FAQ to Preimplantation Genetic Diagnos Performed (PGD)

Who would be benefited by PGD?

Couples with a history of recurrent miscarriage.
Women with history of repeated IVF failures
Family history of with genetic abnormality like beta thalassemai
previously affected child with genetic abnormailty like downs syndrome
known carriers of genetic defects like chromosome translocations.
women who have had to undergo repeated terminations because od an abnormality diagnosed prenatal.
couples morally and religiously opposed to termination of affected pregnancies diagnosed prenatal.

In couples with severe male factor infertility.


How is PGD diff from other prenatal diagnostic techniques?
There are various prenantal diagnostic techniques which presently help in the detection of certain genetic disorders in the fetus. These includes :
1- Chronic Villus biopsy (doen at 10-12 weeks of gestation)
2- Amniocentesis (done at 16-18 weeks if gestation)
Cordocentesis (done at 20-22 weeks of gestation)

The main genetic disorders tested include

1- cytogenetic anomalies i.e structural & numerical chromosomal abnormalities - diagnosed by karyotyping or FISH
2- Molecular genetic disrders i.e single gene disorders - diagnosed by PCR

As opposed to these prenetal diagnostic techniques, in PGD, the diagnostic techniques like FISH & PCR can be performed on the blastomere obtained from the embryo prior to implantaion. PGD thus helps to obviote the physical, emotional & psychological trauma assosiated with termination of pregnancy.

What is FISH?
FISH is molecular cytogenesis technique, which can be performed in non-dividing inter phase cells. FISH involves a chromosome specific fluorescently labeled DNA probes to hybridize to complementary genomic DNA. the signals are visualized using a fluorescene microscope. In case of normal diploid cell. tow signals are send for each chromosome. Presence of one signal for the chromosome being analyzed suggests monosomy and presence of three signals for the chromosome indicated trisome. FISH is rapid, reliable and useful genetic diagnostic test.

Which disorders can be diagnosed by FISH?
FISH can be used for

1- Detection of aneuploidies (numerical abnormalities)
2- Detection of translocations (structural abnormalities)
3- Sexing of the embryo (for x-linked disorders)



In PGD, which chromosomes are usually tested by FISH analysis?
the chromosomes most commonly tested include chromosomes 12, 18, 21, X & Y. The other chromosomes can be tested as and when indicated.

Incase of translocations in either of the parents specific to the patient need to be ordered.


What is PCR?
PCR is a powerful molecular diagnostic technique that allows in vitro amplification of a particular DNA fragmant, allowing generation of millions of copies in a short duration.


Which disorders can be diagnosed by PCR?
PCR is useful in the disgnosis of single gene disorders like beta thalassemia, cystic fibrosos, Ducchenne's muscular dystrophy, etc.

Can PGD be used for sex selection?
PGD can be used for proconception sex selection at the embryonic stage. according to the prenatal diagnostic techniques act, 1994, sexing of embryos is prohibited in India. However sexing of embryos can be performed for medical indications as in case of X linked disorders, where only female embtos (normal or carriers) may be tranfered.

What may be the problems encountered during performing PGD?
- Th case the embryo has started compacting, then it may be difficuilt to perform teh embryo biopsy.
- The cell may be lost during the procedure of embryo biopsy.
- The cell may be lost during fixation of cell on teh slide.
-The FISH signals may not be clear/ overlapping / split in some cases, making the diagnosis difficuilt.
- Mosaicism


Advantages of PGD?
- It is performed at the preimpantation state i.e befire the onset of pregnancy. PGD helps to increase the chances of having a normal baby.
- PGD helps to increase teh pregnancy rate and decrease the miscarriage rate.