Digit ratio (2D:4D) and circulating testosterone, oestradiol, and progesterone levels across the menstrual cycle

Background: Digit ratio (2D:4D) is used by researchers as an indicator of prenatal sex hormone exposure. Twoprevious studies have examined associations between 2D:4D and circulating sex steroid concentrations acrossthe menstrual cycle in adult females. One reported that digit ratio correlated positively with oestradiol levels,whereas the other found no such effect; neither observed significant associations with progesterone.Aims: To examine associations between 2D:4D, as well as asymmetry (i.e. right minus left 2D:4D), and circulatingsex steroids across the menstrual cycle.Study design: Correlational.Subjects: 32 naturally cycling adult females from rural southern Poland.Outcome measures: Salivary oestradiol, progesterone, testosterone, and testosterone to oestradiol ratio (T:O)measured during the follicular, peri-ovulatory, and luteal phases. Average levels across the cycle were alsoexamined.Results and conclusions: Asymmetry in digit ratio correlated positively with oestradiol at each phase, as well aswith average levels across the cycle. Each association, other than that relating to average levels, remainedstatistically significant after a range of covariates had been controlled for. No other significant correlations wereobserved between digit ratio variables and circulating hormone levels. Our results might suggest that low exposureto androgens and/or high exposure to oestrogens during gestation is a predictor of high oestradiol levelsin naturally cycling females of reproductive age. However, considering that it was asymmetry in digit ratio, andnot either right or left 2D:4D, that was a significant predictor, it is also possible that these effects reflect moregeneral associations between bilateral asymmetry and circulating oestradiol levels