What are chromosomal disorders?
The blueprint of the body is “written” in DNA, and
genes correspond to words. These words are grouped together into chapters
of varying length called chromosomes. We each get one set of 23 chromosomes
from each parent, for a total of 46. When a baby is conceived with an abnormal
number of chromosomes, the most common result is a miscarriage. Sometimes
the baby is carried to term and can have varying degrees of problems after
birth.
What kinds of chromosomal disorders are there?
Down syndrome is a relatively common chromosome abnormality
with which many people are familiar. It results from having three copies of
chromosome number 21, so it also called trisomy 21. People with Down syndrome
almost always have some degree of mental retardation and often have a wide
variety of other medical problems. Although these medical problems can be
treated, there is no treatment or cure for the underlying chromosomal disorder.
Individuals with Down syndrome have an average life expectancy of over 50
years but usually cannot live independently.
How likely is it that my baby has Down syndrome?
The risk of Down syndrome at birth depends on the mother’s
age:
| Mother’s Age at Delivery |
Probability of Down Syndrome |
| Less than 30 |
Less than 1:1000 |
| 34 |
1:500 |
| 37 |
1:200 |
| 40 |
1:100 |
| 43 |
1:50 |
Are there other chromosomal disorders besides Down syndrome?
Yes. Some are more severe than Down syndrome: the most important
example is trisomy 18. Affected individuals can be profoundly retarded and
are often stillborn or die in infancy due to the serious medical problems
that are associated with these conditions. Fortunately, these severe chromosomal
disorders are much less common at birth than Down syndrome. Also, the structural
defects that are associated with these conditions are often recognized at
an ultrasound done after 16 weeks.
There are also chromosomal disorders that have manifestations
that are milder than Down syndrome. They are relatively common, but are not
associated with mental retardation or major physical handicap. Affected individuals
may have infertility and possible learning and/or behavioral problems.
As is the case for Down syndrome, the risk for most other chromosomal
disorders is related to the mother’s age.
Can you determine if my baby has a chromosomal disorder?
Yes, but this requires an invasive procedure to obtain a sample
of fetal cells from the amniotic fluid or the placenta. By examining the fetal
cells, we can be certain whether or not the fetus has normal chromsomes.
What types of invasive procedures are there?
Amniocentesis (also called amnio) is most often done after
15 weeks. It involves inserting a needle into your uterus and withdrawing
a small amount of the fluid that surrounds the baby. However, we think that
there is about a 1:200 or 0.5% risk of miscarriage when amniocentesis is done.
CVS stands for chorionic villus sampling. It is done earlier
than amnio, typically between 10 and 12 weeks, so the results are obtained
before 14 weeks. However, it is more likely than amnio to result in a miscarriage.
We think this happens in 1:100 cases, or 1%.
Who should have an amnio?
Since there is a risk of miscarriage, only women who are at
increased risk of having an affected fetus should consider an amniocentesis.
Since age is such an important factor in determining this risk, women who
will be 35 or older at delivery are offered more testing options than are
younger women.
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