Why Do Women Become Less Fertile As They Age?

Even though most women are aware of this fact, many underestimate the extent of the decline in fertility with age. Some women think that because they are in good health and take care of their body, they are not as susceptible to the effects of age on fertility. Perhaps that’s due to maternal optimism. But on a practical note, such a misconception can make it harder to conceive later in life. It is estimated that a woman aged 25 or younger takes two to three months to get pregnant, whereas it takes a woman aged 35 or older six months. This is also born out in women having artificial insemination, with a 25-year-old or younger having an 11 percent monthly chance of pregnancy while a woman 35 or older has only a 6.5 percent chance. There are several reasons for this decline in fertility.

  • It’s not only the woman that ages, but her eggs as well. Women are born with a finite number of eggs, and no new eggs are produced as the woman gets older. The best quality eggs are released when the woman is younger.
  • As eggs age, the chance of abnormal division and/or abnormal fertilization increases. It’s fortunate that most abnormally fertilized eggs do not survive, because those that do can cause miscarriage and genetically abnormal babies.

A woman’s eggs mature in fluid-filled follicles, and at birth a woman has roughly a million of these follicles. At puberty, she has about 400,000 but only about 400 of these will be ovulated during her reproductive years. The majority of follicles degenerate in an ongoing process called atresia. The result is a progressive decrease in the number of follicles and eggs in the ovary. As a woman’s age increases so do the proportion of menstrual cycles that are anovulatory, meaning ovulation does not occur. Evidence suggests that the endometrium, the lining of the uterus, may become less receptive as women age. Even if the embryo is normal, the chances of it successfully implanting in the uterus are reduced. Incidences of endometriosis and fibroids increase with a woman’s age. Both of these conditions are linked to infertility. Those are the primary reasons why women’s fertility tends to decrease as they age. There are two lessons to take from this knowledge, different lessons but equally valid. One is that in spite of the factors, many normal children are born to women who are older. The other is that because of these factors, many women are deciding to freeze their eggs when they are young so they have better chances of conceiving when they are older using the eggs preserved when they were younger.

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