For pregnancy to occur, every part of the complex human reproduction process — from the ovary's release of a mature egg to the fertilization of the egg to the fertilized egg's implantation and growth in the uterus — has to take place just right. In women, a number of factors can disrupt this process at any stage. Female infertility is caused by one or more of these factors. Causes of infertility can be found in about 90% of infertility cases, but despite extensive tests, about 10% of couples will never know why they cannot conceive. Between 10% and 30% of cases of infertility have more than one cause. Male or female infertility each account for about 30% to 40% of cases. In men, sperm defects (their quality and quantity) are usually responsible. Female infertility is more complex.Female infertility can be caused by a number of factors, including the following: Damage to fallopian tubes. Damage to the fallopian tubes (which carry the eggs from the ovaries to the uterus) can prevent contact between the egg and sperm. Pelvic infections, endometriosis, and pelvic surgeries may lead to scar formation and fallopian tube damage. Hormonal causes. Some women have problems with ovulation. Synchronized hormonal changes leading to the release of an egg from the ovary and the thickening of the endometrium (lining of the uterus) in preparation for the fertilized egg do not occur. These problems may be detected using basal body temperature charts, ovulation predictor kits, and blood tests to detect hormone levels. Cervical causes. A small group of women may have a cervical condition in which the sperm cannot pass through the cervical canal. Whether due to abnormal mucus production or a prior cervical surgical procedure, this problem may be treated with intrauterine inseminations. Pelvic inflammatory disease (PID) is the major cause of infertility worldwide. PID comprises a variety of infections caused by different bacteria that affect the reproductive organs, appendix, and parts of the intestine that lie in the pelvic area. The sites of infection most often implicated in infertility are in the fallopian tubes, a specific condition referred to as salpingitis. Causes of PID. PID may result from many different conditions that cause infections. Among them are the following: Sexually transmitted diseases (cause of most PIDs). Chlamydia trachomatis is an infectious organism that causes 75% of infertility in the fallopian tubes. Gonorrhea is responsible for most of the remaining cases. Of concern is a 2002 report that suggests these disease may be widely under-diagnosed and undertreated. Pelvic tuberculosis (a growing global problem as tuberculosis cases increase). Nonsterile abortions. Ruptured appendix. Herpesvirus (suggested for some cases, but not confirmed as a causal agent).