What maternal age related risk equation does αlpha use?

The risk at term of an individual woman having a Down's syndrome pregnancy is given by (1,2,3) :

Age related risk

Where age is the woman's age in years at the expected date of delivery. The age is expressed as a decimal fraction, so for example 25 years 6 months is 25.5 years.

Second trimester Down's syndrome risks are approximately 23% higher than term risk on account of the selective fetal loss of Down’s syndrome pregnancies. In the first trimester, the risk of Down’s syndrome is approximately 43% higher than at term (3).

The following table shows the risk at term corresponding to the age at expected date of delivery.

Age at expected date of delivery (years) Risk at term
15 1/1516
16 1/1512
17 1/1508
18 1/1502
19 1/1494
20 1/1484
21 1/1470
22 1/1452
23 1/1430
24 1/1400
25 1/1362
26 1/1315
27 1/1255
28 1/1182
29 1/1095
30 1/994
31 1/881
32 1/759
33 1/634
34 1/513
35 1/403
36 1/308
37 1/231
38 1/172
39 1/129
40 1/98
41 1/76
42 1/61
43 1/50
44 1/43
45 1/38
46 1/34
47 1/31
48 1/29
49 1/28
50 1/27
51 1/26
52 1/25
53 1/25
54 1/25
55 1/24

References

1. Revised estimates of the maternal age specific live birth prevalence of Down’s syndrome (2002) 
2. Corrections to maternal age-specific live birth prevalence of Down's syndrome (2005)
3. Estimating the risk of Down's syndrome in antenatal screening and the gestation at which this risk applies (2007)