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A Guide to Fertility

At what age does fertility begin to decrease?

Girls are born with a fixed number of immature eggs in their ovaries. The number of eggs decreases as women get older. At birth, most girls have about 2 million eggs, at adolescence that number has gone down to about 400, 000, at age 37 there remain about 25,000. By age 51 when women have their menopause they have about 1000 immature eggs but these are not fertile. At every menstrual cycle one of the immature eggs will mature and be released during ovulation. The eggs that are not released die and get re-absorbed into the body. The quality of the eggs also gets poorer as women get older. All other things being equal the number and quality of the woman’s eggs determines her fertility.

Monthly fertility rate by age

Adapted by permission from BMJ Publishing Group Limited. [Delaying childbearing: e ect of age on fecundity and outcome of pregnancy, van Noord-Zaadstra et al., 302, p. 1363, 1991]

Graphic 3 shows that on average there is a decline in female fertility starting in the mid-thirties, with lower fertility especially after the age of 35. Women’s fertility will continue to decrease every year, whether or not she is healthy and fit because the number and quality of the eggs decreases with age. Even if a woman is not ovulating (for example if she is taking the contraceptive pill, or is pregnant), the number of eggs continues to decline at the same rate. How quick a woman’s fertility declines will depend on a combination of genetic and lifestyle (e.g. smoking) factors.

Men are not born with their sperm. Men produce sperm daily. Men’s fertility also starts to decline around age 40 to 45 years. The decrease in fertility is caused by the decrease in the number and quality of the sperm they produce. Men can have fertility problems even if they can still have sex and have an ejaculation.

If you are concerned about your age and your fertility, you may consider having your fertility tested. Fertility tests for men and women are available at pharmacies, online and at fertility clinics. You can discuss your fertility with your doctor.

More Information


This guide gives you information about fertility and infertility. To help understand this information we’ve included an explanation of the main medical terms used.

  • eggs and sperm

    Cells that are produced by the ovary (eggs, oocytes, ova) and testicles (sperm) and that combine after sex to produce a pregnancy. Women produce eggs and men produce sperm. A healthy sperm is motile, which means it has the ability to move. This movement is what makes it possible for sperm to reach the egg.

  • ejaculation

    Semen is the fluid produced by the male sexual organs to protect and carry sperm. The process of discharging this fluid from the penis is called ejaculation.

  • insemination

    Treatment that involves directly inserting sperm into a woman’s womb.

  • menopause

    The menopause is the time when menstrual periods stop permanently, and women are no longer able to have children. For most women this happens at about 51 years. The age a woman will reach menopause generally be similar to the age at which her mother reached menopause.

  • menstrual cycle

    The monthly changes that occur in the female reproductive system (specifically the uterus and ovaries) which make pregnancy possible. The length of the menstrual cycle is calculated as the time from the first day of a woman’s period (bleeding) to the day before her next period or bleeding. The average time between two periods for women is about 28 days but in teenagers it could be longer (up to 45 days) and sometimes 2 to 3 months, becoming shorter as the teenager gets older. There are events that occur during the menstrual cycle which are repeated each month. These are: development of the egg (phase 1), release of the egg from one of the ovaries (phase 2), preparation of the uterus for a pregnancy (phase 3), and menstruation or bleeding (phase 4). The next period then happens if there is no pregnancy. Young women should have regular periods within 3 years of the rst period occurring. Women could have some spotting in early pregnancy.

  • ovaries

    The two oval-shaped organs located in the lower abdomen (right and left side) that produce the female eggs.

  • ovulation

    Is the release of the oocyte (mature egg, sometimes called ovum) from the ovaries, ready for fertilization. Ovulation occurs about two weeks before the next period is due, for example around day 14 of a 28-day cycle or day 21 of a 35-day cycle. The actual day of release could differ between cycles and between women, and is a affected by many factors (e.g. lifestyle).

  • testicles also called testes or balls

    Oval-shaped organs that sit in a sac that hangs behind the penis. A main job of the testicles is to make and store sperm.

‘A Guide to Fertility’ was developed and evaluated by Prof J Boivin, Cardiff University.