Biological Clock

Definition - What does Biological Clock mean?

A woman's biological clock is the relationship between the age of the person and her fertility. The fertile period of a woman starts with the attainment of menarche. Fertility increases as a woman ages, peaks around 20-25 years of age and then gradually starts to decline. A drop in fertility is typically seen around 35 years with a complete cessation of fertility occurring after the woman reaches menopause. The biological clock plays an important role in determining a woman's chances of conceiving successfully and the need for fertility treatment.

FertilitySmarts explains Biological Clock

A woman's fertile period lasts from menarche, which usually takes place around 12-13 years in most females, to menopause that happens in their 40s or 50s. But the success rate of pregnancy does not remain the same throughout this period. During the first year after menarche, 80% of the menstrual cycles are anovulatory, meaning although the menstruation occurs, an egg is not being released from the ovary. After a peak age around 20 years, fertility begins to decline. The chances of conceiving naturally drop to around 20% after the age of 30 years, which further drops to 5% after 40 years of age.

As a woman ages, the reserve of ovarian follicles that later become eggs depletes. Although a female ovary contains close to one million follicles at birth, a woman ovulates only about 300 eggs and the rest undergo degeneration. A decrease in the number and quality of eggs in the ovarian reserve is the main reason for the natural decline in fertility with age.

A study conducted in 2004, showed that the estimated success rate of a woman getting pregnant naturally without any fertility treatment at different ages are as follows:

  • At 30 years of age
    • 75% of women will have a successful pregnancy within one year
    • 91% of women will have a successful pregnancy within four years
  • At 35 years of age
    • 66% of women will have a successful pregnancy within one year
    • 84% of women will have a successful pregnancy within four years
  • At 40 years of age
    • 44% of women will have a successful pregnancy within one year
    • 64% of women will have a successful pregnancy within four years

There are different methods for women to overcome fertility issues related to biological clock and conceive a child. Fertility drugs can stimulate maturation and release of eggs from the ovary if there is a sufficient ovarian reserve. Intrauterine insemination (IUI) and In vitro fertilization (IVF) with eggs extracted from the patient are helpful for women who do not respond to fertility medications. Use of donor eggs for In Vitro Fertilization (IVF) is available as the last option.


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