Table I.

Description of five approaches to measuring infertility prevalence identified from the systematic review.

	Prospective time-to-pregnancy design1	Retrospective time-to-pregnancy design	Current duration design	Self-reported infertility measure (direct)	Constructed infertility measure (indirect)
Description	Participants are enrolled prior to period of unprotected intercourse (PUI) (incident cohort) or during a period of unprotected intercourse (prevalent cohort). Participants are followed until pregnancy, infertility treatment, or study conclusion (administrative censoring).	Participants are asked to recall the PUI or pregnancy attempt time prior to becoming pregnant (pregnancy-based approach). Alternatively, participants may be asked about a PUI and/or time-to-pregnancy (TTP) in a specified time regardless of outcome (historical prospective approach).	Participants are enrolled during a current PUI or pregnancy attempt. Current duration (CD) is calculated as the interval between when the PUI or pregnancy attempt began and date of interview. CD values are used to estimate a summary TTP distribution for the population using survival methods and under certain analytic assumptions.	Participants are queried directly about their ability to conceive either within a specified duration of time (e.g. 12 months) or based on their subjective evaluation.	Infertility status is determined based on the presence or absence of a pregnancy or live birth among couples exposed to conception for a defined period. Exposure to conception is inferred from survey questions and/or a reproductive calendar.
Sample questions for querying respondents	[For those planning to conceive] Are you pregnant (asked at specified intervals during follow-up)? [For those planning to conceive] pregnancy is ascertained by pregnancy testing over follow-up period	How long had/have you been trying to become pregnant? How many months did you have regular intercourse without contraception before you became pregnant?	[For those at risk of pregnancy at interview]: Series of questions on dates of last use of contraception, pregnancy, or birth. Current duration is calculated from start of at-risk interval to date of interview. [Among those at risk of pregnancy at interview] How long have you been trying to become pregnant? (number of months or years)	Have you ever experienced a period of at least 12 months where you were having unprotected intercourse (or attempting to become pregnant) but did not become pregnant? Have you and a partner ever had difficulty conceiving?	Constructed based on a series of questions or reproductive calendar on relationship status, birth history, contraceptive use, and, in some instances, sexual activity and desire to have another child.
Common research objectives	Assess the biologic capacity for reproduction (i.e. fecundity) Examine the relationship between risk factors on fecundity	Estimate fecundity or measure infertility prevalence Identify and/or examine risk factors, which need to be anchored around the start of the PUI or pregnancy attempt	Generate population-based estimates of infertility prevalence Identify and/or examine risk factors, which need to be anchored around the start of the PUI or pregnancy attempt	Estimate infertility prevalence Assess association between infertility and risk factors, outcomes and/or treatment seeking behavior	Generate population-based estimates of infertility prevalence with nationally representative demographic and reproductive health survey data
Common applications for2:
Type of prevalence3	Period prevalence	Period or lifetime prevalence	Period prevalence	Period or lifetime prevalence	Period prevalence
Duration cut-off for infertility	12 months 24 months	12 months 24 months	12 months 24 months	12 months No duration (subjective measure)	12 months 60 months
Pregnancy intentions considered in numerator	Always considered	Sometimes considered	Sometimes considered	Sometimes considered	Commonly not considered
Denominator considered	Those attempting to conceive	Those ever at risk of pregnancy or attempting to conceive	Those at risk of pregnancy or attempting to conceive at time of interview	Ever and not at risk of pregnancy (e.g. all women of reproductive age)	Ever and not at risk of pregnancy (e.g. all women of reproductive age)

¹The prospective time-to-pregnancy design approach is considered the gold standard.

²Common applications are summarized based on the studies included in the systematic review.

³Period prevalence is defined as the proportion of individuals/couples with infertility at a given point or interval in time, which may be current or past depending on the study aims. Lifetime prevalence is defined as the proportion of individuals/couples who have ever experienced infertility in their life.