Female egg quality is synonymous with the probability of embryo implantation.
The concept of a woman’s “egg quality” is derived from the observation that the probability of embryo implantation is strongly related to the age of the woman who provides the egg and to her ovarian reserve. Therefore, “egg quality” is synonymous with “the probability of embryo implantation”.
Egg quality cannot be determined by looking at the egg, measuring its receptivity to fertilization by sperm, or observing initial embryo division. Just because an embryo looks good in the laboratory, does not mean that it will implant. Just as the proof of the pudding is in the eating, the proof of egg quality is in the embryo implantation.
Some of the noteworthy contributing factors to embryo implantation failure are diminished ovarian reserve, advanced maternal age and diminished egg quality.
Diminished ovarian reserve
A woman who has elevated FSH levels on the third day of menses is said to have diminished “ovarian reserve”. What this means physiologically is that the ovary is producing less feedback signal to the pituitary gland, and the body responds by making more FSH in an effort to stimulate the ovary.
It has been observed over the past ten years in thousands of cycles of fertility treatment that women with elevated FSH levels have markedly decreased egg quality and rarely conceive with their own eggs (they do, however, become pregnant readily with donor eggs). The precise physiological reason for this is unclear. However, we do know that when eggs are obtained from women with elevated FSH levels, they appear normal, they fertilize normally, and undergo initial embryonic cleavage at a normal rate. However, they rarely divide beyond the 8 cell stage and almost never implant.
Advanced maternal age
Even if FSH levels are normal, the age of the woman providing the eggs plays a major role in determining egg quality. Just as with women with elevated FSH levels, eggs obtained from women in their late 40’s appear normal, fertilize normally, and undergo initial embryonic cleavage in a normal manner. However, such embryos almost never implant. Because of low implantation rates in women over 45 years old, normal FSH levels are not considered “reassuring”.
For more information, see our published scientific studies: pregnancy in the sixth decade of life: obstetric outcomes in women of advanced reproductive age. JAMA 2002 and Motherhood after age 50: an evaluation of parenting stress and physical functioning.
Diminished egg quality
One way to think of diminished egg quality is “The Battery Analogy”. Consider each egg as possessing a number of batteries that provide its energy stores. The batteries are analogs of tiny organelles called mitochondria, which are the energy producers of mammalian cells. As we grow older, the energy-producing capacity of the mitochondria decreases. (This is why older people run slower than young people.)
The egg is connected to the circulation prior to ovulation, and it is connected again after embryo implantation. But during the seven days between ovulation and implantation, the egg and the embryo which results from it are contained within the zona pellucida and are dependent on energy coming from the mitochondria which were in the egg at the moment of ovulation (no mitochondrial replication takes place until after implantation).
The older egg usually looks normal at the time of ovulation and its initial fertilization and embryonic development remain normal. This is because its energy stores are still adequate. However, it soon runs out of batteries and stops dividing. Implantation is not achieved because the embryo stops dividing before it reaches the implantation stage. We do not yet know how to increase the energy stores to an egg prior to ovulation. When egg quality is low, the only therapy that has a proven track record and produces reliable results is egg donation.