Conditions Screened

What are Chromosome Differences?

Inside each of the cells in our body, there are string-like structures called chromosomes which package all of our DNA. DNA contains genes that instruct our body how to grow, develop and function on a day to day basis. Some people have extra or missing chromosomes, which means that they have a chromosome difference. When the unborn baby has a chromosome difference there may be long term effects on health and learning. Sometimes having a chromosome difference can have severe effects and results in the loss of the pregnancy.

How Often do Chromosome Differences Happen?

Two chromosome differences that can happen are trisomy 21 (Down syndrome) or trisomy 18 (Edwards syndrome).
Any pregnant person has a chance to have a baby with trisomy 21 or trisomy 18. This chance increases with the age of the mother (or the age of egg donor).
Trisomy 21 and trisomy 18 usually happen by chance in the sperm or the egg that made the pregnancy, and do not generally run in families.

Chance of Chromosome Differences based on Maternal Age
Maternal Age/ Age of Egg Donor *	Chance of Trisomy 21 (at birth)	Chance of Trisomy 18 (at birth)
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

*Ages at expected time of birth.

Adapted from:

JK Morris and NJ Wald. J Med Screen 2007;14:5-7

Which Chromosome Differences does Prenatal Screening Test for?

Currently, multiple marker screening in Ontario (eFTS and MSS) will offer information about:

trisomy 21 (Down syndrome)
trisomy 18

NIPT that is funded by the Ministry of Health (MOH) or paid for out of pocket will offer information about:

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