What is Preimplantation Genetic Diagnosis? (PGD)

Understanding the Differences Between PGD and PGT-A, How Genetic Testing Works, Why It’s Essential for Gender Selection and Why Genetic Testing Matters in IVF

If you are considering in vitro fertilization (IVF) and have heard about preimplantation genetic testing (PGT) or use of preimplantation genetic diagnosis (PGD), you might feel overwhelmed by the terminology. Many clinics still use the term PGD, but is it the same as preimplantation genetic diagnosis for aneuploidy (PGT-A)? More importantly, how does genetic screening impact your chances of a successful pregnancy?

One of the biggest concerns for many families pursuing IVF with gender selection is ensuring they transfer a healthy embryo with the desired sex. PGT-A plays a critical role in this process by screening embryos for chromosomal abnormalities, increasing implantation rates, and helping select the most viable embryo. But not all clinics offer the same level of testing. Some analyze only a few chromosomes, while others screen all 23 pairs. Understanding these differences is essential for making an informed decision.

In this article, we break down PGD vs. PGT, explain how genetic analysis works, explore different testing methods such as FISH (Fluorescent In Situ Hybridization) and NGS (Next-Generation Sequencing), and give you the knowledge to choose the right IVF clinic for your journey.

What Is Preimplantation Genetic Diagnosis (PGD)?

Preimplantation Genetic Diagnosis (PGD) is a genetic testing method used during IVF to analyze embryos for severe genetic disorders before implantation. Originally, PGD may be referred to any genetic testing performed on preimplantation embryos, but modern terminology has evolved. Today, PGD is mostly replaced by Preimplantation Genetic Testing (PGT), which is classified into specific categories based on the type of testing performed.

The Role of PGD in IVF Success

The primary goal of PGD was to help identify embryos free of genetic diseases before implantation, improving the chances of a successful pregnancy and reducing the risk of genetic abnormalities in embryos. This method has been particularly beneficial for couples with a family history of inherited genetic disorders.

Why PGD Is an Outdated Term

Over time, the term PGD has been replaced by PGT to provide a clearer classification of genetic testing techniques. PGT is now divided into several categories:

• PGT-A (preimplantation aneuploidy testing): Detects chromosomal abnormalities and is commonly used for gender selection.
• PGT-M (Monogenic Disorders Testing): Screens for single-gene disorders such as cystic fibrosis or sickle cell anemia.
• PGT-SR (Structural Rearrangements Testing): Identifies structural chromosome issues, such as translocations.

Many clinics still refer to PGT as PGD, which can lead to confusion. If you see the term PGD used, it’s essential to ask the clinic what type of testing they are referring to.

Understanding the Difference Between Preimplantation Genetic Screening (PGS) and Genetic Testing

Previously, PGS (Preimplantation Genetic Screening or genetic diagnosis for aneuploidy screening) was used to describe the screening of embryos for chromosomal abnormalities without testing for specific genetic disorders. Like PGD, the term PGS has been phased out in favor of PGT-A, which provides a more precise classification.

PGT-A (Preimplantation Genetic Testing for Aneuploidies) is the most relevant test for IVF with gender selection, as it identifies whether an embryo has the correct number of chromosomes and determines if it is XX (female) or XY (male).

PGD vs. PGT: What’s the Difference in Genetic Testing?

Preimplantation Genetic Diagnosis (PGD) and Preimplantation Genetic Testing (PGT) are often used interchangeably, but they refer to different approaches in the field of reproductive medicine. Understanding their differences is crucial for anyone considering IVF with genetic screening.

Preimplantation Genetic Diagnosis (PGD) – The Historical Term

PGD was originally developed in the 1990s as a breakthrough method to screen embryos for specific genetic disorders before implantation. It was primarily used by couples with a known risk of passing on severe genetic diseases such as cystic fibrosis, Tay-Sachs disease, or sickle cell anemia. The process involved removing a few cells from an embryo at an early stage and analyzing them for inherited conditions.

However, PGD as a broad term became outdated as genetic testing techniques evolved. Today, PGD has largely been replaced by Preimplantation Genetic Testing (PGT), which offers more specialized screening options.

Preimplantation Genetic Testing (PGT) – The Modern Classification

PGT is the umbrella term for modern genetic screening techniques applied to embryos before implantation. Unlike PGD, which was a general term, PGT is divided into several specific categories:

• PGT-A (Preimplantation Genetic Testing for Aneuploidies) – Screens for chromosomal abnormalities, including missing or extra chromosomes, which can affect implantation success and lead to conditions such as Down syndrome. This is the primary test used in gender selection.
• PGT-M (Preimplantation Genetic Testing for Monogenic Disorders) – Detects single-gene disorders in embryos, helping couples who are carriers of genetic diseases.
• PGT-SR (Preimplantation Genetic Testing for Structural Rearrangements) – Identifies structural chromosomal changes that could lead to miscarriage or failed implantation.

Why Some Clinics Still Use the Term PGD (and Why This Can Be Confusing)

Many clinics continue to use PGD when referring to genetic testing of embryos, even though the field has shifted to more precise terms like PGT-A, PGT-M, and PGT-SR. This can lead to misunderstandings about what is actually being tested.

If a clinic advertises PGD, it is important to ask for clarification. Are they referring to screening for genetic disorders (PGT-M), or are they checking for chromosomal abnormalities (PGT-A)? Understanding this distinction helps ensure you receive the right type of genetic testing for your needs.

Types of Preimplantation Genetic Testing (PGT): Which One Do You Need?

Preimplantation Genetic Testing (PGT) is divided into three main categories, each serving a distinct purpose in the IVF process. Understanding these differences can help you choose the right test for your specific needs.

PGT-A (Preimplantation Genetic Testing for Aneuploidies) – The Most Common Test

PGT-A is used to screen embryos for aneuploidies, which are chromosomal abnormalities involving missing or extra chromosomes. This is especially important because embryos with chromosomal abnormalities often fail to implant or result in miscarriage.

PGT-A is widely used for:

• Increasing pregnancy success rates by ensuring only chromosomally normal embryos are transferred.
• Reducing miscarriage risks by eliminating embryos with genetic abnormalities.
• Gender selection, as PGT-A can identify whether an embryo is XX (female) or XY (male).

Since aneuploidies increase with maternal age, PGT-A is particularly recommended for women over 35.

PGT-M (Preimplantation Genetic Testing for Monogenic Disorders) – For Inherited Diseases

PGT-M is used to detect single-gene disorders that parents might pass on to their children. It is recommended for couples who are carriers of serious genetic disease and conditions, such as:

• Cystic fibrosis
• Tay-Sachs disease
• Sickle cell anemia
• Huntington’s disease

PGT-M ensures that only embryos without the specific inherited disease are selected for transfer, providing peace of mind for families with a history of genetic disorders.

PGT-SR (Preimplantation Genetic Testing for Structural Rearrangements) – For Chromosomal Abnormalities

PGT-SR is designed for individuals or couples with known chromosomal rearrangements, such as translocations or deletions, which can lead to miscarriage or birth defects. This test helps:

• Identify embryos that have balanced chromosomal structures.
• Reduce the risk of implantation failure or pregnancy loss.
• Improve the chances of a successful pregnancy for those with chromosomal translocations.

Which Type of PGT Is Right for You?

• If you are undergoing IVF for gender selection, PGT-A is essential.
• If you have a family history of a specific genetic disease, PGT-M is recommended.
• If you or your partner have chromosomal abnormalities, PGT-SR is the best option.

As genetic testing technology continues to evolve, PGT allows for more precise embryo selection, increasing IVF success rates and providing families with greater control over their reproductive choices.

PGT-A and Gender Selection: How Genetic Testing Helps You Choose the Sex of Your Baby

Preimplantation Genetic Testing for Aneuploidies (PGT-A) is the most effective and reliable method for selecting the sex of an embryo before implantation. It ensures that only embryos with the correct number of chromosomes are transferred, improving pregnancy success rates and reducing the risk of miscarriage.

How PGT-A Identifies XX (Female) and XY (Male) Embryos

PGT-A involves analyzing embryos created through in vitro fertilization (IVF) at the blastocyst stage (usually on day 5 or 6). A few cells are carefully removed from the embryo and tested to determine whether they contain XX (female) or XY (male) chromosomes.

Unlike older methods such as sperm sorting, which are less accurate, PGT-A provides nearly 100% accuracy in sex determination. This makes it the preferred method for couples who are looking to balance their family by choosing the sex of their child.

Why PGT-A Is the Most Accurate Method for Gender Selection

• Screens for all chromosomal abnormalities – ensuring only healthy embryos are selected.
• Nearly 100% accuracy in determining the sex of the embryo.
• Reduces the risk of miscarriage by eliminating embryos with chromosomal abnormalities.
• Improves implantation success rates, leading to a higher chance of a successful pregnancy.

The Role of Genetic Screening in Improving IVF Success Rates

Beyond gender selection, PGT-A significantly increases the success rates of IVF by identifying the healthiest embryos. Embryos with chromosomal abnormalities are often the cause of failed implantation and early miscarriage. By selecting euploid (chromosomally normal) embryos, PGT-A helps:

• Increase clinical pregnancy rates.
• Decrease the number of failed IVF cycles.
• Reduce the emotional and financial burden of multiple unsuccessful transfers.

For couples considering IVF with gender selection, choosing a clinic that offers comprehensive PGT-A testing is essential. However, not all clinics test the same number of chromosomes, so it is important to confirm whether all 23 pairs are being analyzed for the highest accuracy.

Mosaic Embryos: What If Your Embryo Has Genetic Abnormalities?

Mosaic embryos are embryos that contain both normal (euploid) and abnormal (aneuploid) cells. This means that some cells have the correct number of chromosomes, while others have genetic abnormalities. Mosaicism can be detected through preimplantation genetic testing (PGT-A), but interpreting the results can be complex.

Can a Mosaic Embryo Be Transferred After Preimplantation Genetic Screening?

Many fertility clinics once considered mosaic embryos unsuitable for transfer, but recent research suggests that some low-level mosaic embryos can still result in a healthy pregnancy. Whether a mosaic embryo should be transferred depends on several factors:

• The percentage of abnormal cells within the embryo.
• Which chromosomes are affected by mosaicism.
• The patient’s age and previous IVF history.

Why Mosaicism Is a Controversial Topic in Genetic Analysis

Mosaic embryos exist on a spectrum, from low-level mosaicism (where only a few abnormal cells are detected) to high-level mosaicism (where a large percentage of cells are abnormal). This variability makes predicting their implantation potential difficult.

Some clinics refuse to transfer mosaic embryos, while others may consider it under specific circumstances. If you are offered a mosaic embryo transfer, it’s crucial to discuss the risks and potential outcomes with your doctor and a genetic counselor.

How to Interpret Mosaic Results After PGT-A

When PGT-A results indicate mosaicism, clinics often classify embryos as:

• Low-level mosaic: Less than 30% of cells show abnormalities. These embryos have a higher chance of implantation.
• High-level mosaic: More than 50% of cells are abnormal. The chances of successful implantation are lower, and there may be an increased risk of miscarriage.

In some cases, embryos initially identified as mosaic can still lead to successful pregnancies, but decisions should always be made based on expert consultation.

Not All Clinics Test the Same Number of Chromosomes in Embryos!

Not all fertility clinics perform the same level of preimplantation genetic testing (PGT-A). Some only screen a limited number of chromosomes, while others analyze all 23 pairs. This difference can significantly impact the accuracy of the results and the success of an IVF cycle.

Do They Test 5, 7, 9, or All 23 Chromosomes?

Some clinics offer limited PGT-A, screening only a subset of chromosomes, such as the most commonly affected ones (e.g., chromosomes 13, 18, 21, X, and Y). While this can help detect major genetic abnormalities, it may miss others that can still lead to failed implantation or miscarriage.

More advanced clinics perform comprehensive PGT-A, testing all 23 chromosome pairs to ensure the highest accuracy in embryo selection. The more chromosomes analyzed, the lower the risk of transferring an embryo with undetected abnormalities.

Why Incomplete Preimplantation Genetic Screening Can Lead to Wrong Results

If a clinic only screens a few chromosomes, an embryo may still have abnormalities in untested chromosomes. This means:

• The embryo may appear normal but carry undetected genetic defects.
• The risk of miscarriage remains higher than with full chromosome screening.
• The chance of implantation failure increases.

For patients investing in IVF with PGT-A, choosing a clinic that offers full 23-chromosome testing is crucial for maximizing success rates and reducing risks.

What to Ask Your Clinic Before Choosing an IVF Center

Before deciding on a fertility clinic, ask the following questions about their PGT-A process:

• How many chromosomes do you test?
• Do you use FISH or NGS for genetic analysis?
• What percentage of embryos tested show abnormalities?
• How do you handle mosaic embryos?
• What are the success rates for PGT-A-tested embryo transfers?

By getting clear answers, you can make an informed choice and improve your chances of a successful pregnancy.

Genetic Testing Methods: FISH vs. NGS – Which One Is More Accurate?

Understanding the different methods used for preimplantation genetic testing (PGT-A) is crucial when selecting an IVF clinic. The two primary techniques are Fluorescence In Situ Hybridization (FISH) and Next-Generation Sequencing (NGS). Each method has its advantages and limitations.

FISH (Fluorescence In Situ Hybridization) – The Older and Faster Method

FISH is an older method of genetic screening that examines a limited number of chromosomes in embryos. This technique:

• Is performed on day 3 of embryo development when the embryo has only a few cells.
• Allows for a fresh embryo transfer on day 5, eliminating the need for freezing and storage.
• Typically tests only a select number of chromosomes (often 5 to 9), meaning some chromosomal abnormalities may go undetected.
• Is considered less accurate compared to newer technologies like NGS.

Although FISH provides faster results, it is increasingly being replaced by more comprehensive methods, as it does not analyze all 23 chromosome pairs.

NGS (Next-Generation Sequencing) – The Gold Standard for Genetic Testing

NGS is the most advanced method for preimplantation genetic testing and offers significant advantages over FISH:

• Performed on day 5 (blastocyst stage), when the embryo has more cells available for testing.
• Requires embryo freezing, as results take 2–3 weeks to process.
• Tests all 23 chromosome pairs, providing the highest level of accuracy in identifying genetic abnormalities.
• Increases clinical pregnancy rates by ensuring that only euploid (chromosomally normal) embryos are transferred.

Which Method Should You Choose?

• FISH may be an option if you prefer a fresh transfer and need rapid results, but be aware of its limitations.
• NGS is recommended for patients who want the most comprehensive genetic screening with the highest success rates in IVF.

Most leading fertility clinics have shifted to NGS due to its superior accuracy and reliability. When considering an IVF clinic, always ask which method they use for PGT-A and whether they test all 23 chromosomes to ensure the best possible outcome.

Should You Do Preimplantation Genetic Testing (PGT-A)? Pros and Cons

Preimplantation Genetic Testing for Aneuploidies (PGT-A) is a powerful tool in assisted reproduction, but it is not necessary for everyone undergoing in vitro fertilization (IVF). Understanding the benefits and potential drawbacks can help you make an informed decision.

Who Benefits Most from PGT-A?

PGT-A is particularly recommended for:

• Women over 35 – The risk of chromosomal abnormalities increases with maternal age, and PGT-A helps ensure only euploid embryos (chromosomally normal) are transferred.
• Couples with recurrent miscarriages – Many pregnancy losses result from undetected chromosomal abnormalities. PGT-A helps identify viable embryos, reducing the risk of miscarriage.
• Families pursuing gender selection – PGT-A is the only scientifically validated method for accurately determining an embryo’s sex before implantation.
• Patients with previous failed IVF cycles – If multiple IVF attempts have failed, selecting only genetically normal embryos through PGT-A can improve success rates.
• Carriers of balanced translocations – Individuals with structural chromosome rearrangements benefit from PGT-A to select embryos without abnormalities.

When PGT-A May Not Be Necessary

PGT-A is not always recommended in every IVF case. Some scenarios where it may not provide a significant advantage include:

• Young patients with high-quality embryos – Women under 35 with no history of miscarriage or genetic conditions often have a high likelihood of success without PGT-A.
• Limited number of embryos – If a patient produces very few embryos, testing may not be necessary, as all embryos will be considered for transfer regardless.
• Cost considerations – PGT-A adds additional costs to the IVF cycle. If budget constraints are a concern, prioritizing standard IVF treatment may be an alternative.

Potential Risks and Considerations of PGT-A

• Embryo biopsy risks – Though generally safe, the biopsy process involves removing cells from the embryo, which may pose minimal risks to its development.
• False positive/negative results – No test is 100% perfect. Occasionally, embryos may be misclassified, leading to potentially viable embryos being discarded.
• Ethical considerations – Some patients feel uncomfortable with embryo selection based on genetic screening results.

Making an Informed Decision

Discussing PGT-A with your fertility specialist can help you determine whether it aligns with your goals and medical history. If gender selection or reducing miscarriage risk is a priority, PGT-A is a valuable tool that significantly improves IVF success rates.

How to Choose the Right IVF Clinic for in vitro fertilization (IVF) with Preimplantation Genetic Screening

Choosing the right IVF clinic for preimplantation genetic screening (PGT-A) is essential for ensuring the best possible outcomes. Not all clinics offer the same level of genetic testing, so knowing what to look for can make a significant difference in your IVF journey.

Key Questions to Ask Before Starting Genetic Testing

Before selecting an IVF clinic, consider asking the following questions to ensure you are receiving the highest quality genetic screening:

• Which genetic testing method do you use? – Ask whether they use Next-Generation Sequencing (NGS) or Fluorescence In Situ Hybridization (FISH), as NGS provides more accurate results.
• Do you test all 23 chromosome pairs? – Some clinics test only a limited number of chromosomes, which can lead to undetected abnormalities.
• What are the success rates for PGT-A tested embryos? – Request clinic-specific data on pregnancy rates for embryos that have undergone genetic testing.
• How do you handle mosaic embryos? – Not all clinics allow mosaic embryo transfers, so it’s essential to understand their policy.
• What is the cost of PGT-A testing? – Pricing can vary significantly between clinics, so ask about total costs, including the biopsy, freezing, and additional procedures.

Why Some Clinics Still Use FISH Instead of NGS

While NGS is the gold standard for genetic screening, some clinics still use FISH because it is faster and allows for fresh embryo transfers. However, FISH does not test all chromosomes, making it less reliable in detecting genetic abnormalities. If a clinic only offers FISH, consider whether the convenience of a fresh transfer outweighs the benefits of comprehensive genetic analysis.

The Importance of Transparent Genetic Analysis and Screening

A reputable clinic should provide clear and transparent information about their genetic testing procedures. Look for clinics that:

• Offer comprehensive preimplantation genetic testing covering all 23 chromosome pairs.
• Provide detailed reports on embryo testing results.
• Are open about their IVF and genetic testing success rates.
• Have experienced embryologists and genetic counselors available to discuss results.

By choosing a clinic with advanced PGT-A technology and high success rates, you maximize your chances of a healthy pregnancy and a successful IVF outcome.

• Key questions to ask before starting genetic testing
• Why some clinics still use FISH instead of NGS
• The importance of transparent genetic analysis and screening

Final Thoughts: What You Need to Know Before Doing Genetic Analysis with PGT-A in IVF

Preimplantation Genetic Testing for Aneuploidies (PGT-A) has revolutionized the field of in vitro fertilization (IVF), making it possible to select healthy embryos and improve pregnancy outcomes. For couples considering gender selection or looking to minimize the risk of genetic abnormalities, PGT-A offers a scientifically validated method to enhance success rates.

The Most Important Takeaways About Preimplantation Genetic Testing

• PGT-A is the most reliable way to identify chromosomally normal embryos, ensuring higher implantation rates and lower miscarriage risks.
• Not all clinics offer the same level of genetic screening, so it's essential to verify whether all 23 chromosomes are tested.
• Choosing between FISH and NGS can impact accuracy, with NGS being the superior option for most patients.
• Mosaic embryos can sometimes be viable, and understanding the clinic’s policy on their transfer is crucial.
• PGT-A is the best method for gender selection, offering nearly 100% accuracy in determining an embryo’s sex.

Why PGT-A Is the Best Method for Gender Selection

For families considering gender selection, PGT-A provides the most precise method for choosing between XX (female) and XY (male) embryos. Unlike older techniques such as sperm sorting, which have a lower accuracy rate, PGT-A delivers nearly guaranteed results.

Additionally, by ensuring that only euploid embryos (chromosomally normal) are transferred, PGT-A significantly reduces the risk of implantation failure and miscarriage.

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