You’ve probably heard the term ‘A woman’s biological clock’, but what does it actually mean and at what age does it start to matter? Whilst a man will make sperm at virtually the same rate throughout his life, the story is very different for women…

  • You are born with all the eggs you are ever going to have.
  • You don't make any new eggs during your lifetime.

A woman's biological clock

Let’s start with the biology. You are born with all the eggs you are ever going to have. You don't make any new eggs during your lifetime; in fact, the highest number of eggs you possessed was while you were still in your mother's uterus: a 20-week-old female foetus has about seven million eggs. When you are born, this number has reduced to around two million and by the time you reach puberty and begin menstruation (start your periods) you will have somewhere between 300,000 and 500,000 eggs remaining.

The fact that your supply of eggs is continuously reducing needn't be cause for concern, however, as it is a natural and continuous process, completely independent of birth control pills, pregnancies, nutritional supplements, or even health or lifestyle.

You are at your most fertile during your 20s and early 30’s and from your mid-30’s, fertility drops off until menopause. After menopause, it is not possible to get pregnant naturally.

Follicle number over age

During most menstrual cycles, one of your eggs ripens and is released from an ovary (ovulation) in preparation for fertilisation. However, the decline in the number of your eggs (also called ‘ovarian reserve’) continues faster than that, and from the time you start your periods approximately 1,000 eggs are destined to die each month. Over the course of a lifetime your ovaries will release about 500 eggs in their mature form. When the supply of eggs runs out, your ovaries cease to make estrogen, and you will go through the menopause. For most women this happens around the age of 50: the average age in the developed world is 51.4 years1. From this point onwards you will no longer be able to get pregnant naturally.

I have heard there is a test to measure how many eggs I have left? I read this is measured by Anti-Mullerian hormone and FSH blood tests - what does this mean?

Professor Michael Thomas

In women over the age of 35 who are trying to conceive there are a number of tests that can be carried out to predict ovarian reserve (how many eggs she has left). Blood tests include an AMH or Anti-Mullerian Hormone test, which can be taken at any time in the menstrual cycle and even if you are on birth control pills. Anti-Mullerian hormone is made by the cells in the follicles of the ovaries and may be an early way of determining how much reserve is remaining in your ovaries. Interpretation of the results of this test may vary between health care providers.
Also, on day three of the menstrual cycle (two days after you start your period), you can obtain a blood test for Follicle Stimulating Hormone (FSH) and estradiol. These two tests may be a way of determining declining ovarian function at the beginning of the menstrual cycle when a dominant follicle is being prepared for ovulation. Another test for poor ovarian reserve is an ‘antral follicle count’. During this test, a transvaginal ultrasound is used to determine the number of follicles that are ready on day three of the cycle. More detailed information on ovarian reserve testing can be found at www.reproductivefacts.org


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