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Prevalence of Chromosome Differences

Prevalence of Trisomy 21 in Singleton and Twin Pregnancies

Age (years) 1Singleton Dizygotic twins 2Monozygotic twins 3
20 1/1484 1/1107 1/4364
21 1/1470 1/1097 1/4324
22 1/1452 1/1084 1/4272
23 1/1430 1/1067 1/4205
24 1/1400 1/1045 1/4118
25 1/1362 1/1017 1/4007
26 1/1315 1/981 1/3866
27 1/1255 1/937 1/3691
28 1/1182 1/882 1/3477
29 1/1095 1/817 1/3221
30 1/994 1/742 1/2924
31 1/881 1/657 1/2590
32 1/759 1/566 1/2231
33 1/634 1/473 1/1864
34 1/513 1/383 1/1509
35 1/403 1/301 1/1185
36 1/308 1/230 1/906
37 1/231 1/173 1/681
38 1/172 1/129 1/507
39 1/129 1/96 1/379
40 1/98 1/73 1/288
41 1/76 1/57 1/224
42 1/61 1/45 1/179
43 1/50 1/38 1/148
44 1/43 1/32 1/126
45 1/38 1/28 1/111

 

1. Refers to maternal age at expected time of birth. In the case of an IVF pregnancy, maternal age is guided by the age at egg retrieval (whether own egg or donor egg)

2. Dizygotic twins: Both twins originate from a separate oocyte and each oocyte is fertilized by its own spermatocyte. All dizygotic twins are dichorionic/diamniotic.

3. Monozygotic twins: Both twins develop from a single zygote which then cleaves to form two embroys. Majority of monozygotic twins are monochorionic/diamniotic. Depending on the timing of the cleavage, monozygotic twins may be dichorionic/diamniotic or monochorionic/monoamniotic.

Prevalence of Trisomy 21 and Trisomy 18 in Singleton Pregnancies 

Age (years) *Trisomy 21Trisomy 18
 21  1/1470 1/15291
22 1/1452 1/15038
23 1/1430 1/14684
24 1/1400 1/14245
 25 1/1362 1/13699
26 1/1315 1/13055
27 1/1255 1/12270
28 1/1182 1/11351
29 1/1095 1/10341
30 1/994

1/9234

31 1/881 1/8084
32 1/759 1/6935
33 1/634 1/5831
34 1/513 1/4812
35 1/403 1/3905
36  1/308 1/3121
37 1/231 1/2463
38 1/172  1/1923
39 1/129 1/1489
40 1/98 1/1145
41 1/76 1/875
42 1/61 1/667
43 1/50  1/506
44 1/43 1/383
45 1/38 1/289

*Refers to maternal age at expected time of birth. In the case of an IVF pregnancy, maternal age is guided by the age at egg retrieval (whether own egg or donor egg).

Tables Generated Based on Data From:

1. Morris JK, Wald NJ. Estimating the risk of Down's syndrome in antenatal screening and the gestation at which this risk applies. J Med Screen. 2007;14(1):5-7. doi: 10.1258/096914107780154549. PMID: 17362564.

2. Boyle B, Morris JK, McConkey R, Garne E, Loane M, Addor MC, Gatt M, Haeusler M, Latos-Bielenska A, Lelong N, McDonnell R, Mullaney C, O'Mahony M, Dolk H. Prevalence and risk of Down syndrome in monozygotic and dizygotic multiple pregnancies in Europe: implications for prenatal screening. BJOG. 2014 Jun;121(7):809-19; discussion 820. doi: 10.1111/1471-0528.12574. Epub 2014 Feb 4. PMID: 24495335; PMCID: PMC4234000.

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