Definition, Epidemiology and Diagnostic Criteria for Infertility

Infertility is most commonly defined as the failure to achieve a clinical pregnancy after twelve months or more of regular unprotected sexual intercourse. The definition has clinical thresholds that vary with age and known risk factors, a distinction between primary and secondary infertility, and an epidemiology marked by substantial global prevalence and regional variation.

Definition

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 intercourse; evaluation may begin earlier (after about 6 months) for women aged 35 years and older or when a cause is already known or suspected.

Scope

This topic covers the standard duration-based definition and its modifications, the distinction between primary and secondary infertility, the related concept of subfertility, and the global and regional epidemiology. It treats definitions and frequencies as reference knowledge and does not give diagnostic or treatment instructions.

Core questions

• What duration of unprotected intercourse defines infertility, and how is the threshold modified by age?
• How do primary and secondary infertility differ?
• How common is infertility globally, and how does prevalence vary by region?
• How do clinical and demographic definitions of infertility differ?

Key concepts

• Twelve-month duration criterion
• Earlier evaluation at age 35 and older
• Primary versus secondary infertility
• Subfertility
• Fecundability and fecundity
• Clinical versus demographic definitions
• Regional variation in prevalence

Mechanisms

The duration-based definition reflects fecundability, the per-cycle probability of conception: because most fertile couples conceive within a year, persistent failure beyond twelve months marks a population in whom an underlying barrier is more likely. Female fecundity declines with age, which is why a shorter threshold is applied for older women. Demographic definitions, used in population surveys, identify women exposed to the risk of pregnancy who do not conceive over a defined interval, and they can yield different prevalence estimates from clinical definitions.

Clinical relevance

A shared definition determines who is counted as infertile, when evaluation is offered, and how research populations are compared. As a reference topic it explains the criteria and their rationale; it is not a basis for deciding whether a particular couple should be investigated or treated.

Epidemiology

Infertility affects a substantial share of couples of reproductive age. A systematic analysis of 277 health surveys estimated that primary and secondary infertility together affect a large number of women worldwide, with the highest burdens of secondary infertility in some low- and middle-income regions (Mascarenhas et al., 2012). Definitional and methodological differences between clinical and demographic studies contribute to the range of reported prevalence figures (Inhorn & Patrizio, 2015).

Evidence & guidelines

The International Glossary on Infertility and Fertility Care provides harmonised clinical and demographic definitions (Zegers-Hochschild et al., 2017), and the ASRM committee opinion sets out the duration-based clinical criteria and age modifications (Practice Committee, 2020). National guidance such as NICE CG156 (2013, updated 2017) operationalises the criteria for service provision.

History

Definitions of infertility were historically inconsistent across clinical and demographic literatures, complicating comparison of prevalence estimates. The development of internationally agreed glossaries by ICMART and WHO, revised across successive editions, established the now-standard twelve-month clinical criterion alongside demographic definitions for population research (Zegers-Hochschild et al., 2017).

Debates

Should infertility be defined clinically or demographically?

Clinical definitions count couples seeking care after a defined period of trying, while demographic definitions count women exposed to pregnancy who do not conceive; the two yield different prevalence estimates and serve different purposes.

Seminal works

• zegers-hochschild-2017
• mascarenhas-2012
• practice-committee-asrm-2020-definitions

Frequently asked questions

What is the difference between primary and secondary infertility?

Primary infertility describes a couple who have never achieved a pregnancy, whereas secondary infertility describes inability to conceive after a previous pregnancy.

How common is infertility?

It affects a substantial minority of couples of reproductive age worldwide, with the exact figure depending on the definition used and the region studied.