Women are born with about 2 million undeveloped eggs which sit inside follicles in the ovaries. Most of these eggs are lost before birth.

By puberty (10-12 years old), the number of eggs they have declines to about 300,000-500,000, and by the age 30 about 35,000 eggs remain. Menopause happens when the ovaries run out of eggs.

During each menstrual cycle, around 5-20 eggs begin to mature, but usually only one fully matures and is released (ovulated). The rest of the eggs are absorbed by the body.

To become pregnant a woman needs to release a mature egg. The egg is then fertilised by a sperm to create an embryo. Pregnancy happens when the embryo attaches inside the womb.

As women get older the number and quality of their eggs gradually decline, reducing their fertility.

Egg quality declines because genetic defects in each egg add up over time. Older eggs also have less energy to support a developing embryo.

This makes it harder for an embryo to grow into a baby, which is why the risk of miscarriage increases with age. The risk of miscarriage is about 11% for women aged 30-34 years and 17% for women aged 35-39 years (4).

Generally, fertility decline starts in a woman’s early 30’s and speeds up in her mid-late 30’s (particularly after 37 years).

There are currently no tests for egg quality.

The chances of getting pregnant each month are about (5):

• 20% for a 30 year old (1 in 5 chance)
• 16% for a 35 year old (1 in 6 chance)
• 9% for a 40 year old (1 in 11 chance)

Because fertility declines with age, some women choose to freeze their eggs. These eggs can then be used to try and have a baby in the future.

Note: Understanding how age impacts female fertility is important when deciding about egg freezing. This information is not intended to create shame or embarrassment for anyone.

Some lifestyle factors can reduce fertility in both women and men. These include smoking and other substance use, poor diet, drinking too much alcohol, not getting enough exercise, and being over or under-weight (6, 7).

Health conditions such as endometriosis, PMOS and pelvic infection can affect fertility as well (8).

There is growing evidence linking environmental toxins with reduced fertility (8, 9). These are toxins that may be found in the air, food, soil, water, and many everyday products. The American Society for Reproductive Medicine have a podcast and fact sheet for more information.

To learn more about what can impact your fertility, visit the Your Fertility website.