• Sexual issues. Often treatable, problems with sexual intercourse or technique may affect fertility. Difficulties with erection of the penis (erectile dysfunction), premature ejaculation, painful intercourse (dyspareunia), or psychological or relationship problems can contribute to infertility. Use of lubricants such as oils or petroleum jelly can be toxic to sperm and impair fertility.
• Retrograde ejaculation. This occurs when semen enters the bladder during orgasm rather than emerging out through the penis. Various conditions can cause retrograde ejaculation including diabetes, bladder, prostate or urethral surgery, and the use of psychiatric or antihypertensive drugs.
• Blockage of epididymis or ejaculatory ducts. Some men are born with blockage of the part of the testicle that contains sperm (epididymis) or ejaculatory ducts. And some men who seek treatment for infertility lack the tubes that carry sperm (vasa deferentia).
• No semen (ejaculate). The absence of ejaculate may occur in men with spinal cord injuries or diseases. This fluid transports sperm through the penis into the vagina.
• Misplaced urinary opening (hypospadias). A birth defect can cause the urinary (urethral) opening to be abnormally located on the underside of the penis. If not surgically corrected, this condition can prevent sperm from reaching the cervix.
• Anti-sperm antibodies. Antibodies that target sperm and weaken or disable them usually occur after surgical blockage of part of the vas deferens for male sterilization (vasectomy). Presence of these antibodies may complicate the reversal of a vasectomy.
• Cystic fibrosis. Men with cystic fibrosis often have missing or obstructed vasa deferentia.

General health and lifestyle. A man's general health and lifestyle may affect fertility. Some common causes of infertility related to health and lifestyle include:

• Emotional stress. Stress may interfere with certain hormones needed to produce sperm. Your sperm count may be affected if you experience excessive or prolonged emotional stress. A problem with fertility itself can sometimes become long term and discouraging, producing more stress. Infertility can affect social relationships and sexual functioning.
• Malnutrition. Deficiencies in nutrients such as vitamin C, selenium, zinc and folate may contribute to infertility.
• Obesity. Increased body mass may be associated with fertility problems in men.
• Cancer and its treatment. Both radiation and chemotherapy treatment for cancer can impair sperm production, sometimes severely. The closer radiation treatment is to the testicles, the higher the risk of infertility. Removal of one or both testicles due to cancer also may affect male fertility. You may want to consider freezing (cryopreserving) your sperm before cancer treatment to ensure future fertility.
• Alcohol and drugs. Alcohol or drug dependency can be associated with general ill health and reduced fertility. The use of certain drugs also can contribute to infertility. Anabolic steroids, for example, which are taken to stimulate muscle strength and growth, can cause the testicles to shrink and sperm production to decrease.
• Other medical conditions. A severe injury or major surgery can affect male fertility. Certain diseases or conditions, such as diabetes, thyroid disease, HIV/AIDS, Cushing's syndrome, anemia, heart attack, and liver or kidney failure, may be associated with infertility.
• Age. A gradual decline in fertility is common in men older than 35.

Environmental exposure. Overexposure to certain environmental elements such as heat, toxins and chemicals can reduce sperm count either directly by affecting testicular function or indirectly by altering the male hormonal system. Specific causes include:

• Pesticides and other chemicals. Herbicides and insecticides may cause female hormone-like effects in the male body and may be associated with reduced sperm production. Exposure to such chemicals also may contribute to testicular cancer. Men exposed to hydrocarbons, such as ethylbenzene, benzene, toluene, xylen and aromatic solvents used in paint, varnishes, glues, metal degreasers and other products, may be at risk of infertility. Men with high exposure to lead also may be more at risk.
• Testicular exposure to overheating. Frequent use of saunas or hot tubs can elevate your core body temperature. This may impair your sperm production and lower your sperm count.
• Substance abuse. Cocaine or heavy marijuana use may temporarily reduce the number and quality of your sperm.
• Tobacco smoking. Men who smoke may have a lower sperm count than do those who don't smoke.

Causes of female infertility
The most common causes of female infertility include fallopian tube damage or blockage, endometriosis, ovulation disorders, elevated prolactin, polycystic ovary syndrome, early menopause, benign uterine fibroids and pelvic adhesions:

Fallopian tube damage or blockage. This condition usually results from inflammation of the fallopian tube (salpingitis). Chlamydia is the most frequent cause. Tubal inflammation may go unnoticed or cause pain and fever.

Tubal damage with scarring is the major risk factor of a pregnancy in which the fertilized egg is unable to make its way through the fallopian tube to implant in the uterus (ectopic pregnancy). One episode of tubal infection may cause fertility difficulties. The risk of ectopic pregnancy increases with each occurrence of tubal infection.

Endometriosis. Endometriosis occurs when the tissue that makes up the lining of the uterus grows outside of the uterus. This tissue most commonly is implanted on the ovaries or the lining of the abdomen near the uterus, fallopian tubes and ovaries. These implants respond to the hormonal cycle and grow, shed and bleed in sync with the lining of the uterus each month, which can lead to scarring and inflammation. Pelvic pain and infertility are common in women with endometriosis.

Infertility in endometriosis also may be due to:

• Ovarian cysts (endometriomas). Ovarian cysts may indicate advanced endometriosis and often are associated with reduced fertility. Endometriomas can be treated with surgery.
• Scar tissue. Endometriosis may cause rigid webs of scar tissue between the uterus, ovaries and fallopian tubes. This may prevent the transfer of the egg to the fallopian tube.

Ovulation disorders. Some cases of female infertility are caused by ovulation disorders. Disruption in the part of the brain that regulates ovulation (hypothalamic-pituitary axis) can cause deficiencies in luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Even slight irregularities in the hormone system can affect ovulation.

Specific causes of hypothalamic-pituitary disorders include:

• Direct injury to the hypothalamus or pituitary gland
• Pituitary tumors
• Excessive exercise
• Anorexia nervosa

Elevated prolactin (hyperprolactinemia). The hormone prolactin stimulates breast milk production. High levels in women who aren't pregnant or nursing may affect ovulation. An elevation in prolactin levels may also indicate the presence of a pituitary tumor. In addition, some drugs can elevate levels of prolactin. Milk flow not related to pregnancy or nursing (galactorrhea) can be a sign of high prolactin.

Polycystic ovary syndrome (PCOS). An increase in androgen hormone production causes PCOS. In women with increased body mass, elevated androgen production may come from stimulation by higher levels of insulin. In lean women, the elevated levels of androgen may be stimulated by a higher ratio of luteinizing hormone (LH). Lack of menstruation (amenorrhea) or infrequent menses (oligomenorrhea) are common symptoms in women with PCOS.

In PCOS, increased androgen production prevents the follicles of the ovaries from producing a mature egg. Small follicles that start to grow but can't mature to ovulation remain within the ovary. A persistent lack of ovulation may lead to mild enlargement of the ovaries.

Without ovulation, the hormone progesterone isn't produced and estrogen levels remain constant. Elevated levels of androgen may cause increased dark or thick hair on the chin, upper lip or lower abdomen as well as acne and oily skin.

Early menopause (premature ovarian failure). Early menopause is the absence of menstruation and the early depletion of ovarian follicles before age 35. Although the cause is often unknown, certain conditions are associated with early menopause, including:

• Autoimmune disease. The body produces antibodies to attack its own tissue, in this case the ovary. This may be associated with hypothyroidism (too little thyroid hormone).
• Radiation or chemotherapy for the treatment of cancer.
• Tobacco smoking.

Benign uterine fibroids.Fibroids are benign tumors in the wall of the uterus and are common in women in their 30s. Occasionally they may cause infertility by interfering with the contour of the uterine cavity, blocking the fallopian tubes.

Pelvic adhesions. Pelvic adhesions are bands of scar tissue that bind organs after pelvic infection, appendicitis, or abdominal or pelvic surgery. They may limit the functioning of the ovaries and fallopian tubes and impair fertility. Scar tissue formation inside the uterine cavity after a surgical procedure may result in a closed uterus and ceased menstruation (Asherman's syndrome). This is most common following surgery to control uterine bleeding after giving birth.

Other causes. A number of other causes can lead to infertility in women:

• Medications. Temporary infertility may occur with the use of certain medications. In most cases, fertility is restored when the medication is stopped.
• Thyroid problems. Disorders of the thyroid gland, either too much thyroid hormone (hyperthyroidism) or too little (hypothyroidism), can interrupt the menstrual cycle and cause infertility.
• Cancer and its treatment. Certain cancers — particularly female reproductive cancers — often severely impair female fertility. Both radiation and chemotherapy may affect a woman's ability to reproduce. Chemotherapy may impair reproductive function and fertility more severely in men than in women.
• Other medical conditions. Medical conditions associated with delayed puberty or amenorrhea, such as Cushing's disease, sickle cell disease, HIV/AIDS, kidney disease and diabetes, can affect a woman's fertility.