-21. Conversely, amniocentesis is not recommended for women under 35. As a consequence, women aged 35 and higher are more likely to learn that a baby will be born with Down syndrome and choose to terminate the pregnancy. This prenatal testing therefore reduces the incidence of babies born with Down syndrome among women 35 and over, but not among women under 35. Among women under 35, the rate of babies born with Down syndrome is much more likely to be almost the same as the incidence of fetuses having trisomy-21.