In addition to the medical factors mentioned previously, lifestyle factors may also affect a person's fertility: 

Weight

Being overweight or obese reduces male and female fertility and, in women, being significantly underweight can affect regular ovulation. Women undergoing rigorous athletic training may notice that their monthly periods become irregular or they may miss a month.

Infections

In men, mumps can lead to inflammation of the testicles and reduced sperm count. In women, certain sexually transmitted infections (STIs) can damage the oviducts.

Age

As mentioned previously, infertility in women is linked to age. For instance, according to NHS Choices, among women who are 35 years of age, about 95 per cent will get pregnant after three years of regular, unprotected sexual intercourse. For women who are aged 38, that number drops to 75 per cent.

Alcohol

Drinking too much alcohol can damage the quality of a man's sperm and increase a woman's risk of infertility. Couples should follow guidelines and limit their alcohol intake to three to four units a day for men and two to three units for women.

Stress

Day-to-day stresses can contribute to a loss of libido (sex drive) and, in severe cases, can even affect fertility.

Smoking

In addition to all of the other detrimental health effects of smoking, research suggests that it may also adversely affect fertility.

Medicines and drugs

Some prescribed medication, such as those administered during chemotherapy, can affect ovulation and sperm production. In women, antipsychotic medicines can sometimes cause missed periods, while in men, long-term use of anabolic steroids can affect sperm count and mobility. Illegal drugs also have detrimental effects.

Environmental factors

Exposure to certain hazardous chemicals can cause fertility issues.