For many years, preimplantation genetic diagnosis (PGD) was the only term used for all genetic testing of the embryos. With the explosion of new technology in field of genetic testing, clinicians have introduced preimplantation genetic screening (PGS), which is used for screening genetic abnormalities in patients without known genetic disorders. The public commonly uses PGD and PGS interchangeably but they are not the same.
Preimplantation genetic diagnosis (PGD)
PGD is typically done in couples who are known carriers of genetic disorders or have a family history of genetic disorders. This test is done to look for a specific genetic abnormality or abnormalities that the embryo may carry. There are over 4,000 single-gene disorders but only about 200 disorders and some forms of inherited cancers (i.e. retinoblastoma and BRCA2) can be diagnosed using PGD-PCR
Here are some indications for PGD in single-gene disorders according to European Society of Human Reproduction and Embryology (ESHRE) data:
- B-thalassemia
- Cystic fibrosis
- Huntington’s disease
- Fragile X X-linked
- Myotonic dystrophy
- Spinal muscular atrophy
- Neurofibromatosis type I
- Duchenne’s muscular dystrophy
- Marfan syndrome
- Hemophilia A
- Tuberous sclerosis
Preimplantation genetic screening (PGS)
Unlike PGD that test for specific genetic conditions, PGS looks at the overall chromosomes. PGS has been recently used to improve pregnancy success by screening embryos for abnormal number of chromosomes.
Humans have 46 chromosomes, in which 23 are inherited from our mothers and 23 are inherited from our fathers. But frequently, embryos do not have the correct number of chromosomes (aneuploidy) due to a variety of known and unknown reasons.
Embryos with aneuploidy will either stop developing in the early stages, cause spontaneous abortion if the embryo implants onto the uterus or in rare cases, a child will be born with abnormal chromosomes. According to a study by Munné et al., at least 40% to 60% of human embryos are abnormal, and this number increases to 80% in women 40 years or older.
By using PGS to screen for the correct number of chromosomes and to ensure only the normal embryos are transferred, the clinicians can try to increase the pregnancy success rates. Unlike PGD, PGS can be used for all patients, especially women with advanced maternal age or couples with reoccuring, unexplained abortions. PGS can also be also used to determine the sex of the embryo by looking for the X and Y chromosomes (XX for female and XY for male).
Picture shows blood chromosome analysis. (Left) A normal female with 46 chromosomes. (Right) A male with Down syndrome, three 21 chromosomes. Available at: http://geneticsandfertility.com/pgs-and-pgd-ivf/ (Accessed April 22nd, 2017)
Reference
Offt K, Kohut K, Clagett B, Wadsworth EA, Lafaro KJ, Cummings S, et al. Cancer genetic testing and assisted reproduction. J Clin Oncol. 2006;24:4775–82.
Moutou C, Goossens V, Coonen E, De Rycke M, Kokkali G, Renwick P, et al. ESHRE PGD Consortium data collection XII: cycles from January to December 2009 with pregnancy follow-up to October 2010. Hum Reprod. 2014;29:880–903.
Munné S. Preimplantation genetic diagnosis for aneuploidy and translocations using array comparative genomic hybridization. Curr Genomics. 2012;13:463–70.