The most common overall cause of female infertility is the failure to ovulate, which occurs in 40% of women with infertility issues.1 Not ovulating can result from several causes, such as:
- Ovarian or gynecological conditions, such as primary ovarian insufficiency (POI) or polycystic ovary syndrome (PCOS)
- Aging, including "diminished ovarian reserve," which refers to a low number of eggs in a woman's ovaries due to normal aging
- Endocrine disorders, such as thyroid disease or problems with the hypothalamus, which affect the hormones produced by the body so that there might be too much or too little of a hormone or group of hormones
- Lifestyle and environmental factors
Problems with the menstrual cycle, the process that prepares the female body for pregnancy, can lead to infertility. The menstrual cycle includes several phases, and problems at any one of the stages can lead to difficulty getting pregnant or to infertility.
To learn about a normal menstrual cycle, visit the Menstruation section of this website.
To learn about common problems that may occur during the menstrual cycle, visit the What are menstrual irregularities? page.
Structural problems usually involve the presence of abnormal tissue in the fallopian tubes or uterus.
If the fallopian tubes are blocked, eggs are not able to move from the ovaries to the uterus and sperm is not able to reach the egg for fertilization. Structural problems with the uterus, such as those that may interfere with implantation, can also cause infertility.
Some specific structural problems that can cause infertility include:
- Endometriosis, when tissue that normally lines the inside of the uterus is found in other places, such as blocking the fallopian tubes
- Uterine fibroids, growths that appear within and around the wall of the uterus, although most women with fibroids do not have problems with fertility and can get pregnant. However, some women with fibroids may not be able to get pregnant naturally or may have multiple miscarriages or preterm labor.
- Polyps, which are noncancerous growths on the inside surface of the uterus. Polyps can interfere with the function of the uterus and make it difficult for a woman to remain pregnant after conception. Surgical removal of the polyps can increase the chances for a woman to get pregnant.
- Scarring in the uterus from previous injuries, infections, or surgery. Scarring may increase the risk of miscarriage and may interfere with implantation, thus leading to infertility.
- An unusually shaped uterus, which can affect implantation and the ability to carry a pregnancy to term
Infections can also cause infertility in men and women.
Untreated gonorrhea and chlamydia in women can lead to pelvic inflammatory disease, which might cause scarring that blocks the fallopian tubes. Untreated syphilis increases the risk for a pregnant woman to have a stillbirth. More information about infections that may affect fertility can be found on the sexually transmitted infections (STIs) health topic page.
Chronic infections in the cervix and surgical treatment of cervical lesions associated with human papillomavirus (HPV) infection can also reduce the amount or quality of cervical mucus. Problems with this sticky or slippery substance that collects on the cervix and in the vagina can make it difficult for women to get pregnant.1
The Centers for Disease Control and Prevention recommends that all boys and girls age 11 or 12 be vaccinated against HPV. Men and women who weren't vaccinated as preteens can also get the vaccine into their early to mid-20s.2
Eggs may not mature properly for a variety of reasons, ranging from conditions such as PCOS, to obesity, to a lack of specific proteins needed for the egg to mature.
An immature egg may not be released at the correct time, may not make it down the fallopian tubes, or may not be able to be fertilized.
Implantation failure refers to the failure of a fertilized egg to implant in the uterine wall to begin pregnancy. While the specific cause of implantation failure are often unknown, possibilities include:3,4,5
- Genetic defects in the embryo
- Thin endometrium (pronounced en-doh-MEE-tree-uhm)
- Embryonic defects
- Endometriosis
- Progesterone resistance
- Scar tissue in the endometrial cavity
Endometriosis occurs when the cells that normally line the uterine cavity, called the endometrium, are found outside the uterus instead. A more detailed description of endometriosis can be found at the NICHD endometriosis topic page.
Research has found a link between infertility and endometriosis. Studies show that between 25% and 50% of infertile woman have endometriosis and between 30% and 40% of women with endometriosis are infertile.6,7,8 Scientists do not know the exact cause of infertility in women with endometriosis.
Current theories on how endometriosis causes infertility include the follow:
- Changes in the structure of the female reproductive organs may occur. Endometriosis can cause pelvic adhesions made of scar tissue to form between nearby structures, such as between the ovary and pelvic wall. This can obstruct and affect the release of the egg after ovulation. Scarring in the fallopian tube can interrupt block the egg's movement through the fallopian tube.
- The lining of the abdomen, which is called the peritoneum (pronounced pair-ih-tuh-NEE-uhm), may go through changes that affect its function:
- In women with endometriosis, the amount of fluid inside the peritoneum often increases.
- The fluid in the peritoneum contains substances that can negatively affect the functions of the egg, sperm, and fallopian tubes.
- Chemical changes in the lining of the uterus that occur as a result of endometriosis may affect an embryo's ability to implant properly and make it difficult for a woman to stay pregnant after conception.
PCOS is one of the most common causes of female infertility.9 It is a condition in which a woman's ovaries and, in some cases, adrenal glands produce more androgens (a type of hormone) than normal. High levels of these hormones interfere with the development of ovarian follicles and release of eggs during ovulation. As a result, fluid-filled sacs, or cysts, can develop within the ovaries. A more detailed description can be found on the NICHD PCOS topic page.
Researchers estimate that 5% to 10% of women in the United States have PCOS.10 The exact cause of PCOS is unknown, but current research suggests that a combination of genetic and environmental factors leads to the disease.
POI is a condition in which a woman's ovaries stop producing hormones and eggs at a young age. Women with POI ovulate irregularly, if at all, and may have abnormal levels of ovarian and pituitary hormones due to problems with their ovaries.
Women with POI often have trouble getting pregnant. However, pregnancy is still possible, though rare. About 5% to 10% of women with POI get pregnant without medical treatment.11
A more detailed description of POI, including possible causes, can be found on the NICHD POI topic page.
Uterine fibroids are noncancerous growths that form inside the uterus. Uterine fibroids can cause symptoms in some cases, depending on their size and location. Scientists do not know what causes fibroids to form, but it is believed that there may be a genetic basis.
Fibroids can contribute to infertility and are found in 5% to 10% of infertile women.12 Fibroids located in the uterine cavity (as opposed to those that grow within the uterine wall) or those that are larger than 6 centimeters in diameter are more likely to have a negative effect on fertility. Fibroids are more likely to affect a woman's fertility if they12:
- Change the position of the cervix, which can reduce the number of sperm that enter the uterus
- Change the shape of the uterus, which can interfere with the movement of sperm or implantation
- Block the fallopian tubes, which prevents sperm from reaching the egg and keeps a fertilized egg from moving to the uterus
- Interfere with blood flow to the uterus, which can prevent the embryo from implanting
A more detailed description of uterine fibroids can be found on the NICHD uterine fibroids topic page.
Autoimmune disorders cause the body's immune system to attack normal body tissues it would normally ignore. Autoimmune disorders, such as lupus, Hashimoto's and other types of thyroiditis, or rheumatoid arthritis, may affect fertility. The reasons for this are not fully understood and differ between diseases, but they are thought to involve inflammation in the uterus and placenta or medications used to treat the diseases. Both men and women can make antibodies that attack sperm or the reproductive organs.13
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