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Conditions screened

Chromosomes are instructions found in every cell in our body.  Since we have millions of cells in our bodies, we have millions of copies of our chromosomes as well.  Typically, individuals have two copies of each chromosome in every cell.  There are 23 chromosomes and therefore, 46 paired chromosomes in each cell.
Chromosomes are found in pairs. Humans have 23 pairs of chromosomes, 22 that are numbered according to size and the 23rd pair determining gender. XX for female, XY for male.
When an egg or sperm are created, the chromosomes need to line up and divide as each egg or sperm only requires 23 chromosomes.  Sometimes during this process the chromosomes do not line up or divide properly.  This can lead to too many chromosomes (such as a trisomy – 3 copies of a particular chromosome instead of 2 copies) or too few chromosomes (monosomy – 1 copy of a particular chromosome).  Having too many or too few chromosomes can lead to problems with how our bodies grow and develop, and can sometimes lead to spontaneous pregnancy losses.
Down syndrome and trisomy 18 are two of the more common chromosome problems that can occur during the creation of the egg or the sperm.  There are many other chromosome problems that can happen but many of them will result in a pregnancy loss.
Having too much or too little chromosome material usually occurs by chance alone.  It also typically happens well before conception.  There is nothing that an individual does or does not do to cause these types of problems.  We do not know what causes these errors to occur, but we know that they happen more often in the eggs as a female ages.
Prenatal screening provides a pregnant person with their individual risk of having a child with a chromosome difference. It does not screen for all chromosome abnormalities, so some may be missed.

Down syndrome

Babies and children with Down syndrome are individuals who are just as unique and highly variable as anyone else.  An individual with Down syndrome has an extra copy of chromosome 21 (trisomy 21).  This usually causes mild to moderate intellectual disability, which means that adults will typically function at the level of an 8-10 year old.  An individual with Down syndrome is expected to learn to read and write and to be physically active, however major developmental milestones will be delayed.  Individuals with Down syndrome have a greater possibility of health conditions than the average person, such as heart, stomach, bowel, thyroid, vision and hearing problems.  Treatment is available for many of these conditions. There is no way to predict how serious or mild these differences will be. People with Down syndrome generally live into their fifties. In general, about one in 1000 babies born has Down syndrome. The chance of having a child with Down syndrome increases with the pregnant person/egg donor’s (in the case of a donated egg) age (See Table below for specific information).  For more information on Down syndrome, visit the Canadian Down Syndrome Society website.

Trisomy 18 (Edward syndrome)

Babies with trisomy 18 have an extra chromosome 18. Every individual with trisomy 18 is different, however serious intellectual disabilities and congenital anomalies that may affect many organ systems (e.g. heart, kidneys) are expected. Many pregnancies with trisomy 18 will miscarry.  Most babies born with trisomy 18 do not survive past the first few months of life.  About 10% of infants will survive up to 5 years of age, or occasionally longer. Long-term survivors are described as socially interactive with significant physical and intellectual disabilities (e.g. having few words).  In general, about one in 6,000 babies born has trisomy 18. The chance of having a child with trisomy 18 increases with the pregnant person/egg donor’s age.  For more on trisomy 18, visit The Trisomy 18 Foundation website.

Additional conditions which may be screened for by prenatal screening:

Trisomy 13 (Patau syndrome)

Babies with trisomy 13 have an extra chromosome 13. In general between 1/8,000 and 1/15,000 of babies born has trisomy 13. The chance of having a child with trisomy 13 increases with a pregnant person/egg donor’s age. Every individual with trisomy 13 is different; however each individual is expected to have both serious intellectual disabilities and congenital anomalies, including differences of the brain, heart, eyes and face.  Most pregnancies with trisomy 13 will miscarry.  About 5-10% of infants will survive the first year of life.  There are some individuals who have lived to adulthood. For information on trisomy 13, visit Support Organization for Trisomy 18, 13 and Related Disorders (SOFT) website.

Sex chromosome differences

Typically, each mature human cell contains 23 chromosome pairs (46 chromosomes in total).  One of these chromosome pairs, the 23rd pair, is the sex chromosomes.  A female cell will usually have two X chromosomes and a male cell will have one X chromosome and one Y chromosome.  Some prenatal screening and diagnostic testing can identify extra or missing X and/or Y chromosomes.  There are several disorders with sex chromosome differences.  Generally, these types of genetic differences are quite common occurring in about 1 in 500 individuals.  Expected features are variable and depend on which chromosome is extra or missing.  Some features may be as mild as tall or short stature.  There may be differences in development such as delayed speech or learning differences.  There may be associated health issues involving puberty or the heart. For a brief overview of chromosomes and some chromosome disorder visit the Genetics Home Reference

Chance of Chromosome Abnormalities in Liveborn Infants at Term by Pregnant Person’s Age

                Pregnant Person/                Donor’s Age (yrs)

Chance of Down Syndrome

Chance of ANY Chromosome Abnormality

20

1/1,650

1/530

21

1/1,650

1/530

22

1/1,430

1/500

23

1/1,430

1/500

24

1/1,250

1/480

25

1/1,250

1/480

26

1/1,175

1/480

27

1/1,110

1/450

28

1/1,050

1/430

29

1/1,000

1/420

30

1/950

1/390

31

1/900

1/390

32

1/770

1/320

33

1/625

1/285

34

1/500

1/240

35

1/385

1/180

36

1/300

1/150

37

1/225

1/125

38

1/175

1/100

39

1/135

1/80

40

1/100

1/65

41

1/80

1/50

42

1/60

1/40

43

1/50

1/30

44

1/40

1/25

45

1/30

1/19

46

1/23

1/15

47

1/18

1/11

48

1/14

1/9

49

1/11

1/7

Adapted from:
Hook EB, Cross PK, Schreinemachers DM,  Chromosomal abnormality rates at amniocentesis and in live-born infants. JAMA 1983; 249: 2034-38.
Hook EB. Rates of chromosomal abnormalities at different maternal ages. Obstet Gynecol 1981; 53: 282-85.