PGS v. PGD
Some confusion exists between these two.
PGD tests for genetic issues.
PGS tests for chromosomal abnormalities.
Websites and even my clinic use the terms interchangeably so just make sure you make clear what/why you are testing and you should be fine.
Here is some helpful information from my clinic's website:
What is PGS?
PGS is preimplantation genetic screening (PGS) of embryos after IVF. PGS can identify which embryo are chromosomally normal. Embryos with the correct number of chromosomes can be transferred to uterus (womb) or can be frozen for future use. PGS can improve the chance of getting pregnant and carrying to term, and it can reduce the chance of having a baby with a condition like Down syndrome. It can also decrease the chance miscarriage due to aneuploidy.
What are Chromosomes?
Chromosomes are string-like structures found in the center of every cell (the nucleus). Chromosomes contain genes that are made of DNA. Therefore, our inherited information is housed on the chromosomes. Normal human cells (embryo, fetus, baby or adult) contain 46 Chromosomes or 23 pairs. We receive 23 chromosomes from each parent. The first 22 pairs are the same for men and women and labeled largest to smallest: 1 through 22. The 23-rd pair determines our gender. To test for a chromosome abnormality such as Down syndrome, the chromosomes are studied.
Gender Selection or Sex Selection
The PGS report will indicate which embryos are males and which ones are females. If an embryo has one X chromosome and one Y chromosome, the gender is male. If there are two X chromosomes, then the gender is female.
Many Embryos have an abnormal number of chromosomes
Studies from many different IVF clinics have shown that even embryos with good physical appearance or “morphology” can have chromosome problems. Even in women younger than 35, at least one third of the embryos have abnormal numbers of chromosomes. The number of embryos that have abnormal chromosomes increases each year as a women ages.
Chromosomal Aneuploidy
Spermatozoa or eggs that have extra or missing chromosomes will pass this problem on to the embryo after fertilization. This situation is known as aneuploidy. There can be extra (trisomy) or missing (monosomy) chromosomes. Both conditions are a problem. If the aneuploidy involves the larger chromosomes, the embryo may not attach to the wall of the uterus or may stop developing soon after and miscarry. In some cases, however, the aneuploidy may cause the fetus to be abnormal but carry to birth. Down syndrome is an example of this, but there are several other types. The features of the chromosome condition depend upon which chromosome is extra or missing, but can include physical abnormalities and mental retardation.
Risk of Aneuploidy and Maternal Age
As a woman advances in age, the chance of aneuploidy in her pregnancies increases. This association is due to the fact that a woman’s eggs are as old as she is. Females have all of their eggs from the fetal stage on, therefore they are born with all the eggs they will have in their lifetime. As such, the theory regarding aneuploidy risk and advancing maternal age is that, over time, the chromosomes in the egg are less likely to divide properly leading to the egg having an extra or missing chromosome. The risk of conceiving an abnormal baby increases with maternal age, but the frequency of aneuploidy in embryos is much higher than at delivery. This difference in percentages of affected embryos versus live born is due to the fact that a pregnancy with aneuploidy is less likely to attach to the uterus or go to term. Most will not implant or will be miscarried. The percentage of affected pregnancies is reduced over the course of the pregnancy. The lack of implantation and loss rate of aneuploid embryos are believed to be the main reasons why pregnancy rates decrease with advancing maternal age.
Avoiding Transfer of Chromosomally Abnormal Embryos
Aneuploid embryos are mostly indistinguishable morphologically and developmentally from chromosomally normal ones. The PGS report will indicate which embryos have the correct number of chromosomes for transfer.
The PGS Procedure
PGS has been developed to test embryos prior to the embryo transfer. This technique involves removing (biopsy) one or more cells from each embryo, followed by a very fast genetic analysis using a technique called array CGH (aCGH) for fresh transfers or Next Generation Sequencing (NGS) for embryos that will be frozen for a subsequent cycle. Embryos with a normal number of chromosomes can be transferred back to the uterus. Euploid embryos (those having the correct number of chromosomes) have a higher chance of implanting and resulting in a healthy pregnancy.
Blastocyst Biopsy on Day 5 or 6 of Culture
Main Line Fertility is one of the few centers in the USA that has the specialized equipment and trained embryologists to perform biopsy of blastocysts. Blastocysts are embryos that have hundreds of cells that have differentiated into an inner cell mass (that will become the fetus) and the trophectoderm (that will become the placenta). During blastocyst biopsy, a hole is made in the shell of the embryo and several cells are removed from the trophectoderm. Chromosomally normal blastocysts can be frozen for future attempts at pregnancy.
In addition, it is possible to thaw cryopreserved embryos, perform PGS or PGD, and then transfer the healthy embryo/s in a frozen embryo transfer (FET) cycle.
Advantages of the PGS
Most chromosomally abnormal embryos either do not implant or spontaneously abort shortly after implantation. Thus, if only normal embryos are replaced, which have higher chances of implanting and reaching term, the probability of delivering a healthy child may increase if PGS is applied.
PGS of aneuploidy has been proven to double implantation rates in several studies, reduce the rate of pregnancy loss by half, and increase take-home baby rates.
Advantages of Freezing Embryos after PGS or PGD
Embryos can be frozen after biopsy and then later transferred in a frozen embryo transfer (FET) cycle. An advantage of this is the best embryo can be selected from day 5 and day 6 blastocysts. Moreover, NGS testing can be performed which provides more detailed genetic information through complete genome sequencing. In addition, the uterus may be more receptive.
Banking or Batching
Patients have the opportunity to freeze and bank (or batch) multiple biopsied embryos from several IVF cycles. When patients have the desired number of biopsied blastocysts, they can “run” (or analyze) the frozen biopsied cells at the genetics lab, and pay for testing only once. One or two tested embryo/s can be transferred back to the uterus in a frozen embryo transfer (FET) cycle.
Risks of the PGS Procedure
While PSS is a relatively new procedure in IVF, the micromanipulation or biopsy techniques required to perform the procedure have been in use for many years. The risk of accidental damage to an embryo during the removal of the cell(s) is less than 1% in experienced fertility centers. Additionally, no part of the future fetus will be compromised or missing because of the removal of a cells.
The test may occasionally classify an abnormal embryo as normal. Very few of such pregnancies have occurred. The reverse may happen, too – a normal embryo that is tested may be classified as abnormal by mistake, though the chance of this is also small. Again, due to the small chance of misdiagnosis as well as the presence of conditions not tested for via PGD, prenatal testing is still recommended.
Not all genes or chromosomes can be studied by PGS and one cannot test for both genes and chromosomes from the single cells concurrently. Neither test is 100% accurate because we can only biopsy a single cell from the embryo, thus follow-up prenatal testing via chorionic villous sampling (CVS) or amniocentesis is highly recommended.
Which Patients Benefit the Most
- Women 37 and older: Any IVF patient 37 years of age or older may benefit from PGS, provided that they produce 5 or more embryos.
- Women with a prior history of multiple miscarriage or aneuploid pregnancies: Regardless of age, these patients could benefit from PGS. In all these patients, higher implantation rates, reduced pregnancy loss and reduced risk of chromosomally abnormal conceptions are expected after PGS. It is not clear yet if patients with repeated IVF failure benefit from PGS.
- Patients with a chromosome condition: Individuals with certain chromosome conditions can reduce their chance of passing the condition to their child via PGS.
- Severe male infertility: A high rate of chromosome abnormalities has been seen in embryos from men with non-obstructive azoospermia. PGS may also be indicated for other cases of very severe male infertility.
- Couples who are interested in family balancing
What About Cost?
This procedure may add several thousand dollars to the cost of IVF. Few insurance policies cover the expense.
