When people reach middle age, many people become “fat”, especially after the age of 40, work and life pressure increases, busy work every day, exercise less, plus smoking, drinking too much, the body began to obese. This time to have a child, can be easy to get on? People may have heard that women are too fat to have children easily, but in fact, men are also obese can also lead to infertility. The mechanism that causes male infertility is also multifaceted. The effect of obesity on male fertility mainly comes from five aspects: (1) obesity can cause excessive peripheral adipose tissue, in the body under the action of aromatase enzymes lead to the conversion of androgens into estrogen, causing the level of estrogen to increase, so that the level of testosterone in the body decreases, and testosterone is an important condition for the production of sperm, testosterone reduction will lead to sperm production is affected, and cause secondary male hypogonadism. (2) obesity can lead to high blood pressure, high blood sugar and high blood fat, that is, the metabolic syndrome. Modern research shows that the metabolic syndrome can affect male gonadal function, resulting in sperm production disorders. (3) Excessively obese men have fat accumulation in the lower abdomen, which affects the heat dissipation function of the scrotum and can increase the temperature of the scrotum. Normally the scrotal temperature is 2~3℃ lower than the body temperature, which is suitable for sperm production. Increased scrotal temperature decreases semen parameters and increases sperm DNA damage and oxidative stress. The accumulation of fat around the scrotum also inhibits heat dissipation and increases the local temperature of the testes, which can cause a decrease in sperm quality. In addition, obese patients usually exercise less than normal people, and long-term inactivity may also have a certain impact on sperm quality. (4) obesity also affects male libido and sexual function, can lead to male erectile dysfunction or ejaculation dysfunction, thus affecting male reproductive function. (5) Obese people are often accompanied by sleep apnea syndrome, which leads to lack of oxygen in the body and affects the production and vitality of sperm. So how can you tell if you are obese? Generally through the body mass index (BMI) to determine, BMI is weight kilograms divided by height meters squared derived from the number, is currently the international common measure of the human body group fat and thin degree and whether a standard of health. BMI values for adults: too light: below 18.5; normal: 18.5~23.9; overweight: 24~27; obese: 28~32; very obese: above 32. Men preparing for pregnancy should exercise more and control their weight strictly.