Human reproduction is complex, and successful pregnancy relies heavily on the genetic integrity of the embryo. Chromosomes carry the complete set of genetic instructions and must be present in the correct number for proper development. This leads to a central question: can an embryo with a chromosomal abnormality successfully implant in the uterus? The answer is nuanced; initial implantation is possible, but the genetic error often determines the ultimate fate of the pregnancy.
Defining Aneuploidy and Embryo Viability
An embryo with the correct number of chromosomes is described as euploid, meaning it possesses 46 chromosomes arranged in 23 pairs, which is the standard genetic complement for a human cell. Embryos that contain an abnormal number of chromosomes are termed aneuploid. This abnormality can manifest as having an extra copy of a chromosome, known as a trisomy, or a missing copy of a chromosome, referred to as a monosomy.
Aneuploidy arises from errors in cell division, most commonly during the formation of the egg or sperm, a process called meiosis. These errors result in the gamete carrying too many or too few chromosomes, which is then passed on to the resulting embryo after fertilization. The most recognized examples of aneuploidy that can result in a live birth are trisomies, such as Trisomy 21 (Down Syndrome), Trisomy 18 (Edwards Syndrome), and Trisomy 13 (Patau Syndrome).
The presence of aneuploidy significantly affects the embryo’s viability, or its capacity to develop normally. Most aneuploidies are severe and incompatible with sustained development, causing the embryo to fail early on. The risk of aneuploidy is strongly correlated with increasing maternal age, particularly after age 35, because egg cell quality declines over time.
Implantation Success Versus Sustained Pregnancy
Chromosomally abnormal embryos often achieve initial implantation in the uterine wall. Implantation is primarily driven by the outer layer of the blastocyst, known as the trophectoderm, which eventually forms the placenta. The inner cell mass (ICM), which develops into the fetus, plays a less immediate role in the initial attachment process.
The trophectoderm cells may be sufficiently functional to interact with the uterine lining and initiate implantation, even if the inner cell mass carries a severe genetic error. This difference is due to mosaicism, where an embryo contains a mixture of both euploid (normal) and aneuploid (abnormal) cells. If the trophectoderm is largely or entirely euploid, it can successfully attach to the uterus.
Initial implantation is distinct from a sustained, viable pregnancy, and this is where the chromosomal abnormality becomes problematic. Once attached, the embryo must undergo rapid, organized cell division and differentiation, but the genetic error prevents necessary developmental milestones. The aneuploid cells cannot execute the complex genetic program required for fetal organogenesis. This failure leads to a halt in development, often termed developmental arrest.
The vast majority of pregnancies involving aneuploid embryos result in failure soon after implantation. This failure can present clinically as a biochemical pregnancy, where human chorionic gonadotropin (hCG) is detected but the pregnancy does not progress, or as an early miscarriage. Chromosomal abnormalities are the single most common cause of pregnancy loss, accounting for 50% to 60% of all first-trimester miscarriages. The embryo’s inability to organize into a healthy fetus is the primary mechanism of loss, rather than a failure of the uterus to accept the embryo.
Preimplantation Genetic Testing for Aneuploidy (PGT-A)
Given the high incidence of aneuploidy as a cause of pregnancy loss, a clinical tool known as Preimplantation Genetic Testing for Aneuploidy (PGT-A) is used to screen embryos created via In Vitro Fertilization (IVF). This testing, previously called Preimplantation Genetic Screening (PGS), evaluates the chromosomal status of the embryo before transfer. The goal of PGT-A is to identify and select euploid embryos for transfer, increasing the chance of a successful, ongoing pregnancy.
The procedure involves performing a biopsy on the blastocyst, removing a small number of cells from the outer trophectoderm layer. These cells are analyzed to count the number of chromosomes, and only embryos confirmed to be euploid are prioritized for transfer. By transferring only chromosomally normal embryos, PGT-A significantly reduces the risk of miscarriage and decreases the time required to achieve a viable pregnancy.