Unlike men, as women age, the odds of conceiving decline significantly. This phenomenon is largely attributed to reproductive aging, in which both the quantity and quality of eggs, or ovarian reserve, decline over time.
A baby girl has 1-2 million eggs when she is born. Egg number keeps decreasing until exhausted during menopause. When a female enters her late 30s, her egg number decreases even faster, and more in her early 40s (Figure 1).
Egg quality also declines with aging, which means when a female enters her late 30s, the proportions of abnormal eggs which could not result in a healthy pregnancy are getting higher and higher. Therefore, it will be more and more difficult for women to conceive naturally with aging. Also, women are facing higher and higher miscarriage rate caused by abnormal pregnancies with aging (figure 2 and 3).