Infertility is defined as the inability to conceive after a year of unprotected intercourse, or the inability to carry pregnancies to a live birth.
Infertility affects around one in six couples with the causes being shared equally among men and women.
The main causes of female infertility include failure to ovulate, hormone imbalance, obesity, polycystic ovarian syndrome (PCOS), blocked tubes, endometriosis and low ovarian reserve. Ovarian reserve depletion is particularly common after reaching the age of 35. This is due to women being born with all the eggs they will ever produce. Therefore female fertility declines slightly at 30 years and there is a significant decline around 37 to 38 years of age. By the time a woman is 40 years old her fertility is a quarter of when she was 30. The miscarriage rate increases with age from about one in seven for women aged less than 25 years to about one in two at 40 years of age.
Lifestyle factors play an important role with obesity, smoking affecting a woman’s fertility. Genetic factors also have a significant role, and examples include Turner Syndrome where a woman is missing an X chromosome and translocations where genetic material is exchanged between chromosomes.
Male infertility may be due to inadequate numbers of sperm being produced by the testicles, obstruction of the vas deferens preventing sperm from entering the ejaculate or poorly functioning sperm reducing the ability to fertilise an egg (oocyte). Azoospermia is the term used to describe an ejaculate that contains no sperm at all. The causes of male infertility include hormonal disorders, lifestyle factors causing oxidative stress such as obesity and cigarette smoking, genetic disorders such as Klinefelter syndrome, and failure of the testicle to produce sperm due to testicular damage. Drugs and some medications can adversely impact upon sperm health too.
女性不孕的主要原因包括排卵障碍，内分泌失调，肥胖，多囊卵巢综合征(PCOS)，输卵管堵塞，子宫内膜异位，卵巢储备低。特别是在平均年龄达到35岁以后卵巢储备基本耗尽是很普遍的。这是因为女性出生后卵子数量是固定的，因此女性的生殖能力在30岁以后有轻微的下降，在37-38岁时出现明显下降。女性在40岁以后，她的生育能力只有她30岁时的1/4, 流产的几率随着年龄的增加而上升，流产几率从25岁以下的1/7 ，到40岁时达到50%。
生活方式也是一个重要的因素。肥胖，抽烟都会影响女性的生育能力。基因也是一个重要因素，比如Turner 特纳综合症，女性的染色体少了一个X 染色体，而且基因材料在染色体之间交换而导致染色体异位。