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Conditions screened

Prenatal screening is used to detect three conditions:
  1. Down syndrome (also called trisomy 21)
  2. open spina bifida (open neural tube defects).
Down syndrome and trisomy 18 are due to chromosomal problems.  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.
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 women age.
Risk of Chromosome Abnormalities in Liveborn Infants at Term by Maternal Age

Maternal Age (yrs)

Risk of Down Syndrome

Risk of ANY Chromosome Abnormalities

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.