Can you tell just by looking where a woman's at in her reproductive cycle? For other primate species, such as the red-bottomed baboon, it's pretty easy to see. For humans, the changes are more subtle.
But there are some scientists out there who claim it's possible to tell the stage of a woman's cycle simply by measuring the shape of her face - leading to the idea of a distinctive 'ovulation face' or even a 'period face'. Now, a new study has contradicted this idea.
Back in the 1990s, several small studies showed that on the day of ovulation, the symmetry of a woman's soft tissue, such as her breast size, ear size and digit length were subtly shifted by hormones to make them more attractive to men.
Now, however, a more thorough replication of this research has found no such fluctuations in facial shape during a woman's menstrual cycle.
"Our results suggest that the previously found increased facial attractiveness of women in the most fertile phase of the menstrual cycle is not driven by changes in facial shape," the authors write, "but might instead stem from other changes in facial appearance, such as a more attractive skin tone; and underline the importance of replication of studies with new methods."
Using a larger sample size and more detailed hormonal tests than previous studies, the new research set out to test whether reported changes in a woman's attractiveness at different points in their menstrual cycle might be linked to the shape of their face.
As ovulation approaches, studies have shown the female body goes through a bunch of changes. Voices, for instance, can rise and body odour can become more attractive.
But while there is some evidence that women's faces are more attractive to both men and women near ovulation, these results are flimsier and their conclusions have recently come under contention.
In 2019, for instance, researchers improved upon older methods and found no effects of symmetry and femininity on attractiveness.
This year's research is yet another follow-up with an equally skeptical take.
Testing 75 healthy women with regular cycles, the authors measured various components of facial appearance - symmetry, sex-typical features and attractiveness - at three different stages in the menstrual cycle.
Unlike previous research, ovulation was determined not by an arbitrary length of time after menstruation, but from peaks in crucial hormones such as oestradiol and luteinising hormone. This direct measurement along with the much larger sample size sets the study apart from many previous methods.
"[T]hese results do not support reports of symmetry fluctuations in facial images and other body measurements across the menstrual cycle," the authors conclude.
In the new paper, the researchers admit they do not know why previous studies turned up such different results, only that we may have landed on the wrong explanation.
Rather than facial symmetry changing, they suggest, women could appear more attractive at certain times of their cycle because of changes in their skin tone.
"The changes in attractiveness judgements found in some of the previous studies might also be a by-product of changes in hormonal levels," the authors suggest.
And that, they say, is exactly why we need to replicate studies like this with novel methods and novel samples - so we don't go assuming something's true when it very well might be false.
The study was published in Proceedings of the Royal Society B.