At 16 to 20 weeks of gestation a female has 6 to 7 million eggs, but by the time she is born a good portion of those will have wasted away, leaving 1 to 2 million. This is the most a female will ever have in her life. When she reaches puberty only 300,000 eggs remain, but only a very small percentage of eggs will mature. Of these, only approximately 400 eggs will mature for release during a woman’s monthly cycle. Most of the eggs will degenerate, with the degeneration rate increasing in the decade before menopause. By the time menopause occurs, all eggs will be gone.
According to the Association of Reproductive Medicine, women over the age of 40 only has a 5% chance of natural conception during a single month, compared to the 20 year old woman with a 40% chance per cycle. This is because egg quality declines as a woman ages. Most people believe that the major factor involved in conception is the number of available eggs, however, the quality of the eggs is equally important. Number and quality of eggs is referred to as ovarian reserve. Eggs that are prepared to develop into embryos after fertilization are considered good quality and healthy.
An egg requires the correct number of chromosomes to combine with the chromosomes contained in a sperm. As some eggs develop they do not contain the right number of chromosomes, which can either cause a birth defect or make it impossible to carry the pregnancy to term.
After fertilization, an egg uses the mitochondria inside of it to generate energy to fuel its survival. Older eggs have mitochondrial that lack the ability to produce energy adequately. This causes the egg to eventually run out of energy to divide, causing the embryo to arrest development.
Poor quality eggs can be the sole cause of infertility. Conception or carrying the pregnancy to term may be difficult because embryos may fail to fertilize, grow and divide or implant. If there is an abnormality the pregnancy may be lost early in the pregnancy. Multiple miscarriages may be a sign of poor quality eggs.
Lab tests for egg quality may include the anti-mullerian hormone (AMH) or follicle stimulating hormone (FSH). Decreased AMH indicates the pool of eggs available for conception is lower (diminished ovarian reserve), while an increase in FSH indicates the same. AMH may be considered a more reliable indicator of the ovarian reserve as it does not vary greatly throughout the monthly cycle as FSH does.
Antral follicle counts (AFC) are done by ultrasound to measure the number of resting follicles in the ovaries. Resting follicles contain immature eggs that may develop in the future. A high AFC indicates an adequate ovarian reserve, while a low AFC indicates fewer eggs are available. An AFC of less than 7 indicates a reduced count. In contract, an AFC over 30 may indicate polycystic ovarian syndrome (PCOS) and a risk of hyperstimulation during a controlled ovarian stimulation cycle. Age decreases the AFC.