In humans
fertility is a medical term for a woman's capacity to conceive or a man's capacity to induce conception. Human
fertility begins during puberty, when females start menstruating and males develop viable sperm. Men can remain fertile into their 70s, but a woman's
fertility peaks in her mid-20s and decreases sharply after 35, ending at menopause, around age 51. Humans are fertile year-round, but recent studies suggest that annual fertility peaks when there are 12 hours of sunlight and temperatures between 10¡ to 21¡ C (50¡ to 70¡ F). Others attribute this to close timing with holidays and vacations.
Fertility can be affected by health and environmental
problems. Sexually transmitted diseases (STDs) such as human papillomavirus (causing genital warts) or chlamydia can cause scarring of the reproductive tissues in women and men, keeping the sperm or egg apart or preventing implantation of the embryo in the uterus. Women's fertility can also be affected by hormone and ovulation disturbances, scar tissue from previous abdominal surgery, use of the intrauterine device for birth control, and antibodies to sperm. Psychological stress can temporarily stop ovulation. A woman can stop menstruating if her proportion of body fat goes below 20 percent of body weight, a condition affecting athletes such as runners, ballet dancers, and women with anorexia nervosa.
In men, low or no sperm count can result from physical conditions such as varicocele (varicose veins in the scrotum), alcohol and drug use, exposure to certain chemicals, some medications, and higher temperatures around the testicles, caused by tight clothing.
Some fertility problems can be treated with surgery or drugs, as well as techniques like artificial insemination and in vitro fertilization.