Advancements in genetic testing have transformed prenatal care, offering prospective parents insights into their child’s health before and during pregnancy. These tools provide valuable information but can also raise questions about which tests are suitable or if multiple screenings are necessary. This article aims to clarify the distinct roles of Preimplantation Genetic Screening (PGS) and Non-Invasive Prenatal Testing (NIPT) and address why NIPT might still be considered even after an embryo has undergone PGS.
Understanding Preimplantation Genetic Screening
Preimplantation Genetic Screening, often referred to as PGT-A (Preimplantation Genetic Testing for Aneuploidy), is a procedure conducted as part of in vitro fertilization (IVF). This screening examines embryos for chromosomal abnormalities, known as aneuploidies, before implantation. The primary goal of PGT-A is to identify embryos with the correct number of chromosomes, typically 46, to enhance IVF success rates and reduce the likelihood of miscarriage or the birth of a child with a chromosomal condition.
The process involves taking a small biopsy, usually a few cells from the outer layer of the embryo called the trophectoderm, around day 5 or 6 of its development. These cells are then analyzed for missing or extra chromosomes, such as Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), or Trisomy 13 (Patau syndrome). Selecting chromosomally normal embryos for transfer can improve implantation rates and lead to healthier pregnancies.
Understanding Non-Invasive Prenatal Testing
Non-Invasive Prenatal Testing (NIPT) is a blood test performed during pregnancy, typically starting around 10 weeks of gestation. This screening analyzes cell-free DNA (cfDNA) circulating in the mother’s bloodstream, a portion of which originates from the placenta. NIPT primarily screens for common chromosomal conditions like Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18), and Patau syndrome (Trisomy 13).
The test can also screen for aneuploidies involving the sex chromosomes, such as Turner syndrome (45, X) or Klinefelter syndrome (47, XXY). NIPT is a screening test, meaning it indicates a higher or lower risk for certain conditions, but it does not provide a definitive diagnosis. Positive NIPT results typically require confirmation through diagnostic procedures like amniocentesis or chorionic villus sampling.
Distinguishing Between PGS and NIPT
PGS and NIPT serve different purposes and are performed at distinct stages of the reproductive journey. PGS is conducted before pregnancy, during the IVF process, directly on the embryo. It involves sampling cells from the embryo’s trophectoderm to identify chromosomally normal embryos, aiming to improve IVF success rates.
NIPT, conversely, is a prenatal screening performed during an ongoing pregnancy, typically from 10 weeks onward. It analyzes cell-free DNA from the placenta found in the mother’s blood to screen for chromosomal conditions in the developing fetus. A key distinction is that PGS assesses the embryo prior to implantation, while NIPT screens the ongoing pregnancy. Furthermore, NIPT provides a risk assessment, not a definitive diagnosis, meaning positive results require further diagnostic testing for confirmation.
When NIPT May Still Be Recommended After PGS
Even after an embryo has undergone PGS, NIPT may still be recommended due to several factors. PGS involves biopsying cells from the trophectoderm, the outer layer of the embryo that becomes the placenta, not directly from the fetus. While accurate for the sampled cells, mosaicism within the embryo (where some cells are normal and others aneuploid) might be missed if abnormal cells are not sampled.
New chromosomal changes, known as de novo mutations, can also occur after implantation and initial embryo development, which PGS would not detect. These spontaneous mutations arise during cell division in the developing fetus or placenta. Another consideration is confined placental mosaicism (CPM), where chromosomal abnormalities are present in the placenta but not in the fetus. Since NIPT analyzes placental DNA, CPM can lead to a positive NIPT result even when the fetus is chromosomally typical, necessitating further investigation.
NIPT may also screen for a broader range of microdeletions or duplications not always covered by standard PGS panels, offering an additional layer of screening for other genetic changes that arise post-implantation.
Deciding on Further Genetic Screening
The decision to undergo NIPT after PGS is a personal one, best made in collaboration with healthcare professionals. Consulting with an obstetrician, fertility specialist, or a genetic counselor is highly recommended. These experts can evaluate individual risk factors, review specific PGS results, and discuss the benefits and limitations of additional screening. They can also help individuals weigh factors like maternal age, family medical history, and specific concerns, guiding them through the complexities of genetic information to ensure an informed choice.