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In natural conception, fertilization of the egg occurs in the
fallopian tube. The initial cell divisions of the fertilized
egg also take place there and result in an embryo of 4 to 8
cells. As the newly-formed embryo continues to develop, it moves
toward the uterine cavity where additional cell divisions result
in formation of the "blastocyst", an embryonic stage
characterized by up to 150 cells and having the appearance of
a hollow ball. When the embryo reaches the "blastocyst"
stage, it is ready to implant.
"Blastocyst culturing" became an important part of
IVF treatment when, in the late 1990s, sequential embryo culture
media were developed to approximate the changes the embryo encounters
in going from fallopian tube to the uterus.
The advantage of blastocyst culture in IVF is that pregnancy may be
established with the transfer of 1 or 2 blastocysts compared
to the 2 or more embryos that are more typically transferred
on the 3rd day after fertilization at the 4- to 8-cell stage
of development. The transfer of fewer embryos can reduce the
chance for multiple pregnancy and the associated complications
that sometimes arise.
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