Although society should do everything possible to support
families with disabled children, prenatal screening offers
parents who have the ultimate responsibility of looking
after the children the option to terminate the pregnancy
in the event of an abnormal prenatal test. Because mid-trimester
abortion is traumatic, as pointed out in the article "Study
backs early prenatal screening", earlier diagnosis
is preferable. In the same vein, pre-implantation genetic
diagnosis (PGD) allows genetic abnormalities to be screened
in the embryo so that termination of pregnancy may be
avoided. PGD can also be used to diagnose not only identified
genetic diseases in IVF embryos, but also for a non-disease
trait such as a certain HLA type to save an affected sibling.
PGD is now more widely used in fertility clinics to improve
the pregnancy rate and reduce the miscarriage rate.
The significant decline in pregnancy
in women of advanced maternal age is mostly attributed
to chromosome abnormality. Most of these chromosomally
abnormal embryos will not implant and of those implanted,
a large proportion will lead to spontaneous abortion.
Through PGD, now only chromosomally normal embryos are
transferred in women undergoing fertility treatment,
increasing the chances of healthy pregnancy and reducing
the chances of pregnancy losses and affected offspring.
The same technical approach can also be used for a parent
who is a carrier for chromosome rearrangements, who
otherwise may never achieve a viable pregnancy without
the use of PGD.
One of the limitations of PGD hitherto
has been that all female patients have to undergo IVF
treatment to induce multiple oocyte development. But
now patients have the option of a new procedure involving
collection of immature eggs from the ovary and maturing
them in vitro (IVM) without taking fertility
drugs. The embryos generated by IVM can then be tested
for genetic abnormality through PGD. This approach has
been successfully applied in our centre resulting in
the birth of a healthy child. The main advantage is
to avoid the risk of ovarian hyperstimulation syndrome
for selective patients; it also reduces the cost of
taking expensive infertility drugs. With further advances
in oocyte vitrification, we can even freeze eggs from
several cycles in so-called "poor responders" to obtain
a cohort of embryos for PGD.
With the advancement of reproductive
technology comes an ethical concern involving abuse
of technology, which needs to be discussed openly with
patients, doctors and policy makers. However, PGD technology
has immense potential to reduce human suffering and
to improve human life, and should not be curtailed because
of a theoretical concern of possible eugenic selection
of embryos.
References:
- Heffner,
LJ. (2004). Advanced Maternal Age — How Old Is
Too Old? N Engl J Med, 351:1927-1929
- Munne,
S (2005). Analysis of chromosome segregation during
preimplantation genetic diagnosis in both male and
female translocation heterozygotes. Cytogenet Genome
Res.;111(3-4):305-9
- Chian
RC, Gulekli B, Buckett WM, Tan SL (1999). Priming
with human chorionic gonadotropin before retrieval
of immature oocytes in women with infertility due
to the polycystic ovary syndrome. N Engl J Med.18;341(21):1624,
1626
- Ao A,
Jin S, Rao D, Son WY, Chian RC, Tan SL (2006). First
successful pregnancy outcome after preimplantation
genetic diagnosis for aneuploidy screening in embryos
generated from natural-cycle in vitro fertilization
combined with an in vitro maturation procedure. Fertil
Steril. Apr 7; (Epub ahead of print)
- Rao, GD, Chian RC, Son WS, Gilbert
L, Tan SL (2004). Fertility preservation in women
undergoing cancer treatment. Lancet. 363(9423):1829-30
— Asangla Ao, PhD, Departments of Obstetrics
and Gynecology & Human Genetics and Seang Lin
Tan, MD, Department of Obstetrics and Gynecology,
McGill University, Montreal, QC
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