Infertility overview

In biology, infertility is the inability of a male and female organism to reproduce. It is usually not the natural state of a healthy organism that has reached sexual maturity. As a medical term, infertility does not apply before and after the normal period of natural, age-dependent reproductive function: in children before puberty, or in women after menopause.
In humans, infertility is defined as the inability to become pregnant after at least one year of unprotected and regular sexual intercourse involving a male and female partner. There are many causes of infertility, including some that medical intervention can treat. Estimates from 1997, suggest that worldwide about five percent of all heterosexual couples have an unresolved problem with infertility. That figure has been on the rise, with the World Health Organization (WHO) reporting in 2023 that about 17.5% of people experience infertility during their lifetime, while prevalence at a given time point is 12.6%. Many more couples, however, experience involuntary childlessness for at least one year, with estimates ranging from 12% to 28%.
Male infertility is responsible for 20–30% of infertility cases, while 20–35% are due to female infertility, and 25–40% are due to combined problems in both partners. In 10–20% of cases, no cause is found.
The most common causes of female infertility are hormonal in nature, including low estrogen, imbalanced gonadotropin-releasing hormone (GnRH) secretion, polyendocrine metabolic ovarian syndrome (PCOS), and aging, which generally manifests in sparse or absent menstrual periods leading up to menopause. As women age, the number of ovarian follicles and oocytes (immature eggs) decline, leading to a reduced ovarian reserve. Some women undergo primary ovarian insufficiency (premature menopause) or the loss of ovarian function before age 40, leading to infertility. 85% of infertile couples have an identifiable cause, and 15% is designated unexplained infertility. Of the 85% of identified infertility, 25% is due to disordered ovulation (of which 70% of the cases are due to polycystic ovarian syndrome). Tubal infertility (structural issues with the fallopian tubes) is responsible for 11–67% of infertility in women of childbearing age, with the large range in prevalence due to different populations studied. Endometriosis, the presence of endometrial tissue (which normally lines the uterus) outside of the uterus, accounts for 25–40% of female infertility.
During the menstrual cycle, there is a limited physiological fertile window of up to seven days before and during ovulation, when conception may be possible. Both the length of the menstrual cycle and the length of the fertile window are highly variable. Fertility awareness methods are used to discern when these changes occur by tracking changes in cervical mucus, basal body temperature, or hormone measurements.
Male infertility is most commonly due to deficiencies in the semen, and semen quality is used as a surrogate measure of male fecundity. Male infertility may also be due to retrograde ejaculation, low testosterone, functional azoospermia (in which sperm is not produced or not produced in enough numbers) and obstructive azoospermia in which the pathway for the sperm (such as the vas deferens) is obstructed.