This information was a response to a question I asked Joyce Hammel July 24, 2000, the then HDSA - Medical & Scientific Manager:
In response to your question on pre-natal testing, I provide the following information:
There are several ways prospective parents can determine if their child will develop HD. The specific method of determining the embryo's genetic status depends on whether it was conceived naturally or through in vitro fertilization (IVF).
the embryo was conceived naturally,
parents can use prenatal testing or non-disclosing
prenatal testing to determine the child's genetic status.
During in vitro fertilization procedures, Preimplantation Genetic Diagnosis (PGD)
can be used test embryos prior to their transfer to the uterus.
The prenatal procedures currently available are amniocentesis and chorionic villus
sampling (CVS). Individuals or couples considering prenatal testing are advised
seek genetic counseling prior to becoming pregnant. Prenatal testing is usually
requested in one of the two following circumstances:
1. In case I, a prospective parent has been diagnosed with HD or has been found
to be a gene-carrier and will eventually develop HD. In this instance, prenatal
testing can determine whether the child is
at 0 or 100% risk of developing HD.
2. In case II, the prospective parent is at 50% risk of developing HD and is pre-symptomatic. This means that one of his/her parents had HD but s/he isn't showing symptoms. Prenatal testing will reveal the embryo's and the parent's genetic
The test results in this case can be over-whelmingly
traumatic if both are positive for HD. Prospective parents should be counseled about
possible emotional ramifications before undergoing the tests.
Nondisclosing Prenatal Testing
If the parent who has a 50% risk for HD does not wish to know his/her gene
status but the couple wants prenatal testing, then non-disclosing prenatal testing can
be performed. In this situation, the embryo is at a 25% risk of developing HD.
Using linkage analysis, a procedure that compares
the chromosomes of family members with that of the embryo, this test may reduce the
fetus' risk from 25% to zero
or increase it to 50% Couples or individuals seeking
this testing option are encouraged
to seek counseling and prepare for the test prior to conception.
Preimplantation Genetic Diagnosis (PGD) can be used during in vitro fertilization
procedures to test embryos for HD or other genetic disorders. This allows couples or individuals that have serious genetic disorders to decrease the risk of having a
child who is affected with the same problem.
At present, PGD is only offered in a few centers, usually under the supervision of an
institutional ethics review board, but its use may become more widespread in the near future.
There are only a few prenatal tests for HD done each year. The centers that do these
tests, especially the Genetics and In Vitro Fertilization Institute (GIVF) are experienced
with preimplantation diagnosis as well as CVS and amniocentesis testing for HD. GIVF
can also offer testing when knowledge of parental carrier status is not desired.
Please contact the following person at GIVF with additional questions about testing.
Fallon, MS, CGC
Supervisor, Genetic Counseling
Genetics & IVF Institute
Fairfax, VA 22031
Hope that this information is helpful!
Prenatal testing for HD is possible, and should be performed in conjunction with detailed genetic counseling. Affected or at-risk individuals or couples should be informed of all of their reproductive options (shown in table 1), with
the understanding that different options are appropriate or desirablefor different people.
For those who desire prenatal testing, the best time to make arrangements is prior to the pregnancy. Chorionic villus sampling can be performed very early, at 8-10 weeks, and a non-disclosing prenatal test, which determines only
whether the fetus received a chromosome from the affected grandparent
or the unaffected grandparent, without determining whether the fetus or at-risk parent actually carries the HD gene, requires samples from several individuals.
TABLE 1: REPRODUCTIVE OPTIONS
- Natural reproduction without genetic testing
- Prenatal testing by amniocentesis or chorionic villus sampling
- Non-disclosing prenatal test
- Decision not to reproduce (may include sterilization)
- Artificial insemination
- Surrogate mother
- Pre-implantation genetic testing and embryo selection
Pre-natal and Vitro
Fertilization for HD
Testing of a fetus (prenatal testing) presents
special challenges and risks; in fact some centers
do not perform genetic testing
on fetuses. Because a positive test result using
direct genetic testing means the at-risk parent is also a gene carrier, at-risk individuals who are considering a pregnancy are advised
to seek genetic counseling prior to conception.