|
A blastocyst is an embryo that has developed in culture
for at least five days after fertilization and has divided
into two different cell types. The surface cells are
termed the trophectoderm and will eventually become
the placenta and the inner cell mass will become the
fetus. A healthy blastocyst should hatch from its shell
(zona pellucida) by the end of six days or earlier and
begin to implant within the lining of the uterus.
In
the early years of human IVF, embryos were transferred
to the uterus between 24-48 hours (Day-1 or Day-2 transfer)
after fertilization. Later, some laboratories started
to grow the embryos for three days before transfer.
This
was a more physiological approach since it is known
that under in-vivo conditions the embryos reach the
uterine cavity around the end of the third day following
fertilization. Three day old embryos have usually developed
to the 4- to 8-cell stage.
Since
we cannot determine the viability of each embryo at
this stage, and given the high embryonic loss at this
stage, it was common to replace many embryos to compensate
for those that would not implant.
This
practice yields adequate pregnancy rates, however, it
often resulted in high order pregnancies. Usually, two
blastocyst stage embryos are transferred, compared to
four or five day-3 embryos. This dramatically reduces
the probability of multiple pregnancies with higher
pregnancy rates in many cases.
Let
us use a hypothetical case to illustrate our point. A
patient has 13 oocytes harvested. With our current rates
of fertilization, about 10 will become embryos (80%).
At this point, it is not possible to determine which of
these 10 are most likely to implant and develop into a
baby.
By
day three, 7 of the 10 embryos may be developing into
healthy 4- to 8-cell embryos. The others may have slowed
or stopped their development and their prognosis is
not favorable. In this example, we are left with seven
embryos that still look like they may have excellent
potential.
However,
we cannot determine based on their appearance, which
ones are the best to transfer on day 3. Further culture
for two additional days in the blastocyst culture medium
allows the natural selective process to continue.
Thus,
after 5 days of growth in the laboratory, only 4 or
5 of the original ten embryos may remain viable and
we can now identify the best embryos to transfer.
Read
Dr. Hernandez's extensive discussion on blastocyst transfer
in Adobe Acrobat format. |
