PREIMPLANTATION GENETIC DIAGNOSIS
What is Pre-implantation Genetics or PGD?
Genetic diseases can be diagnosed prior to a child being born.
The most common way is to diagnose if there is a problem in
the unborn child is by doing a Chorion Biopsy at 8-9 weeks
at around the 4th-5th month of pregnancy (prenatal diagnosis).
In case a baby is found to be abnormal, it can be aborted.
However this is a traumatic process- physically and psychologically.
Instead of undergoing this trauma, Pre-implantation Diagnosis,
popularly called PGD, can be done. In this a standard ICSI
cycle is carried out, and a lot of embryos are produced. A
small piece of the embryo is then removed and examined for
any genetic or chromosomal defects. The defective embryos
are discarded and the normal embryos are put back in the womb.
Thus with the help of PGD one can have normal babies, without
undergoing the trauma of Amniocentesis and abortion.
The detection of genetic disease in the human embryo before
implantation gives parents a chance to start a pregnancy,
knowing that the baby will be free of inherited disorder that
is prevalent in their family.
What are the Indications?
There are several groups of patients in which PGD is the preferred
1. PGD is used to avoid conception of abnormal babies
in couple who are at high genetic risk: especially in those
who have had a previously affected genetically abnormal babies
or in patients with age more than 35 years.
In India, the most common abnormalities are that of Downs
Syndrome (mentally retarded babies with three chromosomes
21 or 3 chromosomes 18) and Thallasemia (genetic disorder
commonly seen in Kutchis, Lohanas, Sindhis and Greeks). Downs
Syndrome is commonly seen in babies of elderly women above
the age of 35 years.
2. It has been observed that many women with advanced
maternal age, which are Undergoing IVF/ICSI may have a higher
number of genetically abnormal embryos. PGD can be used in
identifying and transferring back normal embryos, thus decreasing
the risk of miscarriages and increasing the chance of normal
3.In many couples suffering from repeated miscarriages,
chromosomal Aneuploidy and Translocations, may play a role.
PGD with multicolor FISH can be used as a screening test in
these couples, thus reducing the chances of abortion.
4. Finally there are those patients with moral or religious
objection to pregnancy termination such as Jains or Catholics.
This group would prefer selection at the pre-implantation
stage, giving them a reasonable chance of starting with a
How is PGD done?
The patient undergoes the standard ICSI cycle. Following ICSI,
the eggs are placed in culture media and kept in the Carbon
Dioxide incubators, for 3 days. On the third day numerous
6-8 celled embryos (earliest form of life) are formed. A small
hole is made in the cover of the embryo called the zona. This
hole is made using a machine called the Micromanipulator.
The hole can be made either mechanically using needles or
chemically using acid Tyrode solution.
Alternately the hole can be made using a Laser. Babies And
Us has had the privilege of having the first Laser machine
in the country to be used for cutting cells.(see Assisted
Laser hatching link).
At least two cells or Blastomeres are removed from the embryo
with the aid of the Micromanipulator These aspirated Blastomeres
are sent to the Genetic Laboratory, for analysis by the two
techniques of FISH or PCR, depending on the condition, which
is being investigated.
Will the removal of Blastomeres damage the embryo?
A frequently asked question is whether the remaining embryos
will survive and will have the same normal complement of genetic
material as the parent embryo. The embryo at this stage is
totipotent; that means each of the individual Blastomere has
the potential to grow independently into an embryo and an
individual, if implantation takes place. Hence removing two
of the eight cells is acceptable.
How are the Blastomeres tested?
There are two kinds of genetic problems. One of these is due
to chromosomal defects. Normally there are 46 chromosomes.
The other defects are due to abnormalities in the genes. Genes
are blocks of DNA that make up the chromosomes. These defects
can be diagnosed by either FISH or PCR.
1. Fluorescent In Situ Hybridisation(FISH):
This is used for analyzing chromosome related Aneuploidy such
as Downs syndrome and sex chromosome related problems such
as Klinefelteror Turners syndrome. It has been extensively
used in X linked recessive disorders such as Hemophilia A,
Duchennes Muscular Dystrophy, Lesh-Nyhans syndrome, Colour
In FISH, the Blastomere is fixed on a slide. The fluorescent
probe is then added to the cell and allowed to hybridize with
the Centromere DNA of the specific chromosomes. The nucleus
is then observed under fluorescent microscope, making use
of various filters. The chromosomes are seen as orange, blue
or green coloured fluorescent dots ,shining against a blue
background .By simply counting the dots one can make out the
problem or disorder. For example in Tisomy 21 there are 3
orange dots (representing 3 chromosome 21s), as compared to
normal, where there are only two dots (representing Chromosome
FISH can be carried out in two hours. If a Embryo biopsy is
done on the morning of day 3, then the FISH result is available
by afternoon. The normal embryos can then be transferred back
by late afternoon of the same day.
2. Polymerase Chain reaction (PCR):
This technique of genetic analysis is performed for diagnosing
single gene defects. There are numerous diseases such as Cystic
Fibrosis, Sickle Cell Anemia, Thalassemia and Swteinerts Myotonic
Dystrophy, which can be diagnosed with this technique. Embryo
sexing can also be done with this method. However unlike FISH,
PCR cannot differentiate between XX and XO.
Genetic disorders may be caused by several changes in the
DNA. These may involve a point mutation or the deletion or
insertion of one or a few base pairs. The DNA mutation from
a single cell is difficult to analyze. The cyclical process
of the PCR makes millions of copies of the small portion of
DNA, which is being screened for. In each PCR, the amount
is doubled. It becomes easy to detect an error in the amplified
portion of the DNA, since millions of copies are present.
The important points in PCR reaction are:
After amplification, it is analyzed by a method called Gel
Electrophoresis, wherein the DNA is analyzed on the basis
of migration of various fragments on the gel, when an electric
current is applied. Though Cystic Fibrosis is very extensively
investigated in the West, Thalassemia will be the main disease
to contend with in our country. Already few successful PGDs
for Thal have been carried out in the Mediterranean. The biggest
problem in Thal at present , is the numerous mutations which
are present in the country.
What is the scope of PGD in India?
Since the first successful clinical PGD, more than 100 centers
worldwide have started performing this technique. More than
a thousand successful births following PGD have been recorded.
In India, few clinics, including Babies And Us at Lilavati
Hospital, have started doing preliminary work in this very
Does Babies And Us have the ability
to accurately perform the complex task of PGD?
Pre-implantation Genetic Diagnosis is a very complex and precise
technique. Its successful clinical application is based on
the unification of a highly successful ICSI program and a
very accurate Genetic Laboratory. At present Clinical PGD
has been done on a very limited number of couples worldwide.
It has a tremendous potential to develop into a successful
clinical procedure, in the future.
We have the latest imported Equipment from Olympus and Vysis
to perform this technique. Furthermore we are at present performing
PGD in collaboration with the IVF Institute in Naples, Italy.
A PGD expert from Naples comes down to Mumbai every 4 months
to perform this task.
How accurate is PGD?
PGD is 95-97% accurate. However one can never be sure. All
patients who conceive after PGD are requested to undergo prenatal
diagnosis to confirm that the pregnancy is normal.
Also the chance of a patient conceiving after PGD is in the
region of 20-25%. That means one in five patient who undergoes
PGD will become pregnant.