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 GnRH secretion, 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 (eggs) decline, leading to a reduced ovarian reserve. Some women undergo primary ovarian insufficiency (also known as 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 her menstrual cycle, a woman generally experiences a physiological fertile window of up to 7 days before and during ovulation, when she may be able to conceive. She is infertile at other times in the menstrual cycle. 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.
Definition "Demographers tend to define infertility as childlessness in a population of women of reproductive age." 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 in young adulthood. It diminishes after age 35, with pregnancy occurring rarely after age 50. A female is most fertile within 24 hours of ovulation. Male fertility peaks usually in young adulthood 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 organizations. 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 differ significantly. A couple that tries unsuccessfully to have a child after a certain period (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 similarly and often used interchangeably. Subfertility is the delay in conceiving within six to twelve months, whereas infertility is the inability to conceive naturally within a full year.
World Health Organization The World Health Organization defines infertility as follows:
Infertility 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 (and there is no other reason, such as breastfeeding or postpartum amenorrhoea). Primary infertility is infertility in a couple who have never had a child. Secondary infertility is the failure to conceive following a previous pregnancy. Infertility may be caused by infection in the man or woman, but often there is no obvious underlying cause." The World Health Organization also adds that "women whose pregnancy spontaneously miscarries, or whose pregnancy results in a stillborn child, without ever having had a live birth, would present with primarily infertility".
United States One definition of infertility that is frequently used in the United States by reproductive endocrinologists, doctors who specialize in infertility, to consider a couple eligible for treatment is:
A woman under 35 has not conceived after 12 months of contraceptive-free intercourse. A woman over 35 has not conceived after six months of contraceptive-free sexual intercourse.
United Kingdom In the UK, previous NICE guidelines defined infertility as failure to conceive after regular unprotected sexual intercourse for two years in the absence of known reproductive pathology. Updated NICE guidelines do not include a specific definition, but recommend 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, with earlier referral to a specialist if the woman is over 36 years of age."
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