Trisomy 21 (also referred to as Down syndrome)
This genetic anomaly occurs at conception when the fetus receives three copies of chromosome 21, rather than two. Trisomy 21 gives rise to intellectual and physical handicaps (including cardiac disorders) for which it is impossible to predict the degree of severity. All pregnant women can bring a pregnancy to term in which the fetus has Down syndrome and there is no curative treatment. According to the Canadian Down Syndrome Society, Down Syndrome occurs in about 1 in 700 births, however the incidence rate varies significantly by age.
According to the Canadian Down Syndrome Society,
80% of babies
born with a trisomy are from mothers aged 35 years and under.
Trisomy 18 (referred to as Edward's syndrome)
Trisomy 18 also occurs at conception. It is the result of three copies of chromosome 18 in the fetus, rather than two. In almost all cases, the fetus does not survive to birth.
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.1
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.
1 Health 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.
