MATERNAL AGE

All pregnant women have a risk of birth defects, including Down syndrome and other chromosome abnormalities. Most women don't know what their risk is, however, and may be surprised to learn what it is. The overall risk for all maternal ages in the U.S. is approximately 1 in 500 pregnancies at the time of a 2nd trimester ultrasound, based on the maternal age distribution. This risk has increased from 1 in 700 two decades ago, reflecting the trend toward delayed child birth.. Of course, the individual risk varies with the individual patient's age. The risk also decreases with gestational age because there is a certain 'attrition rate' due to the increased risk of spontaneous intrauterine death among fetuses with major chromosome abnormalities.

The table below summarizes the risk of fetal Down syndrome based on both maternal age and gestational age. The risk is reported as 1 in __. For example, a 30 year old woman has an approximate risk of 1:703 at 16 weeks.

   

Gestational Age (Weeks)

  10 12 14 16 20 40
20 983 1068 1140 1200 1295 1527
25 870 946 1009 1062 1147 1352
30 576 626 668 703 759 895
31 500 543 580 610 658 776
32 424 461 492 518 559 659
33 352 383 409 430 464 547
34 287 312 333 350 378 446
35 229 249 266 280 302 356
36 180 296 209 220 238 280
37 140 152 163 171 185 218
38 108 117 125 131 142 167
39 82 89 95 100 108 128
40 62 68 72 76 82 97
41 47 51 54 57 62 73
42 35 38 41 43 46 55
43 26 29 30 32 35 41
44 20 21 23 24 26 30
45 15 16 17 16 19 23