By Chioma Umeha
Everyone considering IVF should be aware of PGS,
and those in the following circumstances should be particularly aware: women
who have attempted IVF without becoming pregnant more than once, women who have
experienced recurrent miscarriages or have had a pregnancy involving a
chromosomal abnormality, and women with diminished ovarian reserve (since a
high proportion of their eggs are often chromosomally abnormal).
All embryos do not develop to the blastocyst
stage, and generally only some of those that do will be identied as
chromosomally normal.
Regardless of the number of blastocyst stage
embryos produced, there is always a risk that none will be identied as
chromosomally normal once subjected to PGS. Should this occur, a transfer will
not take place.
Women who produce few eggs in a single IVF cycle
may engage in a process referred to as “embryo banking”. In an effort to
accumulate a more plentiful number of embryos for PGS analysis, their doctors
stimulate their ovaries and retrieve their eggs over more than one menstrual
cycle.
Most clinics freeze the embryos with a method
called vitrification while PGS analysis is being performed. If a normal embryo
is identified it is then thawed and transferred on a subsequent cycle. Some now
believe that embryos are more likely to implant when the woman’s uterus has
recovered from the effects of the drugs used to stimulate her ovaries. In
certain circumstances, some clinics conduct the PGS within 24 hours so that if
a normal embryo is identied it can be transferred fresh.
If chromosomally normal embryos are identified
through PGS, a reputable clinic will transfer no more than two to avoid the
complications that often arise in high order multiple pregnancies (triplets,
quadruplets, etc.). The remainder will remain vitried for potential transfer at
a later date.
Even when chromosomally normal embryos are
identified through PGS and transferred into the woman’s uterus, the embryo is
not absolutely certain to implant. On occasion, a chromosomally normal embryo
will fail to implant or miscarry due to factors unrelated to its genetic
make-up.
In a high quality laboratory the risks of PGS are
low. They include accidental damage to or destruction of the embryo during
biopsy, inaccurate test results including the misdiagnosis of a chromosomally
normal embryo as chromosomally abnormal and the misdiagnosis of a chromosomally
abnormal embryo as normal, as well as the possibility that an embryo may not
survive vitrification.
Because of the possibility of an inaccurate test
result following PGS, all women undergoing PGS who become pregnant should speak
with a genetic counsellor about whether to have prenatal testing, and the
different types of prenatal tests that are available.
PGS is not offered at every fertility clinic and
even among those clinics that do offer it, some have vastly more experience
with PGS than others. Patients should be careful when choosing a fertility
clinic for PGS as they will be better served by those with substantial PGS
experience and high success rates in cycles where chromosomally normal embryos
have been identified and transferred following PGS analysis.
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