Infertility can be caused by a variety of problems. Somewhere around half of all infertility problems are due to "male factor" infertility, and the other half are due to "female factor" infertility. Each partner can have their own difficulties contributing to a successful pregnancy.
The production of sperm is typically at the root of male factor infertility. This an include low sperm count brought on by stress, chemotherapy, high fever, lack of sleep, environmental toxins, over-heating and other causes. It can be an abnormality in the ability of the sperm to move (called motility.) A man may have an abnormal sperm shape, making it difficult if not impossible for the sperm to fertilize the egg. He could have a condition called Germ-cell aplasia, in which sperm cells develop abnormally. Blockages of the vas deferens or other tubes can also restrict movement or production of the sperm, as can gonorrhea and chlamydia. Finally, either a man or a woman can have a sperm allergy, in which the immune system produces antibodies that kill the sperm.
Female factor infertility often relates to a problem with ovulation, such as occurs with polycystic ovarian syndrome (PCOS), tumors, infections, chemotherapy and infections. It may also be caused by an abnormality in cervical mucus, in that it can be too thick or too watery. The uterus itself may not be able to accept implantation of the embryo, or may not be able to sustain it during pregnancy due to fibroids, poor endometrial lining, and scarring or inflammation from endometriosis. Finally, an abnormal pelvic area such as blocked fallopian tubes or excessive scarring near the pelvic organs can cause infertility.