For pregnancy to occur, every step of the human reproduction process has to happen correctly. The steps in this process are:

• One of the two ovaries releases a mature egg.
• The egg is picked up by the fallopian tube.
• Sperm swim up the cervix, through the uterus, and into the fallopian tube to reach the egg for fertilization.
• The fertilized egg travels down the fallopian tube to the uterus.
• The fertilized egg attaches (implants) to the inside of the uterus and grows.

In women, several factors can disrupt this process at any step. Female infertility is caused by one or more of the factors below.

Ovulation disorders: female infertility

Ovulation frequently or not at all accounts for most cases of infertility. Problems with the regulation of reproductive hormones by the hypothalamus or the pituitary gland or problems in the ovary can cause ovulation disorders.

• Polycystic ovary syndrome (PCOS). PCOS causes a hormone imbalance, which affects ovulation. PCOS is associated with insulin resistance and obesity, abnormal hair growth on the face or body, and acne. It’s the most common cause of female infertility.
• Hypothalamic dysfunction. Two hormones produced by the pituitary gland are responsible for stimulating ovulation each month — follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Excess physical or emotional stress, a very high or very low body weight, or a recent substantial weight gain or loss can disrupt the production of these hormones and affect ovulation. Irregular or absent periods are the most common signs.
• Primary ovarian insufficiency. Also called premature ovarian failure, this is usually caused by an autoimmune response or by premature loss of eggs from your ovary, possibly as a result of genetics or chemotherapy. The ovary no longer produces eggs, and it lowers estrogen production in women under age 40.
• Too much prolactin. The pituitary gland can cause excess production of prolactin (hyperprolactinemia), which reduces estrogen production and can cause infertility. This can also be caused by medications you’re taking for another condition.

Damage to fallopian tubes (tubal infertility)

Damaged or blocked fallopian tubes keep sperm from getting to the egg or block the passage of the fertilized egg into the uterus. Causes of fallopian tube damage or blockage can include:

• Pelvic inflammatory disease, an infection of the uterus and fallopian tubes due to chlamydia, gonorrhea, or other sexually transmitted infections
• Previous surgery in the abdomen or pelvis, including surgery for ectopic pregnancy, in which a fertilized egg implants and develops somewhere other than the uterus, usually in a fallopian tube

Endometriosis

Endometriosis occurs when tissue that typically grows in the uterus implants and grows in other places. This extra tissue growth and the surgical removal of it can cause scarring, which can block fallopian tubes and keep an egg and sperm from uniting.

Endometriosis can also disrupt the implantation of the fertilized egg. The condition also seems to affect fertility in less-direct ways, such as damage to the sperm or egg.

Uterine or cervical causes

Several uterine or cervical causes can interfere with the egg implanting or increase the risk of miscarriage:

• Benign polyps or tumors (fibroids or myomas) are common in the uterus. Some can block fallopian tubes or interfere with implantation, affecting fertility. However, many women who have fibroids or polyps do become pregnant.
• Problems with the uterus present from birth, such as an unusually shaped uterus, can cause problems becoming or remaining pregnant.
• Cervical stenosis, a narrowing of the cervix, can be caused by an inherited malformation or damage to the cervix.
• Sometimes the cervix can’t produce the best type of mucus to allow the sperm to travel through the cervix into the uterus.

Unexplained infertility

In some cases, the cause of infertility is never found. A combination of several minor factors in both partners could cause unexplained fertility problems. Although it’s frustrating to get no specific answer, this problem can correct itself with time. But you shouldn’t delay infertility treatment.

Risk factors

Certain factors may put you at higher risk of infertility, including:

• Age. The quality and quantity of a woman’s eggs begin to decline with age. In the mid-30s, the rate of follicle loss speeds, resulting in fewer and poorer quality eggs. This makes conception more difficult and increases the risk of miscarriage.
• Smoking. Besides damaging your cervix and fallopian tubes, smoking increases your risk of miscarriage and ectopic pregnancy. It’s also thought to age your ovaries and deplete your eggs prematurely. Stop smoking before beginning fertility treatment.
• Weight. Being overweight or significantly underweight may affect ovulation. Getting to a healthy body mass index (BMI) might increase the frequency of ovulation and the likelihood of pregnancy.
• Sexual history. Sexually transmitted infections such as chlamydia and gonorrhea can damage the fallopian tubes. Having unprotected sex with multiple partners increases your risk of a sexually transmitted infection that may cause fertility problems later.
• Alcohol. Excess alcohol consumption can reduce fertility.

Prevention: female infertility

For women thinking about getting pregnant soon or in the future, these tips might help:

• Maintain a healthy weight. Overweight and underweight women are at increased risk of ovulation disorders. If you need to lose weight, exercise moderately. Strenuous, intense exercise of more than five hours a week has been associated with decreased ovulation.
• Quit smoking. Tobacco has multiple negative effects on fertility, as well as your general health and the health of a fetus. If you smoke and are considering pregnancy, quit now.
• Avoid alcohol. Heavy alcohol use may lead to decreased fertility. And any alcohol use can affect the health of a developing fetus. If you’re planning to become pregnant, avoid alcohol, and don’t drink alcohol while pregnant.
• Reduce stress. Some studies have shown that stress can cause couples to have poorer results with infertility treatment. Try to reduce stress in your life before trying to become pregnant.