Egg quality fundamentally refers to the oocyte’s potential to be fertilized and develop into a genetically normal, viable embryo capable of resulting in a healthy live birth. This assessment is crucial for guiding treatment decisions, setting realistic expectations, and selecting the best eggs or embryos for transfer in fertility treatment and assisted reproductive technologies (ART). Evaluating quality involves a combination of indirect clinical screening, immediate laboratory observation, and, most definitively, genetic analysis of the resulting embryo.
Clinical Screening: Indirect Predictors of Quality
Before any eggs are retrieved, clinicians rely on markers of ovarian reserve to estimate both the quantity and the likely quality of the remaining eggs. The most powerful predictor of declining egg quality remains maternal age, as the rate of chromosomal abnormality increases significantly after the mid-30s.
Anti-Müllerian Hormone (AMH)
Testing AMH provides a valuable measure of the ovarian reserve, reflecting the number of small follicles remaining in the ovaries. AMH is produced by the granulosa cells of these early-stage follicles. A higher level generally correlates with a larger pool of available eggs, suggesting a higher chance of retrieving a high-quality one.
Antral Follicle Count (AFC)
The AFC is an ultrasound measurement of the small follicles visible in the ovaries early in the menstrual cycle. These follicles, typically 2 to 10 millimeters in size, represent the cohort that may respond to stimulation in an ART cycle. A higher AFC count predicts a better response to fertility medications and a greater number of eggs available for selection.
Follicle-Stimulating Hormone (FSH)
The FSH level, measured on the third day of the menstrual cycle, also provides insight into ovarian function. High basal FSH levels suggest that the pituitary gland is working harder to stimulate the ovaries, indicating a diminished ovarian reserve. While these hormonal and ultrasound tests are excellent predictors of egg quantity and ovarian response, they offer only an indirect estimate of an individual egg’s true quality.
Laboratory Assessment: Morphology and Maturity
Once eggs are retrieved, embryologists perform an immediate, non-invasive assessment of their physical characteristics under a microscope. The first and most important assessment is determining the egg’s maturity, as only a mature egg can be successfully fertilized.
Oocyte Maturity
A mature egg, known as a Metaphase II (MII) oocyte, is identified by the visible extrusion of the first polar body in the perivitelline space. The presence of the first polar body signifies that the egg has completed the first meiotic division and is ready to combine its chromosomes with sperm. Immature eggs, such as those at the Germinal Vesicle (GV) or Metaphase I (MI) stage, cannot be used for fertilization immediately. This nuclear maturity is a prerequisite for successful intracytoplasmic sperm injection (ICSI) or conventional in vitro fertilization (IVF).
Morphological Assessment
Embryologists also assess the oocyte’s morphology, examining the appearance of the cytoplasm, the zona pellucida, and the first polar body. A high-quality egg typically has clear, homogenous cytoplasm without excessive granularity, vacuoles, or clusters. The zona pellucida (outer shell) is assessed for normal thickness and shape. While morphological grading is a routine practice used to select the best candidates for fertilization, visual features are recognized as imperfect predictors of the egg’s ultimate chromosomal health.
Genetic Health: Determining Chromosomal Integrity
The definitive measure of egg quality is its chromosomal integrity, or whether it possesses the correct number of chromosomes. The presence of an incorrect number of chromosomes, known as aneuploidy, is the single greatest cause of implantation failure, miscarriage, and age-related fertility decline. Chromosomal errors primarily arise from mistakes during meiosis, the cell division process that creates the egg.
As a woman ages, the cellular mechanisms responsible for accurately separating chromosomes during meiotic division become less reliable. The vast majority of aneuploidy originates in the egg, with the incidence rising sharply from approximately 20% in women under 35 to over 70% in women over 42. Since the egg’s DNA cannot be directly tested without destroying the cell, its genetic quality is assessed indirectly through the resulting embryo.
Preimplantation Genetic Testing for Aneuploidy (PGT-A) is the technology used to screen the chromosomal status of an embryo created from the egg. During PGT-A, a small sample of cells is taken from the trophectoderm and analyzed for whole chromosome errors. By selecting a euploid (chromosomally normal) embryo for transfer, PGT-A confirms the genetic health of the oocyte that contributed its DNA, offering the most accurate determination of true egg quality.
Biological Foundations: Cellular Components of Egg Quality
The underlying biological health of the egg supports its ability to mature, fertilize, and divide correctly. A major factor in this support system is the health of the mitochondria, often referred to as the cell’s powerhouses. Mitochondria produce adenosine triphosphate (ATP), the energy currency required to fuel the metabolic demands of the egg during maturation and the early stages of embryo development.
The oocyte contains a unique reserve of mitochondria, as no new mitochondria are generated immediately after fertilization. Dysfunction or a reduced number of mitochondria, often observed in older eggs, can lead to insufficient ATP production. This energy deficit can compromise the formation of the meiotic spindle, which is responsible for accurately separating chromosomes, linking mitochondrial health directly to aneuploidy rates.
Oxidative stress also plays a role in the decline of egg quality by causing accumulated damage from free radicals. The egg’s cellular components, including the mitochondrial DNA and proteins necessary for division, are susceptible to this damage. Furthermore, the egg’s cytoplasm must contain adequate nutrient reserves, such as lipids and proteins, to support the embryo until it can begin to implant.