Everyone can take steps to protect their fertility. However, age is an important factor that affects fertility, especially in women.
If you're trying to conceive, you can find out how to give yourself the best chance by reading Trying to get pregnant.
Even if babies are the last thing on your mind at the moment, you can take steps to help maintain your fertility.
Professor William Ledger, Professor of Obstetrics and Gynaecology at the University of Sheffield, explains the basics.
Age and fertility
When it comes to fertility, age matters.
Many people today wait until they're older to have children. But fertility declines over time, and you should consider this if you plan to have children later.
Both women and men are at their most fertile in their early twenties.
In women, fertility declines more quickly with age. This decline becomes rapid after the age of 35. This has a number of causes, but particularly the decline in the quality of the eggs released by the ovaries.
Around one-third of couples in which the woman is over 35 have fertility problems. This rises to two-thirds when the woman is over 40.
Women over 35 are also less likely to become pregnant as a result of fertility treatments, including IVF, and are more likely to have a miscarriage if they do become pregnant.
Men's fertility gradually declines from around the age of 40, but most men are able to father children into their 50s and beyond.
Good fertility health
Other factors also affect fertility. But in many cases, you can take action to help protect your fertility.
Sexually transmitted infections, such as chlamydia and gonorrhoea, can damage a woman's fallopian tubes, which cause difficulties becoming pregnant. If you think you might have contracted an STI, go to your GP or a sexual health clinic. Find out more about STIs.
"Women who smoke 20 cigarettes a day experience the menopause on average two years earlier," says Professor Ledger. Men who smoke risk damaging their sperm. Get advice on stopping smoking.
Be a healthy weight
Being underweight or overweight can lower your chances of conceiving. One cause of infertility is polycystic ovarian syndrome (PCOS), which is made worse by being overweight or obese. You can check whether you're a health weight with our Healthy weight calculator.
Women who regularly exceed the government's recommended daily alcohol limits of 2-3 units may damage their fertility. Men who exceed 3-4 units a day may damage their sperm. See Drinking and alcohol for advice on safe drinking.
Keep your testicles cool
A man's testes should be one or two degrees cooler than the rest of their body. Tight underwear, hot showers and hot baths can all raise the temperature of the testes.
Avoid radiation and dangerous chemicals
Exposure to radiation and chemicals such as glycol ester, found in some paints, can damage fertility.
If you've been trying to conceive for a year or more by having regular unprotected sex, it's time to see your GP.
Your GP can do tests to identify possible fertility problems, and can provide advice on the next steps.
Learn more in Fertility and your GP .