Common Birth Anomalies

Trisomy 21

Trisomy 21, also called Down syndrome, is a genetic condition that occurs at conception when the fetus receives three copies of chromosome 21 instead of two. Trisomy 21 gives rise to intellectual and physical disabilities for which it is impossible to predict the degree of severity. Trisomy 21 occurs in about 1 in 800 births1, however the incidence rate varies significantly with age. As a woman gets older, her risk of having a pregnancy with a chromosome abnormality increases. However, any woman, regardless of her age, has a risk of having an affected pregnancy. According to the Canadian Down Syndrome Society, Down Syndrome occurs in about 1 in 700 births, however the incidence rate varies significantly by age.

Maternal Age and Risk of Trisomy 21
Maternal Age Risk of Trisomy 21 at Birth
20 1 in 1,530
25 1 in 1,350
30 1 in 900
32 1 in 660
34 1 in 450
35 1 in 360
36 1 in 280
38 1 in 170
40 1 in 100
42 1 in 55
44 1 in 30
         Source: Fetal Medicine Foundation

According to the Canadian Down Syndrome Society, 80% of babies
born with a trisomy are from mothers aged 35 years and under.

Trisomy 18

Trisomy 18, also called Edwards syndrome, is a genetic condition that occurs at conception when the fetus receives three copies of chromosome 18 instead of two. In most cases, the fetus does not survive to birth. It is estimated that trisomy 18 occurs in approximately 1 out of every 6,000 births2. The incidence rate of trisomy 18 also increases with maternal age.

Open neural tube defects*

Open neural tube defects are birth defects that affect the head and spine of the baby. They occur when the neural tube (the structure that later develops into the brain and spinal cord) remains open instead of closed during embryonic growth in the first month of pregnancy. Two common types of open neural tube defects are anencephaly and spina bifida. In Quebec, the incidence of neural tube defects is estimated to be 4 per 1,000 births.3

Anencephaly is a disorder in which much of the brain and head are underdeveloped. Newborns usually do not survive after birth.

Open spina bifida involves an opening in the spine (backbone) of the baby. Open spina bifida is often accompanied by several handicaps with varying degrees of severity including paralysis of the legs, lack of bowel or bladder control and an accumulation of excess fluid in the brain. In a small percentage of open spina bifida newborns, the birth defect is minor.

*Only detected in the second trimester

1Canadian Down Syndrome Society
2Ontario Ministry of Health and Long-Term Care
3Health Canada - Canadian Task Force on the Periodic Health Examination. The Canadian Guide to Clinical Preventive Health Careî. Minister of Supply and Services Canada (1994) ISBN 0-660-15732-2.


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