The biggest cause of male infertility, according to ASRM, is no or too few sperm cells. Other major issues include inadequate sperm motility or movement and abnormally shaped sperm.
Lifestyle can influence the number and quality of a man's sperm. Alcohol and drugs - including marijuana, nicotine, and certain medications - can temporarily reduce sperm quality. Also, environmental toxins, including pesticides and lead, may be to blame for some cases of infertility.
The causes of sperm production problems can exist from birth or develop later as a result of severe medical illnesses, including mumps and some sexually transmitted diseases, or from a severe testicle injury, tumor, or other problem. Sometimes men have a condition called varicocele. This is caused by swollen or congested veins in the testicles. This enlarged vein produces a higher temperature than is normal for the testicles and can lead to decreased movement by the sperm and even fewer sperm. Inability to ejaculate normally can prevent conception, too, and can be caused by many factors, including diabetes, surgery of the prostate gland or urethra, blood pressure medication, or impotence.
For women, the most common cause is some sort of ovulation problem, according to the National Women's Health Information Center (NWHIC). Without ovulation, eggs are not available for fertilization. Problems with ovulation are signaled by irregular menstrual periods or a lack of periods altogether (called amenorrhea). Simple lifestyle factors - including stress, diet, or athletic training - can affect a woman's hormonal balance. Much less often, a hormonal imbalance can result from a medical problem such as a pituitary gland tumor or hypothyroidism.
Another big issue is blocked fallopian tubes, which can be caused by any number of problems. If the fallopian tubes are blocked at one or both ends, the egg can't travel through the tubes into the uterus. Such blockage may result from pelvic inflammatory disease , surgery for an ectopic pregnancy (when the embryo implants in the fallopian tube rather than in the uterus), or other problems, including endometriosis (the abnormal presence of uterine lining cells in other pelvic organs).