Table 1.

List of the considerations related to participant characteristics and selection criteria

Consideration	Rationale (intended to…)	Pros (could…)	Cons (could…)
Define puberty as the onset of menarche [50]	Increase accuracy and validity of the population definition	Reduce between study variability in describing the population studied	Increase timescale of the study in order to recruit participants who fit the criteria
Define peri-puberty as the time around puberty			Need to be aware of other physical and endocrine indicators of puberty outside of menarche (e.g., maturation of the genital organs, development of secondary sex characteristics; Tanner stages; oestrogen and progesterone concentrations)
Define adrenarche as activation of adrenal androgen production; usually occurs before gonadarche [51]	Increase homogeneity of hormonal profiles Increase consistency of terminology	Reduce between participant variability in hormone status Reduce between study variability in describing the population studied	Reduce availability of eligible participants Increase timescale of the study if the condition needs to be confirmed prior to the commencement of data collection Increase number of participants who need to be excluded (during or retrospectively) from the study if the condition was not confirmed prior to commencement of the study
Define gonadarche as activation of reproductive glands leading to menarche [51]
Define primary amenorrhea as failure to reach menarche by age 15 years when development of secondary sexual characteristics is evident, or by age 14 years when no secondary sexual characteristics are present [52, 53]
Define eumenorrhea as menstrual cycle lengths ≥ 21 days and ≤ 35 days resulting in 9 or more consecutive periods per year, plus evidence of LH surge, plus correct hormonal profile, plus no HC use 3 months prior to recruitment
Define anovulatory as those who experience menstruation but do not ovulate (ovulation cannot be detected by urinary LH surge or confirmed by hormone concentrations via blood sample analysis) [49, 55]
Define luteal phase deficiency as cycles with less than 16 nmol·L−1 of progesterone, when a single luteal phase progesterone measurement is taken [48]
Define oligomenorrhea as those with cycle length > 35 days [54]
Define secondary amenorrhea as the absence of ≥ 3 consecutive periods in non-pregnant women with past menses [54], which can be caused by polycystic ovary syndromea, hypothalamic amenorrheab, hyperprolactinemia, or primary ovarian insufficiency [55]
Define naturally menstruating women as those who experience menstruation, with menstrual cycle lengths ≥ 21 days and ≤ 35 days, but without confirmed ovulation [ovulation was not confirmed by urinary LH surge or verified by hormone concentrations via blood sample analysis]	Increase accuracy and validity of the population definition	Reduce unfounded assumption that ovulation, and thus eumenorrhea, has been established Reduce between study variability describing the population studied	Increase timescale of the study in order to recruit participants who fit the criteria
A priori exclusion of participants with self-reported or diagnosed menstrual irregularities for eumenorrheic studies. Menstrual irregularities refer to perturbations of the eumenorrheic menstrual cycle, such as amenorrhea, anovulation, oligomenorrhea etc	Increase homogeneity of hormonal profiles	Reduce between participant variability in hormone status Increase validity of the data	Reduce availability of eligible participants for eumenorrheic studies
A posteriori exclusion of participants with observed or implied menstrual irregularities for eumenorrheic studies			Increase number of participants who need to be excluded (during or retrospectively)
No HC use ≥ 3 months prior to recruitment for study on eumenorrheic participants	Increase likelihood that an eumenorrheic cycle and its typical hormonal profile has been re-established	Reduce occurrence of atypical hormonal profiles not fitting the eumenorrheic definition	Reduce availability of eligible participants for eumenorrheic studies Increase timescale of the study as the condition needs to be met prior to recruitment
HC use ≥ 3 months prior to recruitment for HC studies	Increase likelihood that eumenorrheic cycle has been removed and replaced by a hormonal profile indicative of HC use	Reduce occurrence of atypical hormonal profiles not fitting the characterisation of HC users
Define HC users as those taking any type of contraceptive capable of altering the endogenous hormonal milieu. Please note that HC also influence other aspects of metabolism, which are beyond the scope of this paper	Increase accuracy and validity of the population definition	Reduce between study variability in describing the population studied	N/A
Report the type (e.g., OCPs, implants, injections, intrauterine devices/coils that are hormone releasing and NOT copper-based, vaginal rings, contraceptive transdermal patches) and formulation (e.g., mono, bi or triphasic; combined or progesterone-only; names and concentration of exogenous hormones) of HC used	Increase reliability of studies Increase validity of findings	Reduce between participant variability in hormone status Reduce between study variability in describing the population studied	Increase number of participants who need to be excluded as they do not know the exact type or formulation of HC used
One brand/type of OCP per group of participants [56]. Clearly identify exogenous hormone names and concentrations in each OCP, as well as androgenicity	Increase homogeneity of hormonal profiles (both endogenous and exogenous)		Increase timescale of the study whilst trying to recruit a sufficient sample size on the same brand/type of OCP Reduce generalisability of the findings
Define the first trimester as first 13 weeks of pregnancy [54]	Increase accuracy and validity of the population definition		Increase timescale of the study in order to recruit participants who fit the criteria
Define the second trimester as the time from week 14 to week 27 of pregnancy [54]		Reduce between participant variability in hormone status Reduce between study variability in describing the population studied	Increase timescale of the study in order to recruit participants who fit the criteria
Define the third trimester as the time from 27 weeks of pregnancy onwards [54]
Define full term as when a pregnancy is a normal duration (i.e., 37–42 weeks gestation) [54]
Define postpartum as the 12 months following parturition
State gestation (i.e., the length of time, in days or weeks, that a baby is in the uterus)		Reduce between study variability in describing the population studied	Increase number of participants who need to be excluded as they do not know their exact gestational stage
State gravidity (i.e., number of times that a woman has been pregnant, including miscarriages and abortions)			Increase timescale of the study in order to recruit women willing to state this number (can be a sensitive issue)
State parity (i.e., number of times that a woman has given birth to a foetus with a gestational age of 24 weeks or more, regardless of whether the child was born alive or was stillborn)
State singleton or multiple pregnancy
Define peri-menopause as the time around the occurrence of the menopause when the ovaries gradually produce less oestrogen [57]	Increase accuracy and validity of the population definition	Reduce between study variability in describing the population studied	Need to be aware of other physical indicators of menopause (e.g., hot flushes, vaginal dryness, emotional changes)
Define menopause as the time when menstruation surceases; i.e., characterised by sporadic amenorrhea. *Please note that this is the transitional time between peri- and post-menopause			Increase timescale of the study in order to recruit participants who fit the criteria
Define post-menopause as the time after which a woman has experienced 12 consecutive months of amenorrhea and is characterised by < 118 pmol∙L−1 of oestrogen, < 4.4 nmol∙L−1 of progesterone and follicle stimulating hormone > 25 IU∙L−1 [57]	Increase homogeneity of hormonal profiles Increase consistency of terminology	Reduce between participant variability in hormone status Reduce between study variability in describing the population studied	Reduce availability of eligible participants Increase timescale of the study if the condition needs to be confirmed prior to the commencement of data collection Increase number of participants who need to be excluded (during or retrospectively) if the condition was not confirmed prior to commencement of the study
Consider menopausal symptoms [58]	To limit the effects of menopausal symptoms on outcomes	Reduce the likelihood that effects are indirectly due to symptoms rather than directly due to changes in hormones	Increase the time burden to identify participants with no, or a consistent set of, menopausal symptoms capable of affecting the intended outcome
Define HRT users as those taking any type of HRT capable of altering the endogenous hormonal milieu (e.g., tablets, skin patches, gels, implants, vaginal creams, pessaries or rings; combined or oestrogen only; cyclical or continuous)	Increase accuracy and validity of the population definition	Reduce between study variability in describing the population studied	N/A
Report the type and formulation of HRT used	Increase reliability of studies Increase validity of findings	Reduce between participant variability in hormone status Reduce between study variability in describing the population studied	Increase number of participants who need to be excluded as they do not know the exact type or formulation of HRT used

Considerations without a reference have been developed by the authors for this statement

HC hormonal contraceptives, HRT hormone replacement therapy, OCP oral contraceptive pill, LH luteinising hormone, NA not applicable

^aPlease note that polycystic ovary syndrome affects between 2.2% and 26% of women worldwide, based on data from the World Health Organisation [59]

^bPlease note that for women with functional hypothalamic amenorrhea (especially with the female athlete triad), this condition should be immediately corrected