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Innermost Secrets / Innermost Living / Body Clock
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Page modified at: 02/01/2010

Biological Body Clock

The 'Biological Body Clock' describes the decline in a woman's fertility and ovarian reserve as she grows older. It is a useful concept, but does not give the whole picture: the clock does not tick away at a constant rate while fertility gradually declines: it speeds up at about the age of 35, then comes to a halt as a woman passes through the menopause. High profile celebrities having children in their 40s give the perception that women’s bodies have changed, that they can all have children later in life, and that science is making it safer. But this is not the case. Although over the last 20 years lifestyles have changed dramatically, women’s biological clocks haven't, and every woman's 'clock' is different: in some women, the clock starts to run down much sooner than average, perhaps in their 20s. In others, it might keep going well into their 40s.

Why Does Age Affect A Woman’s Fertility?

There are many factors affecting a woman's fertility including illness or disease, drug treatments, accidents, smoking and pollution. All can make it more difficult to conceive or carry children. However, a woman's fertility also reduces naturally with age: not only does the number of her eggs decline, but so does their quality, making it less likely that any remaining eggs will become fertilized and develop normally.

How Much Does Age Affect A Woman's Fertility?

In today’s society, age-related infertility is becoming more common. Approximately 20% of women wait until after age 35 to begin their families. Several factors have contributed to this trend: contraception is readily available; women are marrying at an older age; the divorce rate remains high; married couples are delaying pregnancy until they are more financially secure; and many women don’t realize that their fertility begins to decline in their late 20s or early 30s. In addition, stories in the media may lead them to believe that they can delay getting pregnant and then use assisted reproductive technologies (ART) to get pregnant when they are ready. However, age affects the success rates of infertility treatments as well as their natural ability to get pregnant. For example, a healthy 30-year-old woman has about a 20% chance per month to get pregnant however by 40 the chance is only about 5% per month. In many cases, these percentages are true for natural conception as well as conception using artificial reproductive techniques like IVF. It is important to remember that fertility decreases with age, particularly after age 35. Even though women today are healthier and taking better care of themselves than ever before, improved health in later life does not offset the natural age-related decline in fertility.

Declining Egg Number

Most women are born with about 2 million eggs in their ovaries. By the time they reach puberty, almost 90% of these eggs have died and by the time she reaches twenty, only about 50,000 of the original 2 million eggs remain. This decline usually continues steadily until around the age of 35, at which point the rate of loss speeds up dramatically. This means that there are increasingly fewer eggs available for fertilization as the average woman ages beyond 35.

Declining Egg Quality

During a woman's normal cycle, the body selects the 'best' eggs to ripen for fertilization. So, each month at least one healthy egg is passed during menstruation. So gradually, unless a woman conceives, the 'best' eggs are discarded by the body. Those that remain have grown older, and are possibly lower quality, making them less likely to become fertilized and develop into a healthy baby. Although an older egg can sometimes become fertilized, there is a greater risk of miscarriage or birth abnormalities due to chromosomal defects. It may be that the reduced fertility seen with aging is therefore part of the body's natural 'safety mechanism'.