Female fertility is affected by age. Age is thus a major fertility factor for women. Menarche, the first menstrual period, usually occurs around 12-13, although it may happen earlier or later, depending on each girl. After puberty, female fertility increases and then decreases, with advanced maternal age causing an increased risk of female infertility. In humans, a woman's fertility peaks in the early and mid-20s, after which it starts to decline slowly. While many sources suggest a more dramatic drop at around 35, this is unclear since studies are still cited from the nineteenth century and earlier. One 2004 study of European women found fertility of the 27-34 and the 35–39 groups had only a four-percent difference. At age 45, a woman starting to try to conceive will have no live birth in 50–80 percent of cases. Menopause, or the cessation of menstrual periods, generally occurs in the 40s and 50s and marks the cessation of fertility, although age-related infertility can occur before then. The relationship between age and female fertility is sometimes referred to as a woman's "biological clock."

Infertility is the inability of a person, animal or plant to reproduce by natural means. It is usually not the natural state of a healthy adult, except notably among certain eusocial species (mostly haplodiploid insects).

In humans, infertility is the inability to become pregnant or carry a pregnancy to full term. There are many causes of infertility, including some that medical intervention can treat. Estimates from 1997 suggest that worldwide about five percent of all hetersexual couples have an unresolved problem with infertility. Many more couples, however, experience involuntary childlessness for at least one year: estimates range from 12% to 28%." 20-30% of infertility cases are due to male infertility, 20-35% are due to female infertility, and 25-40% are due to combined problems in both parts. In 10-20% of cases, no cause is found. The most common cause of female infertility is ovulatory problems which generally manifest themselves by sparse or absent menstrual periods. Male infertility is most commonly due to deficiencies in the semen, and semen quality is used as a surrogate measure of male fecundity.

Women who are fertile experience a natural period of fertility before and during ovulation, and they are naturally infertile for the rest of the menstrual cycle. Fertility awareness methods are used to discern when these changes occur by tracking changes in cervical mucus or basal body temperature.

Unexplained infertility is infertility that is idiopathic in the sense that its cause remains unknown even after an infertility work-up, usually including semen analysis in the man and assessment of ovulation and fallopian tubes in the woman.

Female infertility refers to infertility in female humans. It affects an estimated 48 million women with the highest prevalence of infertility affecting people in South Asia, Sub-Saharan Africa, North Africa/Middle East, and Central/Eastern Europe and Central Asia. Infertility is caused by many sources, including nutrition, diseases, and other malformations of the uterus. Infertility affects women from around the world, and the cultural and social stigma surrounding it varies.

There is no unanimous definition of female infertility, because the definition depends on social and physical characteristics which may vary by culture and situation. NICE guidelines state that: "A woman of reproductive age who has not conceived after 1 year of unprotected vaginal sexual intercourse, in the absence of any known cause of infertility, should be offered further clinical assessment and investigation along with her partner." It is recommended that a consultation with a fertility specialist should be made earlier if the woman is aged 36 years or over, or there is a known clinical cause of infertility or a history of predisposing factors for infertility. According to the World Health Organization (WHO), infertility can be described as the inability to become pregnant, maintain a pregnancy, or carry a pregnancy to live birth.

A clinical definition of infertility by the WHO and ICMART is “a disease of the reproductive system defined by the failure to achieve a clinical pregnancy after 12 months or more of regular unprotected sexual intercourse.” Infertility can further be broken down into primary and secondary infertility. Primary infertility refers to the inability to give birth either because of not being able to become pregnant, or carry a child to live birth, which may include miscarriage or a stillborn child.

"Demographers tend to define infertility as childlessness in a population of women of reproductive age," whereas "the epidemiological definition refers to "trying for" or "time to" a pregnancy, generally in a population of women exposed to" a probability of conception. Currently, female fertility normally peaks at age 24 and diminishes after 30, with pregnancy occurring rarely after age 50. A female is most fertile within 24 hours of ovulation. Male fertility peaks usually at age 25 and declines after age 40. The time needed to pass (during which the couple tries to conceive) for that couple to be diagnosed with infertility differs between different jurisdictions. Existing definitions of infertility lack uniformity, rendering comparisons in prevalence between countries or over time problematic. Therefore, data estimating the prevalence of infertility cited by various sources differs significantly. A couple that tries unsuccessfully to have a child after a certain period of time (often a short period, but definitions vary) is sometimes said to be subfertile, meaning less fertile than a typical couple. Both infertility and subfertility are defined as the inability to conceive after a certain period of time (the length of which vary), so often the two terms overlap.

During the transition to menopause, menstrual patterns can show shorter cycling (by 2–7 days); longer cycles remain possible. There may be irregular bleeding (lighter, heavier, spotting). Dysfunctional uterine bleeding is often experienced by women approaching menopause due to the hormonal changes that accompany the menopause transition. Spotting or bleeding may simply be related to vaginal atrophy, a benign sore (polyp or lesion), or may be a functional endometrial response. The European Menopause and Andropause Society has released guidelines for assessment of the endometrium, which is usually the main source of spotting or bleeding.

In post-menopausal women, however, any genital bleeding is an alarming symptom that requires an appropriate study to rule out the possibility of malignant diseases.

Symptoms that may appear during menopause and continue through postmenopause include:

- painful intercourse

- vaginal dryness

- atrophic vaginitis – thinning of the membranes of the vulva, the vagina, the cervix, and the outer urinary tract, along with considerable shrinking and loss in elasticity of all of the outer and inner genital areas.

The diagnosis of infertility begins with a medical history and physical exam by a physician, physician assistant, or nurse practitioner. Typically two separate semen analyses will be required. The provider may order blood tests to look for hormone imbalances, medical conditions, or genetic issues.

The history should include prior testicular or penile insults (torsion, cryptorchidism, trauma), infections (mumps orchitis, epididymitis), environmental factors, excessive heat, radiation, medications, and drug use (anabolic steroids, alcohol, smoking).

Sexual habits, frequency and timing of intercourse, use of lubricants, and each partner's previous fertility experiences are important.

Loss of libido and headaches or visual disturbances may indicate a pituitary tumor.

The past medical or surgical history may reveal thyroid or liver disease (abnormalities of spermatogenesis), diabetic neuropathy (retrograde ejaculation), radical pelvic or retroperitoneal surgery (absent seminal emission secondary to sympathetic nerve injury), or hernia repair (damage to the vas deferens or testicular blood supply).

A family history may reveal genetic problems.

In the US, up to 25% of infertile couples have unexplained infertility.

On average, the ovaries supply a woman with eggs until age 51, the average age of natural menopause.

POF is not the same as a natural menopause, in that the dysfunction of the ovaries, loss of eggs, or removal of the ovaries at a young age is not a normal physiological occurrence.

Infertility is the result of this condition, and is the most discussed problem resulting from it, but there are additional health implications of the problem, and studies are ongoing. For example, osteoporosis or decreased bone density affects almost all women with POF due to an insufficiency of estrogen. There is also an increased risk of heart disease, hypothyroidism in the form of Hashimoto's thyroiditis, Addison's disease, and other auto-immune disorders.

Hormonally, POF is defined by abnormally low levels of estrogen and high levels of FSH, which demonstrate that the ovaries are no longer responding to circulating FSH by producing estrogen and developing fertile eggs. The ovaries will likely appear shriveled.

The age of onset can be as early as the teenage years, or can even exist from birth, but varies widely. If a girl never begins menstruation, it is called primary ovarian failure. The age of 40 was chosen as the cut-off point for a diagnosis of POF. This age was chosen somewhat arbitrarily, as all women's ovaries decline in function over time. However an age needed to be chosen to distinguish usual menopause from the abnormal state of premature menopause. Premature ovarian failure has components to it that distinguish it from normal menopause.

By the age of 40, approximately one percent of women have POF. Women suffering from POF usually experience menopausal symptoms that are more severe than the symptoms found in older menopausal women.

Menopause, also known as the climacteric, is the time in most women's lives when menstrual periods stop permanently, and they are no longer able to bear children. Menopause typically occurs between 49 and 52 years of age. Medical professionals often define menopause as having occurred when a woman has not had any vaginal bleeding for a year. It may also be defined by a decrease in hormone production by the ovaries. In those who have had surgery to remove their uterus but still have ovaries, menopause may be viewed to have occurred at the time of the surgery or when their hormone levels fell. Following the removal of the uterus, symptoms typically occur earlier, at an average of 45 years of age.

Before menopause, a woman's periods typically become irregular, which means that periods may be longer or shorter in duration or be lighter or heavier in the amount of flow. During this time, women often experience hot flashes; these typically last from 30 seconds to ten minutes and may be associated with shivering, sweating, and reddening of the skin. Hot flashes often stop occurring after a year or two. Other symptoms may include vaginal dryness, trouble sleeping, and mood changes. The severity of symptoms varies between women. While menopause is often thought to be linked to an increase in heart disease, this primarily occurs due to increasing age and does not have a direct relationship with menopause. In some women, problems that were present like endometriosis or painful periods will improve after menopause.

Menopause is usually a natural change. It can occur earlier in those who smoke tobacco. Other causes include surgery that removes both ovaries or some types of chemotherapy. At the physiological level, menopause happens because of a decrease in the ovaries' production of the hormones estrogen and progesterone. While typically not needed, a diagnosis of menopause can be confirmed by measuring hormone levels in the blood or urine. Menopause is the opposite of menarche, the time when a girl's periods start.

Specific treatment is not usually needed. Some symptoms, however, may be improved with treatment. With respect to hot flashes, avoiding smoking, caffeine, and alcohol is often recommended. Sleeping in a cool room and using a fan may help. The following medications may help: menopausal hormone therapy (MHT), clonidine, gabapentin, or selective serotonin reuptake inhibitors. Exercise may help with sleeping problems. While MHT was once routinely prescribed, it is now only recommended in those with significant symptoms, as there are concerns about side effects. High-quality evidence for the effectiveness of alternative medicine has not been found. There is tentative evidence for phytoestrogens.

Poor ovarian reserve is a condition of low fertility characterized by 1): low numbers of remaining oocytes in the ovaries or 2) possibly impaired preantral oocyte development or recruitment. Recent research suggests that premature ovarian aging and premature ovarian failure (aka primary ovarian insufficiency) may represent a continuum of premature ovarian senescence. It is usually accompanied by high FSH (follicle stimulating hormone) levels.

Quality of the eggs (oocytes) may also be impaired as a 1989 study by Scott et al. of 758 in vitro fertilisation (IVF) cycles showed a dramatic decline in implantation rates between high (> 25 mIU/mL) and low day three FSH (<15 mIU/mL) women even though the ages of the women were equivalent between the two groups (mean age 35 years). However, other studies show no association with elevated FSH levels and genetic quality of embryos after adjusting for age. The decline in quality was age related, not FSH related as the younger women with high day three FSH levels had higher live birth rates than the older women with high FSH. There was no significant difference in genetic embryo quality between same aged women regardless of FSH levels. A 2008 study concluded that diminished reserve did not affect the quality of oocytes and any reduction in quality in diminished reserve women was age related. One expert concluded: in young women with poor reserve when eggs are obtained they have near normal rates of implantation and pregnancy rates, but they are at high risk for IVF cancellation; if eggs are obtained, pregnancy rates are typically better than in older woman with normal reserve. However, if the FSH level is extremely elevated these conclusions are likely not applicable.

The most common words women use to describe how they felt in the 2 hours after being given the diagnosis of primary ovarian insufficiency are "devastated, "shocked," and "confused." These are words that describe emotional trauma. The diagnosis is more than infertility and affects a woman’s physical and emotional well-being. Patients face the acute shock of the diagnosis, associated stigma of infertility, grief from the death of dreams, anxiety and depression from the disruption of life plans, confusion around the cause, symptoms of estrogen deficiency, worry over the associated potential medical sequelae such as reduced bone density and cardiovascular risk, and the uncertain future that all of these factors create. There is a need for an evidence-based integrative medicine program to assist women with primary ovarian insufficiency. Presently such a program does not exist in the community, but a community of practice has formed to address this deficiency. Women with primary ovarian insufficiency perceive lower social support than control women, so building a trusted community of practice for them would be expected to improve their well being. It is important to connect women with primary ovarian insufficiency to an appropriate collaborative care team because the condition has been clearly associated with suicide related to the stigma of infertility. Suicide rates are known to be increased in women who experience infertility.

Terms oligospermia and oligozoospermia refer to semen with a low concentration of sperm and is a common finding in male infertility. Often semen with a decreased sperm concentration may also show significant abnormalities in sperm morphology and motility (technically oligoasthenoteratozoospermia). There has been interest in replacing the descriptive terms used in semen analysis with more quantitative information.

Post-testicular factors decrease male fertility due to conditions that affect the male genital system after testicular sperm production and include defects of the genital tract as well as problems in ejaculation:

- Vas deferens obstruction

- Lack of Vas deferens, often related to genetic markers for Cystic Fibrosis

- Infection, e.g. prostatitis

- Ejaculatory duct obstruction

There is some controversy as the accuracy of the tests used to predict poor ovarian reserve. One systematic review concluded that the accuracy of predicting the occurrence of pregnancy is very limited. When a high threshold is used, to prevent couples from wrongly being refused IVF, only approximately 3% of IVF-indicated cases are identified as having unfavourable prospects in an IVF treatment cycle. Also, the review concluded the use of any ORT (Ovarian Reserve Testing) for outcome prediction cannot be supported. Also Centers for Disease Control and Prevention statistics show that the success rates for IVF with diminished ovarian reserve vary widely between IVF centers.

Amenorrhoea is the absence of a menstrual period in a woman of reproductive age. Physiological states of amenorrhoea are seen, most commonly, during pregnancy and lactation (breastfeeding), the latter also forming the basis of a form of contraception known as the lactational amenorrhoea method. Outside the reproductive years, there is absence of menses during childhood and after menopause.

Amenorrhoea is a symptom with many potential causes.

Primary amenorrhoea (menstrual cycles never starting) may be caused by developmental problems, such as the congenital absence of the uterus or failure of the ovary to receive or maintain egg cells. Also, delay in pubertal development will lead to primary amenorrhoea. It is defined as an absence of secondary sexual characteristics by age 14 with no menarche or normal secondary sexual characteristics but no menarche by 16 years of age.

Secondary amenorrhoea (menstrual cycles ceasing) is often caused by hormonal disturbances from the hypothalamus and the pituitary gland, from premature menopause or intrauterine scar formation. It is defined as the absence of menses for three months in a woman with previously normal menstruation or nine months for women with a history of oligomenorrhoea.

In terms of ovarian reserve, a typical woman has 12% of her reserve at age 30 and has only 3% at age 40. 81% of variation in ovarian reserve is due to age alone, making age the most important factor in female infertility.

The most common methods of checking the status of the ovarian reserve is to perform a blood test on day 3 of the menstrual cycle to measure serum FSH level, alternatively a blood test to measure the serum AMH level can give similar information. Transvaginal ultrasound can also be used to “count the number of follicles” and this procedure is called Antral Follicle Count.

The American College of Obstetricians and Gynecologists recommends ovarian reserve testing should be performed for women older than 35 years who have not conceived after 6 months of attempting pregnancy and women at higher risk of diminished ovarian reserve, such as those with a history of cancer treated with gonadotoxic therapy, pelvic irradiation, or both; those with medical conditions who were treated with gonadotoxic therapies; or those who had ovarian surgery for endometriomas.

It is important to recognize that a poor result from ovarian reserve testing does not signify an absolute inability to conceive and should not be the sole criterion considered to limit or deny access to infertility treatment.

There are two primary ways to classify amenorrhoea. Types of amenorrhoea are classified as primary or secondary, or based on functional "compartments". The latter classification relates to the hormonal state of the patient that hypo-, eu-, or hypergonadotropic (whereby interruption to the communication between gonads and follicle stimulating hormone (FSH) causes FSH levels to be either low, normal or high).

- By primary vs. secondary: Primary amenorrhoea is the absence of menstruation in a woman by the age of 16. As pubertal changes precede the first period, or menarche, female children by the age of 14 who still have not reached menarche, plus having no sign of secondary sexual characteristics, such as thelarche or pubarche—thus are without evidence of initiation of puberty—are also considered as having primary amenorrhoea. Secondary amenorrhoea is where an established menstruation has ceased—for three months in a woman with a history of regular cyclic bleeding, or nine months in a woman with a history of irregular periods. This usually happens to women aged 40–55. However, adolescent athletes are more likely to experience disturbances to the menstrual cycle than athletes of any other age. Amenorrhoea may cause serious pain in the back near the pelvis and spine. This pain has no cure, but can be relieved by a short course of progesterone to trigger menstrual bleeding.

- By compartment: The reproductive axis can be viewed as having four compartments: 1. outflow tract (uterus, cervix, vagina), 2. ovaries, 3. pituitary gland, and 4. hypothalamus. Pituitary and hypothalamic causes are often grouped together.

Women with hypogonadism do not begin menstruating and it may affect their height and breast development. Onset in women after puberty causes cessation of menstruation, lowered libido, loss of body hair and hot flashes. In boys it causes impaired muscle and beard development and reduced height. In men it can cause reduced body hair and beard, enlarged breasts, loss of muscle, and sexual difficulties. A brain tumor (central hypogonadism) may involve headaches, impaired vision, milky discharge from the breast and symptoms caused by other hormone problems.

Pregnancy over age 50 has, over recent years, become more possible for women, due to recent advances in assisted reproductive technology, in particular egg donation. Typically, a woman's fecundity ends with menopause, which by definition is 12 consecutive months without having had any menstrual flow at all. During perimenopause, the menstrual cycle and the periods become irregular and eventually stop altogether, but even when periods are still regular, the egg quality of women in their forties is lower than in younger women, making the likelihood of conceiving a healthy baby also reduced, particularly after age 42. It is important to note, that the female biological clock can vary greatly from woman to woman. A woman's individual level of fertility can be tested through a variety of methods.

Men also experience a decline in fertility as they age, for example, the average time to pregnancy if a man is under 25 is just over 4.5 months but nearly two years if a man is over 40 (if the woman is under 25). The risk of genetic defects is greatly increased due to the paternal age effect. Children with fathers aged 40 or older are more than five times as likely to have an autism spectrum disorder than children fathered by men aged under 30. Researchers estimate that compared to a male fathering a child in his early 20's - there is double the chance of the child getting schizophrenia when the father is age 40, and triple the risk of schizophrenia when the father is age 50 (though, for most people this means the risk goes from approximately 1 in 121 when a man is 29, to 1 in 47 when a man is age 50 to 54). Men's fertility declines throughout the lifespan, with the volume of a man’s semen and sperm motility (the ability of sperm to move towards an egg) decrease continually between the ages of 20 and 80. The risk of dwarfism and miscarriage also increases as men age

In the United States, between 1997 and 1999, 539 births were reported among mothers over age 50 (four per 100,000 births), with 194 being over 55.

The oldest mother to date to conceive, was 71 years, and the youngest mother, 5 years old. According to statistics from the Human Fertilisation and Embryology Authority, in the UK more than 20 babies are born to women over age 50 per year through in-vitro fertilization with the use of donor oocytes (eggs).

Maria del Carmen Bousada de Lara is the oldest verified mother; she was aged 66 years 358 days when she gave birth to twins; she was 130 days older than Adriana Iliescu, who gave birth in 2005 to a baby girl. In both cases the children were conceived through IVF with donor eggs. The oldest verified mother to conceive naturally (listed currently in the Guinness Records) is Dawn Brooke (UK); she conceived a son at the age of 59 years in 1997 while taking oestrogen.

The symptoms of hypogonadotrophic hypogonadism, a subtype of hypogonadism, include late, incomplete or lack of development at puberty, and sometimes short stature or the inability to smell; in females, a lack of breasts and menstrual periods, and in males a lack of sexual development, e.g., facial hair, penis and testes enlargement, deepening voice.

A freemartin or free-martin (sometimes martin heifer) is an infertile female mammal with masculinized behavior and non-functioning ovaries. Genetically the animal is chimeric: Karyotyping of a sample of cells shows XX/XY chromosomes. The animal originates as a female (XX), but acquires the male (XY) component in utero by exchange of some cellular material from a male twin, via vascular connections between placentas: an example of microchimerism. Externally, the animal appears female, but various aspects of female reproductive development are altered due to acquisition of anti-Müllerian hormone from the male twin. Freemartinism is the normal outcome of mixed-sex twins in all cattle species that have been studied, and it also occurs occasionally in other mammals including sheep, goats and pigs.

External genitalia are often ambiguous, the degree depending mainly on the amount of testosterone produced by the testicular tissue between 8 and 16 weeks of gestation.