Ovarian Reserve and Fertility — Ovarian reserve is the number and quality of eggs a woman has available when she tries to become pregnant and is the major factor that links age and infertility. (Human oocyte "egg" pictured at right).
Female fertility is fixed very early in life when she is just a fetus in her mother's womb. Primordial cells that will become eggs (oocytes) begin to develop in the human fetus by the 7th week of gestation, and the total number of eggs peaks at 20 weeks gestation and declines significantly over time.
From puberty to menopause, 99.9% of oocytes are lost due to the monthly process of the reproductive organs, which includes the selection of one oocyte for possible fertilization while others that have developed during that month are victims of apoptosis (programmed cell death). This decades-long process is, in fact, the body’s natural progression to eventual ovarian failure and infertility.
So, excluding any other factors that might prevent pregnancy, age and the decline of ovarian reserve has a lot to do with infertility. The statistics reported by the Centers for Disease Control and Prevention indicate that the following percentage of women who are infertile by age group: