Embryo
Freezing
The
embryo freezing process takes approximately
3 hours. Embryos are sequentially treated
with varying concentrations of a special
solution, called the cryoprotectant. The
cryoprotectant protects the embryos during
the freezing process. Each embryo is then
drawn into a specially designed sterile
straw. The straws containing the embryos
are placed into a freezing machine, slowly
cooled to -35°C and stored in liquid
nitrogen for long-term storage (-196°C).
Why
freeze embryos?
If there are enough embryos of sufficient
quality remaining after embryo transfer,
these can be frozen or cryopreserved for
future use. Using stored embryos for future
treatment avoids the need for stimulation,
egg collection and fertilisation, and
makes maximum use of these very precious
resources.
How
are embryos frozen?
The cryopreservation process is complicated
and takes 2-3 hours, during which time
the embryos are carefully placed in a
series of special protective solutions
before transferring to small sealed plastic
straws and frozen slowly using a computer-controlled
freezing machine. This is followed by
storage in large tanks of liquid nitrogen
at the extremely cold temperature of minus
196 degrees. All tanks are fitted with
alarms which notify us remotely 24 hours
a day of any change in temperature which
may require our attention.
Before
embryos are stored we ask you to make
some decisions about the storage – how
long you wish to have them stored, what
to do with them in the event of divorce,
separation, incapacitating illness or
death. These are all important decisions
for you and your partner to consider,
and we will contact you each year that
your embryos remain stored to check that
your wishes remain the same. If your circumstances
have changed or you change your mind,
you can contact us at any time to vary
the conditions of storage.
Will
my embryos be frozen?
Only
the best grade embryos have the potential
to withstand the freezing process which
is somewhat stressful to the cells, and
we expect an average 70% of embryos to
survive freezing and thawing. Occasionally
none of a patient’s embryos survive and
in order to minimise this risk we recommend
that no fewer than 3 good grade embryos
are frozen. If there are fewer than 3
surplus embryos of good quality we will
keep them in the incubator and can freeze
them if they reach the blastocyst stage.
Embryo
freezing should be regarded as a bonus
- only about a third of couples
will have embryos frozen in any one treatment
cycle. There is no charge for the initial
freezing of embryos and the first year
of storage. Subsequently there is an annual
storage fee (currently £125), payable
in advance.
How
do I use my frozen embryos?
Frozen embryos can be transferred in two
types of treatment cycle: if you ovulate
reliably embryos can be replaced in your
natural cycle, otherwise you will need
an ‘artificial’ cycle using drug therapy
to prepare the endometrium to receive
the embryos. Embryos will be carefully
thawed at a time appropriate to their
cell number, and the embryo transfer will
be scheduled very carefully so that embryo
stage and uterine receptivity coincide
to ensure the optimum chance of success.
The embryo transfer will be performed
in the same manner as for your fresh embryos.
How
successful is freezing embryos?
Pregnancy rates for frozen embryos transfers
are significantly lower than for fresh
embryo transfers but depend on the number
and quality of embryos frozen. Typically
they are between 10-20% per embryo transfer
depending on the number, stage and survival
of embryos frozen.
Are
there any risks with freezing and thawing
embryos?
As already mentioned, the greatest risk
with freezing and thawing embryos is damage
caused by the process itself, despite
the care we take to minimise this. Not
all embryos are able to withstand the
stresses of the necessary dehydration
for freezing and rehydration during thawing,
hence a reduced survival rate and subsequent
failure to resume division and growth
for some embryos. This also accounts for
the lower pregnancy rate following transfer
of frozen-thawed embryos. In
a very few cases no embryos survive, or
they may survive but all stop developing
early. This means for these patients no
embryo transfer takes place.
To
date there is no conclusive evidence that
freezing and thawing embryos causes long-term
damage to them, but as with all assisted
conception procedures the technology is
relatively new and there have been no
really long-term studies carried out.
To the best of our current knowledge the
techniques employed are safe and not harmful
in any way.
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