These days ever more Canadian women have access to early
prenatal screening either through their provincial healthcare
plans (like Ontario) or private clinics (as in Quebec).
In many cases women who get screened can find out the
risk their tiny baby has of developing serious genetic
diseases like Down's syndrome. "New mothers-to-be have
a basic knowledge [of prenatal screening]," says Dr Anne
Summers, chief and medical director of genetics at the
North York General Hospital, "it's partly education and
partly osmosis through the population."
Still, she insists that the primary
care provider, be it a GP or an ob/gyn, should provide
more thorough information to patients. "Women have to
be given the tools. A primary care provider, in most
cases, is the person who broaches the subject." That's
why you need to be equipped to answer these difficult
patient questions: What are the tests and what do they
look for? What does a positive result mean for my baby?
What are my options?
PRENATAL
SCREENING 101
Lola Cartier, a genetic counsellor at the Montreal Children's
Hospital, believes GPs play a very important role in
making sure mums-to-be understand the process. "Doctors
need to explore much further with the patient how this
screening test could be useful in her particular case,"
she says. "They need to spend some time explaining the
difference between screening and diagnostic tests."
Doctors should also point out that a positive test doesn't
mean that the baby has the genetic disorder.
|
Pre-implantation genetic diagnosis
explained
What is it?: PGD tests
embryos for birth defects before they're implanted
into the uterus. Naturally, it's only an option
for those using in vitro fertilization.
Who qualifies?: One of
the potential parents has one of the following
disorders; balanced translocation; single gene
disorders, like autosomal recessive disorders,
autosomal dominant disorders, and X-linked disorders;
or recurrent pregnancy loss due to aneuploidy.
What does it screen for?:
Some IVF centres have successfully tested for
over 58 diseases, including cystic fibrosis, Duchenne
muscular dystrophy, familial amyotrophic lateral
sclerosis, Huntington disease and sickle-cell
anemia.
What are the controversies?:
PGD has the potential to screen for genetic issues
unrelated to medical necessity. It can be used
for sex selection either for the purpose of 'family
balancing' or 'social selection' or to conceive
'saviour siblings', when one baby is conceived
to be the genetic match of an already living child
in order to save its life.
What about regulations?:
Health Canada is currently working on policy for
PGD that should be released in 2008. "It will
be a controlled activity," says Francine Manseau,
who is responsible for policy development at Health
Canada. Interested physicians can check out the
consultation document on the Health Canada website.
For more on prenatal screening
see the editorial "Breast
cancer advances bloom in April".
|
Screening tests look for biochemical
markers in the mother. The most commonly used maternal
markers include maternal serum β-fetoprotein
(MSAFP), unconjugated estriol and human chorionic gonadotropin
(hCG) measured in the second trimester, according to
the Society of Obstetricians and Gynecologists. Other
markers include inhibin A and urinary β-hCG-core fragment,
also measured in the second trimester, and pregnancy
associated plasma protein-A (PAPP-A) and free β-hCG
in the first trimester.
"A screening test determines risk,"
specifies Dr Summers. In many cases first and second
trimester maternal serum screening are provided as an
integrated test. In many cases with the integrated test
women will only get results in the second trimester.
"In terms of accuracy of the tests
the integrated is the most accurate and has the lowest
rate of false positives," says Ms Cartier. "If you talk
about safety and efficiency that would be the best test
to consider." But, she adds, other things should be
considered, especially psychological well-being. "One
of the arguments of first trimester screening is that
[if it's done] earlier on it's easier to deal with [mentally]."
Diagnostic tests, like chorionic
villus sampling (CVS) and amniocentesis, are more invasive.
They're also more accurate at determining if the baby
is sick. Most troublingly, these tests also carry a
risk of miscarriage, albeit a low one. In most cases
positive results from a screening test will dictate
whether diagnostic testing is considered.
IS
EARLIER BETTER?
A study published in the November 10, 2005 edition of
New England Journal of Medicine points out that
first-trimester combined screening has similar results
to second-trimester screening. The authors add that
fully integrated screening has the highest rate of detection
with low false positives.
An accompanying editorial, by Dr
Joe Leigh Simpson from the Baylor College of Medicine
in Houston, went so far as to say that "earlier is better."
Dr Simpson's reasoning is that first trimester screening
picks up about as many positive results as a combination
of screening tests done in both first and second trimesters.
Also, he points out that an uncertain number of patients
will return for tests in the second trimester. But his
ultimate argument is that with first trimester testing
"pregnancy terminations are earlier, more private and
far safer than in the second trimester."
This argument touches on the burning
issue in genetic screening: termination of a pregnancy.
Some people in the medical community suggest that genetic
counselling is biased towards termination of the fetus
in the event that a test comes back positive. "I think
that it's crap," says Dr Summers. She won't deny that
genetic counsellors talk to patients. "We do discuss
whatever the disorder is," she says. "But we are very
careful to not pressure people to make one decision
or another."
THE
DAWN OF DESIGNER BABIES
Still, the potential for such bias might exist. "Have
I seen [this bias]?" muses Dr Kerry Bowman, PhD, a bioethicist
at the University of Toronto, "No. I haven't ever seen
the question being asked in a very weighted way." He
believes it's less about the profession and more about
general societal trends. "I will say that as a society
we're less and less tolerant towards disabilities. There
is a kind of subtext in society that says you want to
try to get the best you can."
He worries more about the social
message parents are getting rather than stuff they are
hearing from healthcare workers. "There is this movement
towards perfection and a sort of shallowness in our
culture," he says. "A lot of narcissism is really playing
out in reproductive issues."
Which might explain why many are
concerned that all this early screening paves the way
for parents to conceive custom-made or designer babies.
A technology, called pre-implantation genetic diagnosis,
can actually identify genetic defects in embryos created
through in vitro fertilization (IVF) before transferring
them into the uterus (for more see "Pre-implantation
genetic diagnosis explained"). "I think there is a slippery
slope," admits Ms Cartier.
Dr Bowman couldn't be more opposed
to the concept of designer babies. "One of the elements
of parenthood and humanity is the unconditional love
of a child, independent of how his or her genes may
have played out," he says. "When you get in to 'I want
this one and not that one' all of that is greatly eroded."
|
