The main symptom of infertility is the inability to get pregnant. A menstrual cycle that's too long (35 days or more), too short (less than 21 days), irregular or absent can mean that you're not ovulating. There may be no other outward signs or symptoms.

Each of these factors is essential to become pregnant:

• You need to ovulate. To get pregnant, your ovaries must produce and release an egg, a process known as ovulation.
• Your partner needs sperm. For most couples, this isn't a problem unless your partner has a history of illness or surgery.
• You need to have regular intercourse. You need to have regular sexual intercourse during your fertile time.
• You need to have open fallopian tubes and a normal uterus. The egg and sperm meet in the fallopian tubes, and the embryo needs a healthy uterus in which to grow.

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 implants and grows in the uterus.

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

Ovulation disorders

Ovulation disorders, meaning you ovulate infrequently or not at all, account for infertility in about 1 in 4 infertile couples. 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 — (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 production of these hormones and affect ovulation. Irregular or absent periods are the most common signs.
• Premature ovarian failure. Also called primary ovarian insufficiency, this disorder is usually caused by an autoimmune response or by premature loss of eggs from your ovary (possibly from genetics or chemotherapy). The ovary no longer produces eggs, and it lowers estrogen production in women under the age of 40.
• Too much prolactin. The pituitary gland may cause excess production of prolactin (hyperprolactinemia), which reduces estrogen production and may cause infertility. Usually related to a pituitary gland problem, this can also be caused by medications you're taking for another disease.

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 in a fallopian tube instead of the uterus
• Pelvic tuberculosis, a major cause of tubal infertility worldwide, although uncommon in the United States

Endometriosis

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

Endometriosis can also affect the lining of the uterus, disrupting 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 impact fertility by interfering with implantation or increasing the likelihood of a 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.
• Endometriosis scarring or inflammation within the uterus can disrupt implantation.
• Uterine abnormalities present from birth, such as an abnormally 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

Sometimes, 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 may correct itself with time. But, you shouldn't delay treatment for infertility.

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 increasing 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 normal ovulation. Getting to a healthy body mass index (BMI) may increase the frequency of ovulation and likelihood of pregnancy.
• Sexual history. Sexually transmitted infections such as chlamydia and gonorrhea can damage the fallopian tubes. Having unprotected intercourse with multiple partners increases your risk of a sexually transmitted infection that may cause fertility problems later.
• Alcohol. Stick to moderate alcohol consumption of no more than one alcoholic drink per day.