Limited ovarian reserve
In contrast to men, that have the capacity to form spermatozoa as long as they live, women are born with a limited ovarian reserve; at birth, they will the stock of ovarian follicles set for all their lives.
Some of these follicles will be ovulated and will become oocytes, feminine germ cells, but several of them will suffer from a natural degeneration process named atresia.
Oocyte quality and characteristics
On another note, as women age, the features of the oocyte as a cell are disturbed. This induces the occurrence of some genetic anomalies named aneuploidies; consisting on an increase or decrease in the number of chromosomes inside the oocyte.
The alteration on the number of chromosomes may affect the fetus and cause genetic anomalies, such as the Down Syndrome, or even cause the inviability of the fetus and provoking an abortion.
Feminine fertility is considered to decrease progessively from 20 years old and on, notably after 35 years, and before menopause
At 45 years old the probability to conceive is reduced to a 5% each cycle, regardless of whether it is sought through a natural method or via in vitro fertilization procedures. Therefore, this age is considered the ending of their reproductive years.