What is the difference between Preimplantation Genetic Diagnosis (PGD) and Preimplantation genetic screening (PGS)?

Preimplantation Genetic Diagnosis or PGD is the technique of identifying genetic defects in embryos created through in vitro fertilization (IVF) before pregnancy. It is used when one or both sets of parents have a known genetic marker that could indicate some kind of abnormality and/or have a previous child affected by a genetic disease for which one/both parents are carriers of the disease. The embryo is tested to ascertain if it is affected with the disease.

While there are definite advantages of PGD and it has reduced the chances of certain genetic disorders, it cannot completely eliminate it. Besides in some genetic diseases, the symptoms and markers do not begin to show till much later in life and it is still very important to have prenatal testing.

Preimplantation genetic screening or PGS also known as aneuploidy screening is a procedure in which the genes of embryos created through in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) are examined for chromosomal abnormalities. Pregnancies where embryos show chromosomal abnormalities often end in miscarriages or could result in conditions such as Down Syndrome.

 PGS is recommended in cases when the women opting for pregnancies are over the age of 35. A fertility specialist may also recommend it if there has been a history of some chromosomal abnormalities or more than a couple of failed IVF attempts or even a few failed miscarriages.

For both PGD and PGS

1. Once the embryos are 5 days old,  few cells are micro-surgically removed from the outer layer called as trophectoderm layer for testing. DNA of the cells is examined to determine if the disease-carrying or the problematic gene is present within the cell. This process may take up 6-7 days. 
2. Once the PGD/PGS is completed and it has been established that the embryo(s) are healthy, it is implanted in the uterus. 
3. The fertility specialist often recommends that embryos that are not being implanted and free of genetic issues should be frozen for later use and the problematic embryos are destroyed. 

PGD is a big step in planning healthy babies as well as ensuring successful IVF terms.

One of the major advantages of PGD has been that parents can get quite a few genetic conditions tested. As the process for PGD is performed and determined before implantation the couple has a choice to ensure their child may be born healthy without some genetic diseases looming on the head.

Clinics advocating PGD or PGS have to have top of the line types of equipment and good lab practices as well expertise of the fertility specialist dictate and hold clout.

You should refer to a fertility specialist to understand your needs and about PGD and PGS.  It is also important to know how to choose your fertility specialists (Read: 6 things to know when choosing a fertility specialist - http://drvyjayanthifertility.com/fertility/6-things-to-know-when-choosing-a-fertility-specialist/).