What Causes Infertility?

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Infertility is usually defined as a couple’s failure to conceive after one year of regular and unprotected intercourse. Intercourse is defined as regular if the couple engages in sex every two to three days. This results in conception for 50% of couples within 3 months, 75% within six months, and 90% within one year.

In the United Kingdom and the United States, around 10% to 15% of couples experience problems conceiving. Infertility may be the result of a single problem affecting either partner or a combination of factors that prevent conception or pregnancy from continuing.

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The reason for infertility may involve one partner or both; however, in general, one-third of cases involve the woman, one-third the man, and the remainder of cases either involve both partners or are unexplained.

Sperm disorders

Sperm disorders are the underlying cause of infertility in up to 35% of couples. The problem may be inadequate quality or quantity of sperm or a problem with sperm emission. The disorder is diagnosed using semen and genetic testing. Moreover, the most effective treatment for a sperm disorder is usually intracytoplasmic sperm injection.

An inadequate quantity or poor quality of sperm can be caused by defective spermatogenesis, which may occur as a result of exposure to toxins, certain drugs, or heat or as a result of endocrine, genetic, or genitourinary disorders.

Sperm emission may be impaired as a result of retrograde ejaculation of sperm into the bladder, which can be caused by neurological dysfunction, diabetes, or a result of surgical procedures such as retroperitoneal dissection or transurethral resection of the prostate. Impaired sperm emission may also be caused by obstruction of the vas deferens or certain congenital disorders such as cystic fibrosis, where the vasa deferentia or epididymis are absent. Sometimes, both seminal vesicles are missing.

Decreased ovarian reserve (DOR) and ovulatory dysfunction

DOR and ovulatory dysfunction are the causes of infertility in up to 20% of couples. DOR refers to a decrease in the number or quality of oocytes. The ovarian reserve can start to decrease at the age of 30 or even earlier and falls rapidly after the age of 40. Ovarian reserve may also be decreased by lesions. Although the risk of DOR increases with older age, DOR and age are each independent predictors of infertility and of poorer response to fertility treatment.

Ovulatory dysfunction refers to ovulation that is irregular, abnormal, or absent altogether. When ovulation is dysfunctional, menstruation is often irregular or absent. The problem is diagnosed by examining the patient’s history, measuring hormones, and performing pelvic ultrasonography.

Long-term ovulatory dysfunction in women who have not yet reached menopause is usually caused by polycystic ovarian syndrome (PCOS), although there are numerous other causes, such as hypothalamic dysfunction and hyperprolactinemia. Ovulatory dysfunction can usually be treated by inducing ovulation with a drug such as clomiphene.

Reasons for infertility

Tubal dysfunction and pelvic lesions

Tubal dysfunction and pelvic lesions are the causes of infertility in around 30% of infertile couples.

Tubal dysfunction describes either blockage of the fallopian tube or epithelial dysfunction that disrupts the motility of the zygote. This condition may occur as a result of an ectopic pregnancy, pelvic inflammatory disease, pelvic infection caused by an intrauterine device, inflammatory disorders, ruptured appendix, or pelvic adhesions caused by lower abdominal surgery.

Pelvic lesions are structural abnormalities that may hinder fertilization or implantation. These lesions may take the form of pelvic adhesions, fibroids that block the fallopian tubes, fibroids that distort the womb cavity, or intrauterine adhesions.

Abnormal cervical mucus

In up to 5% of infertile couples, the cause is abnormal cervical mucus, which inhibits the penetration of sperm or destroys them. Usually, the cervical mucus changes from thick and impenetrable to stretchy and thin during the follicular phase of the menstrual cycle as a result of increased estradiol.

In cases of abnormal cervical mucus, the mucus may stay impenetrable around the time of ovulation or it may increase the destruction of sperm by enabling an influx of vaginal bacteria due to cervicitis, for example. On rare occasions, the mucus contains antibodies against sperm. Abnormal cervical mucus usually only has a significant effect on fertility in cases of chronic cervicitis or cervical stenosis as a result of previous treatment for cervical intraepithelial neoplasia.

Unexplained infertility

In around 10% of infertile couples, no factors causing infertility can be identified. The man’s sperm is normal, as are the fallopian tubes and ovulation in the woman. Some experts believe tests for other conditions should be continued, while others accept the definition of “unexplained infertility” and proceed with fertility treatment.


Further Reading

Last Updated: Apr 5, 2021

Sally Robertson

Written by

Sally Robertson

Sally first developed an interest in medical communications when she took on the role of Journal Development Editor for BioMed Central (BMC), after having graduated with a degree in biomedical science from Greenwich University.


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