Genetic Screening for Embryos: Understanding PGS and PGD

pgs mother daughters

Sometimes I think to myself how amazing it is that we live in a world where we can order, pay for and direct the delivery of our food through an app on our smartphones. It’s also incredible that we use GPS to find a car nearby that offers a ride for a small fee, and that we can pay for that ride via an app on our smartphones. The world is advancing at a rapid pace due to technology, and on most days I think we are better off because of these changes.

This brave new world is also the landscape in which swift advances are being made in assisted reproductive technology. Two of the most common and popular topics in assisted reproductive technology are preimplantation genetic screening (PGS) and preimplantation genetic diagnosis (PGD). These procedures are both used to screen embryos for their genetic makeup before they are implanted into the uterus during in vitro fertilization (IVF). As technology gets better so does our ability to diagnose deadly genetic disorders and ultimately deliver more disease-free babies.

While many patients visit the Women & Infants’ Fertility Center thinking that they understand what these procedures are and what they mean, more often than not they come away learning something new. It’s important that fertility patients understand PGS, PGD, the differences between the two and the benefits that they offer.


Let’s start with the procedure that’s been around for a bit longer. Preimplantation genetic diagnosis (PGD) began in the early 1980s when physicians began performing biopsies on embryos. PGD involves testing an embryo for a known genetic disorder by diagnosing a specific mutation that either one or both parents carry.

For example, if a couple is undergoing IVF and both individuals are carriers for cystic fibrosis, they could use PGD to determine which of their embryos have the disorder and which do not. A fertility specialist can then divide the embryos into which do not have the cystic fibrosis makeup and those that do. The disorder-free embryos are transferred into the uterine cavity in hopes of achieving a successful pregnancy and delivering a disease-free child.

In the instance of cystic fibrosis, it is advantageous for the couple to know which embryos carry the disorder and which do not. Cystic fibrosis can be very devastating for young children and young adults because it causes severe lung problems very early on. There are also some known diseases that can be passed on if only one parent has the disorder. Huntington’s disease is a good example. In the case of Huntington’s disease, a genetic disorder causes the progressive breakdown in the nerve cells of the brain and patients suffer from a progressive deterioration that ultimately results in death at a young age.


Another procedure in the same vein as PGD is preimplantation genetic screening (PGS). PGS is similar to PGD but it takes the screening a step further. The purpose of PGS is to look for more structural defects in the embryo’s chromosome as opposed to PGD which tests for specific, small mutations. PGS looks at the entire chromosomal structure of the embryo.

During PGS we test to see if chromosomes within the embryo’s DNA are normal. An abnormality means that there is an irregular number of chromosomes, which can cause miscarriages or can result in an increased risk of certain chromosomal diseases, such as Down syndrome.

As physicians, we know that the risk of a woman having a child with Down syndrome increases as the woman gets older. If Down syndrome is present, an embryo’s genetic makeup will have an extra chromosome. Scientists and physicians know that if there are three of chromosome 21 versus the normal 2, the embryo has Down syndrome.

Addressing the rumors

In light of all of the amazing things that we’ve been able to do as a result of PGD and PGS, there are some who believe that the science might be taking things too far. There are several over exaggerated stories that use scare tactics and non-evidence based predictions to point toward a “designer baby” culture. Skeptics of gene editing and the negative impact on our future use statements like, “What’s stopping us from choosing our baby’s eye color, hair color or even intelligence level?”

In all reality, we are nowhere near being able to “design,” babies. PGD and PGS are not those kinds of techniques. What we are truly aiming to do is identify what already exists, find embryos that are normal and use the knowledge to optimize a couple’s chances of having a normal, single pregnancy that is free of disease.