PGS (pre-implantation genetic screening) can help couples not only select the gender of
their child but can also determine if an embryo contains the normal number of
chromosomes. PGD (pre-implantation genetic diagnosis) can identify particular genetic
diseases that a person may carry while also assisting couples who could potentially
transmit a sex-linked genetic disease to their children.
With 99.9 percent accuracy in predicting an embryo’s gender, PGD/PGS gives couples
the best odds in determining their baby’s sex.
PGS allows couples to choose their baby’s sex by identifying male and female embryos
conceived in a laboratory, prior to transfer to the woman’s uterus. PGS requires IVF,
fertilization in a laboratory petri dish, along with a minor surgical procedure to remove
eggs from the woman’s ovaries. After fertilization, specialists examine the embryo for its
sex chromosomes (XX for a female or XY for male), and then implant an embryo of the
selected sex into the woman’s uterus.
PGS was first developed in the 1980s. Interestingly, the initial application of PGD in
humans was to determine the gender of embryos to prevent X-linked genetic diseases.
This technique was first described in 1987 by scientists at the University of Edinburgh,
and the first live births, healthy twin girls, were reported in 1990. Preimplantation genetic
diagnosis has been used not only to detect gender but also to detect abnormalities of
chromosomes, such as Down syndrome, but may also be used to “diagnose” serious
single-gene disorders such as cystic fibrosis and sickle-cell anemia prior to implantation.
Embryo Quality
With all methods of PGS, an important fact is worth noting: A surprisingly high
percentage (50-70 percent) of embryos will be found to be abnormal, even in healthy,
fertile couples. A typical PGS case might look something like this: 12 eggs are retrieved,
11 are suitable for fertilization, 9 fertilize, 7 are biopsied, 3-6 are normal, and 2 or 3 of
the normal embryos are of the desired gender. There is no real scientific way to
determine how many embryos will survive, how many embryos will be male or female,
or if they will produce a pregnancy. In few instances, it is not always possible to
determine the gender during the first biopsy. These embryos may require a second
biopsy.
This news can be a shock when couples get the results of their embryo biopsies. But
this is all part of being human and it can help explain the many miscarriages that

women experience as a whole. Not all of our eggs or embryos are healthy or free of
chromosomal abnormalities, and most of them do not have the potential of turning into
perfect little babies. But many embryos do. PGD/PGS can help physicians sort out the
“good eggs from the bad,” to borrow an expression. And it gives couples the opportunity
to produce a healthy child of the gender they’re hoping for.
The Scientific Understanding of Gender Selection
It has been known for many years that the gender of a pregnancy is determined by the
sex chromosome carried by the sperm. Sperm bearing an X chromosome, when united
with the X from the female (females only produce X) will result in an XX pregnancy that
produces a female. If a sperm bearing a Y chromosome (men have both X and Y
bearing sperm) unites with the X chromosome from the female, an XY pregnancy will
result that gives rise to a male offspring.
Armed with this knowledge, science initially worked to allow for an accurate method of
safely separating sperm to allow the majority of those sperm capable of producing the
desired gender (X sperm or Y sperm) to be exposed to the female egg (oocyte). While a
variety of methods of purifying the sperm separation process have been reported and
studied, in reality, very few of these methods have withstood scientific scrutiny that
“checks” the validity of claims made by those employing the procedure.
Because no sperm separation method thus far developed has produced the high level
of sperm separation X (for female) and Y (for male) needed to provide gender outcome
success levels greater than 90 percent, further work to perfect the gender selection
process is being studied.
Sperm that have been filtered by our standard sperm preparation process are allowed
to fertilize the eggs obtained from the female “in vitro” (in our highly specialized fertility
laboratory). The embryos resulting from this specialized fertilization process are then
screened by our genetics team to determine both their gender and that selected
chromosome pairs have resulted in an expected normal genetic pairing outcome (this
process is called “aneuploidy” screening). This gender determination process at the
very early development level as made famous by our center has resulted in the ability to
provide gender selection results for the chosen gender far in excess of 99.9 percent.
The aneuploidy (abnormal chromosome count) screening process also employed at the
time of PGD gender determination also allows for the detection of limited genetic count
abnormalities as a routine or for the optional screening of the embryos for a wide variety
of additional genetic abnormalities. Upon request, we can screen for genetic
abnormalities such as Down syndrome (one extra chromosome 21), Turner’s syndrome
(the absence of one of the two X chromosomes normally found in a female), and

Kleinfelter’s syndrome (a male with one Y chromosome and two X chromosomes
instead of the normally found single X chromosome).
New DNA microarray technology also provides us the option of screening embryos for a
full (46 chromosome) genetic count. We are also able to provide those patients known
to carry specific personal or family genetic diseases the ability to screen the embryos for
many specific disorders. All couples meeting our standard, liberal entrance criteria will
qualify for the PGD process.
Aneuploidy screening as described above detects abnormal chromosome numbers and
the diseases associated with those conditions. “Single gene disorders” include a wide
variety of hereditary diseases found on a specific chromosome that can also be
screened for with PGD.
Blastomere Biopsy
Blastomere biopsy (also known as embryo biopsy) is a technique that is performed
during IVF when an embryo has reached the six to eight cell stage (about 72 hours or
day three of embryo culture). One or two cells, or blastomeres, are separated from the
rest of the embryo and removed from the zona pellucida, which is the shell surrounding
the developing embryo. After removal of the cell(s), the developing embryo is placed
back into the culture media and returned to the incubator where it can resume its normal
growth and development. Preimplantation genetic diagnosis (PGD) can be performed
separately on the removed cell(s).
At this early point of embryo development, all of the cells should be identical, and thus,
removal of a cell from the embryo at this stage should not remove anything critical for
normal development. An embryo should be able to compensate for the removed cell
and should continue to divide following blastomere biopsy. However, a recent study
suggested that a biopsy performed at the blastomere stage was responsible for a
decreased chance that the embryo would be able to implant into the uterus later.
After obtaining cells from the embryo, they can then be analyzed using a variety of
different techniques. It doesn’t matter which of the eight cells was removed because, as
the embryo divides, each subsequent generation of cells contains exactly the same
genetic information as the “parent” cell. Thus, each of the eight cells should be identical.
However, at times there can be an aberration in the cell division in which one or more of
the “daughter” cells ends up being slightly different from the parent cell. This is called
mosaicism. Mosaicism is important when performing preimplantation genetic diagnosis
via blastomere biopsy because it means it is possible that the cell that is biopsied may
not be representative of the entire embryo. For example, if during PGD, a blastomere
biopsy is performed and the cell that is obtained is abnormal, the entire embryo would

considered abnormal, even though the remaining cells in the embryo may be normal.
The opposite is also true. An embryo with seven abnormal cells and one normal can be
considered normal if the “eighth” cell happens to be the one that is biopsied.