During conception, the egg is fertilized by sperm in the fallopian tube
and an embryo is formed. Fertilization usually happens in the part of
the fallopian tube that is closest to the ovary and the embryo continues
to develop (through cell division) as it travels back down the tube towards
the uterus. The embryo usually reaches the uterine cavity about 5 or 6
days after fertilization. At this time, it is a blastocyst, or an embryo
made up of about a hundred cells.
In an
IVF cycle, a blastocyst forms in a culture system in a laboratory. Eggs are
retrieved from a woman's ovaries, fertilized with sperm, and an embryo
is created. The embryo divides and multiplies its cells over 5 to 6 days
to become a blastocyst. Embryos that survive to this stage of development
have a high implantation potential once transferred into the uterine cavity.
Embryo Transfer
Allowing the embryos to grow from Day 3 (cleavage stage) to Day 5/6 (blastocyst)
allows us to select the embryos with the highest chance of success. The
primary benefit to the patient of using blastocyst stage embryo culture
is that fewer embryos need be transferred into the patient because each
day 5 embryo has a higher chance of implanting than a day 3 embryo does.
Transferring fewer embryos greatly reduces the risk for twin or triplet pregnancies and the associated
risks to the mother and child. Additionally, embryos that continue to
grow to the blastocyst stage have a lower rate of
chromosomal errors and therefore a higher chance of resulting in a healthy baby.
Grading Blastocysts
Expansion
Blastocysts sizes are measured on a scale of 1 to 6. Early blastocysts,
with fewer cells, are graded as 1 or 2. Because these have fewer cells
and a poorly defined inner cell mass and trophectoderm (see below), they
are not given any letter grades like the more advanced blastocysts are.
Blastocysts that are further along in development are given expansion
grades of 3 through 6, where 6 indicates an embryo that has completely
hatched out of the zona pellucida (shell).
Inner Cell Mass
The inner cell mass represents the collection of cells that are destined
to differentiate into the fetus/baby. This group of cells is given a letter
grade of either A, B, or C. Grades A and B are most desirable and reflect
a good number of cells with normal appearances. Therefore only those cells
graded A or B are selected to be
cryopreserved.
Trophectoderm
The trophectoderm represents the collection of cells that will become the
placenta and are responsible for the initial invasion and implantation
of the embryo into the uterine lining. This is also given letter grades
of A, B, or C. Just as with the inner cell mass, grades A and B are most
desirable.
Though embryos are graded to signify their stage of development, it is
important to remember that the grade does not always reflect the true
potential of an embryo. Specifically, there are embryos that receive less
than ideal grades that frequently create beautiful babies. Embryo grading
helps us determine how many embryos to transfer. It is important to remember
that embryo grading is subjective and that IVF programs may vary somewhat
in assigning grades to embryos.
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