What is preimplantation genetic testing?
7.9 million children are born with a serious birth defect of genetic or partially genetic origin—that accounts to, 6% of total births a year worldwide. The problem has been constantly increasing since 2005 for babies born to mothers of every race and age group. Chromosomal abnormalities, affect about 60% of embryos in women under 35 and 80% of embryos in women over 40 years. Preimplantation genetic testing (PGT) previously known as PGD, is an early form of prenatal genetic diagnosis where abnormal embryos are identified, and only genetically normal embryos are used for implantation. This has become an integral part of Assisted Reproductive techniques.
When to go for PGT?
- The female partner is more than 35 years and so is at higher risk of having a child with chromosomal abnormalities
- H/O recurrent miscarriages
- H/O having a child with a chromosomal abnormality
- Partners are concerned about having a baby with a chromosomal abnormality
- H/O recurrent implantation failure during IVF cycles despite good quality embryos
- Either or both partners have a known chromosomal defect
How is PGT done?
Preimplantation genetic testing is often used with in vitro fertilization. After mature eggs are retrieved and fertilized with sperms, they are further grown in an IVF laboratory Once the developing embryos reach five days of age, known as the blastocyst, a biopsy is done. 5-10 cells are removed from each embryo and sent for genetic testing. They test the sample for abnormalities. It commonly takes 7-10 days for test results to arrive. In the meantime, embryos are frozen. Later after the PGT results arrive the healthy embryos, free from genetic problems, are transferred into the uterus in subsequent cycles to boost the chances of having a healthy baby. By avoiding the transfer of embryos with a genetic abnormality, recurrent implantation failure and also recurrent pregnancy loss.
Types of PGT
Preimplantation genetic testing is often used with in vitro fertilization. After mature eggs are retrieved and fertilized with sperms, they are further grown in an IVF laboratory Once the developing embryos reach five days of age, known as the blastocyst, a biopsy is done. 5-10 cells are removed from each embryo and sent for genetic testing. They test the sample for abnormalities.
It commonly takes 7-10 days for test results to arrive. In the meantime, embryos are frozen. Later after the PGT results arrive the healthy embryos, free from genetic problems, are transferred into the uterus in subsequent cycles to boost the chances of having a healthy baby. By avoiding the transfer of embryos with a genetic abnormality, recurrent implantation failure and also recurrent pregnancy loss.
- Preimplantation genetic testing-aneuploidy (PGT-A)
PGT-A is used to describe screening embryos for sporadic chromosome abnormalities. PGT-A refers to the testing of embryos. The number of chromosomes is counted to look for extra or missing chromosomes (“aneuploidy”). PGT-A is used to select embryos that are normal, and so are the most likely to implant and result in a successful pregnancy.
Examples of aneuploidy include trisomy 21 (Down syndrome) and monosomy X (Turner syndrome)
- Preimplantation genetic testing-monogenic/ single-gene disorders(PGT-M)
PGT-M is used to describe single-gene defects. PGT-M is used to help individuals or couples to reduce their risk to have a child with a known inherited disorder caused by mutations in a single gene (“monogenic”).
Examples of single-gene disorders include sickle cell anaemia, cystic fibrosis, Huntington disease, fragile-X, and spinal muscular dystrophy
- Preimplantation genetic testing-structural rearrangements” or PGT-SR
In PGT-SR, the chromosome abnormalities are hereditary due to one or both parents having a balanced chromosome “structural rearrangement” (such as translocations or inversions). PGT-SR reduces the risk of having a pregnancy or child with an unbalanced structural abnormality, which involves extra or missing genetic material and typically results in a pregnancy loss.
