# menstrual cycle research is confusing and conflicting > This leads to significant confusion in the literature and frustrates attempts at systematic reviews and meta-analysis. For example, in a recent meta-analysis on fluctuations of cardiac vagal activity across the natural menstrual cycle, previous inconsistencies in the literature could be partially resolved by applying a common definition of cycle phases to the 37 included studies (Schmalenberger et al., 2019).[[(Schmalenberger et al., 2021)#^9cmtp]] - Research has not adopted consistent methods of defining different phases - Some research use the term early follicular phase when referring to menses - Others use the term menstruation - Late luteal phase is also referred to as premenstrual phase > They identified inconsistencies in terminology and research design, namely differences in the definition of reproductive status (e.g., inconsistencies in menstrual cycle phase definition), often leading to the grouping of non-homogeneous participants; and quantification of reproductive status, by using inaccurate and subjective methods[[(Elliott-Sale et al., 2021)#^j2mxj]] > Furthermore, the heterogeneity in study populations (e.g., eumenorrheic, OCP user, post-menopausal) and large variation in research designs (e.g., definition of menstrual cycle phase, oral contraceptive pill consumption or withdrawal) has resulted in many conflicting findings in sport and exercise science research with women as participants.[[(Elliott-Sale et al., 2021)#^d8rtv]]