The most common risk factor for Down syndrome is
maternal age. As a woman gets older, her risk to have a pregnancy with a
chromosome abnormality increases. Maternal age risks are often presented in
chart format such as the chart below. In order to really understand the risk
figures presented in this chart, it is important to understand how the data was
collected and what it is actually telling you.
Why Do Maternal Age-related Risk Charts Vary?
You may have seen different versions of maternal
age-related risk charts and you may have noticed that there are differences in
the risks numbers presented. There are a couple of reasons for these
variations. Some charts present the maternal age-related risk for all
chromosome abnormalities rather than age-related risk for Down syndrome. This
chart presents both pieces of information. The middle column below shows the
age-related risk to have a child with Down syndrome (trisomy 21) and the
right-hand column shows the age-related risk for for all chromosome related
abnormalities (trisomy 21 and other trisomies - 13, 18, X and Y).
Maternal Age-related Risks at Birth and at the Time of Prenatal Diagnosis
One other factor that accounts for
some of the differences seen in age-related risk charts is that some charts
present the chance to have a child with Down syndrome at birth (full-term), and
other charts present the chance at the time of prenatal diagnosis (early second
trimester). Since up to 25% of pregnancies with Down syndrome are miscarried or
stillborn, and over 70% of pregnancies with trisomy 13 and trisomy 18 are lost
before birth, the age-related risks at the time of prenatal diagnosis are
higher than the age-related risk at birth. The chart below presents maternal
age-related risks at birth.
Age
at the Time of Conception versus Age at Delivery
Many women wonder why we talk about the age of an egg
at conception, but use an age-related risk based upon your age at the time of
delivery. This is because of how the studies were done to collect this data.
Most of the data came from looking at the diagnosis rate of infants at the time
of delivery. Because of premature births and miscarriages, it was not possible
to accurately go backwards nine months and come up with a maternal-age
conception-related risk to have a child with a chromosome abnormality. The
chart below gives you your risk to have a child with a chromosome abnormality
based upon your age at the time of delivery.
What
this Maternal Age-Related Risk Chart Tells You
This chart tells what your chance to have a child with
Down syndrome, and your chance to have a child with any chromosome abnormality
is, based on a mother’s age at the time of delivery. For example, if you (the
mother) will be 39 years old at the time of delivery, the chance to have a
child diagnosed with Down syndrome at birth is 1 in 137 (a little less than
1%), the chance to have a child diagnosed with any chromosome abnormality
(including Down syndrome) at birth is 1 in 83 (or slightly higher than 1%).
Who
should NOT use this chart?
If you have had a previous child or pregnancy with
trisomy 21 or another trisomy, you risk may be different than the age-related
risks presented in this chart. If you have a family history of a chromosome translocation
or a previous child with translocation Down syndrome, you should consult with a
genetic counselor who can give you more specific information about your risk
depending on the type of translocation and your chromosome results. If you have
conceived a pregnancy using a donor egg, you should use the age of the woman
that donated the egg at delivery. In all of these situations, it is best to
talk to a genetic counselor to get an accurate assessment of your risks.
Maternal Age Related Risks
|
Mom's Age
|
Risk for trisomy 21 (Down syndrome)
|
Risk for
all triomies
|
|
20
|
1 in 1,667
|
1 in 526
|
|
21
|
1 in 1,429
|
1 in 526
|
|
22
|
1 in 1,429
|
1 in 500
|
|
23
|
1 in 1,429
|
1 in 500
|
|
24
|
1 in 1,250
|
1 in 476
|
|
25
|
1 in 1,250
|
1 in 476
|
|
26
|
1 in 1,176
|
1 in 476
|
|
27
|
1 in 1,111
|
1 in 455
|
|
28
|
1 in 1,053
|
1 in 435
|
|
29
|
1 in 1,000
|
1 in 417
|
|
30
|
1 in 952
|
1 in 384
|
|
31
|
1 in 909
|
1 in 384
|
|
32
|
1 in 769
|
1 in 323
|
|
33
|
1 in 625
|
1 in 286
|
|
34
|
1 in 500
|
1 in 238
|
|
35
|
1 in 385
|
1 in 192
|
|
36
|
1 in 294
|
1 in 156
|
|
37
|
1 in 227
|
1 in 127
|
|
38
|
1 in 175
|
1 in 102
|
|
39
|
1 in 137
|
1 in 83
|
|
40
|
1 in 106
|
1 in 66
|
|
41
|
1 in 82
|
1 in 53
|
|
42
|
1 in 64
|
1 in 42
|
|
43
|
1 in 50
|
1 in 33
|
|
44
|
1 in 38
|
1 in 26
|
|
45
|
1 in 30
|
1 in 21
|
|
46
|
1 in 23
|
1 in 16
|
|
47
|
1 in 18
|
1 in 13
|
|
48
|
1 in 14
|
1 in 10
|
|
49
|
1 in 11
|
1 in 8
|
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