perhaps a shift in soft tissue body symmetry?

Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus?
Geoffrey Miller, a, , Joshua M. Tybura and Brent D. Jordana

Evolution and Human BehaviorVolume 28, Issue 6, November 2007, Pages 375-381

Abstract
To see whether estrus was really “lost” during human evolution (as researchers often claim), we examined ovulatory cycle effects on tip earnings by professional lap dancers working in gentlemen's clubs. Eighteen dancers recorded their menstrual periods, work shifts, and tip earnings for 60 days on a study web site. A mixed-model analysis of 296 work shifts (representing about 5300 lap dances) showed an interaction between cycle phase and hormonal contraception use. Normally cycling participants earned about US$335 per 5-h shift during estrus, US$260 per shift during the luteal phase, and US$185 per shift during menstruation. By contrast, participants using contraceptive pills showed no estrous earnings peak. These results constitute the first direct economic evidence for the existence and importance of estrus in contemporary human females, in a real-world work setting. These results have clear implications for human evolution, sexuality, and economics.

very interesting stuff, plus I learned a lot about the world of gentleman's clubs.

from background:

Club patrons will often “sample” several different dancers with one lap dance each before picking one for a more expensive multisong bout of dancing. Thus, patrons can assess the relative attractiveness of different women through intimate verbal, visual, tactile, and olfactory interaction, and those attractiveness judgments can directly influence women's tip earnings, through the number of 3-min dances that patrons request from each dancer. In these ways, estrous attractiveness effects on lap-dancer earnings in gentlemen's clubs may be stronger than in other kinds of psychology research that use photo ratings (e.g., Haselton et al., 2007) or other kinds of sex work (e.g., visual pornography, phone sex) that give fewer fertility cues across fewer modalities.

from discussion:

We found strong ovulatory cycle effects on tip earnings, moderated by whether the participants were normally cycling. All women made less money during their menstrual periods, whether they were on the pill or not. However, the normally cycling women made much more money during estrus (about US$354 per shift)—about US$90 more than during the luteal phase and about US$170 more than during the menstrual phase. Estrous women made about US$70 per hour, luteal women made about US$50 per hour, and menstruating women made about US$35 per hour. By contrast, the pill users had no midcycle peak in tip earnings. As in other previous research, the pill eliminates peak-fertility effects on the female body and behavior by putting the body in a state of hormonal pseudopregnancy (e.g., [Gangestad et al., 2004], [Gangestad et al., 2005] and [Macrae et al., 2002]). This also results in pill users making only US$193 per shift compared to normally cycling women making US$276 per shift—a loss of more than US$80 per shift.

This is the first direct economic evidence for the existence of estrus in contemporary human females.

....
A final limitation is that our study did not identify the precise proximal mechanisms that influence tip earnings. These might include the previously documented shifts in body scent, facial attractiveness, soft-tissue body symmetry, waist-to-hip ratio, and verbal creativity and fluency—or they might include shifts in other phenotypic cues that have not yet been studied. We can, however, exclude some possible mediators based on previous exotic dancer research. Tip earnings are unlikely to be influenced by cycle shifts in stage-dance moves, clothing, or initial conversational content because these cues just do not vary much for professional dancers ([Barton, 2006] and Beasley, 2003 J. Beasley, Lapdancer, PowerHouse Books, New York (2003).[Beasley, 2003]). The tip earnings pattern in Fig. 1 is similar to the pattern of estradiol levels across the cycle (with a main estrous peak and a secondary midluteal peak); hence, it is plausible that estradiol levels might mediate the tip-earning effects.

Perhaps, most importantly, from an evolutionary viewpoint, further research could clarify whether women have evolved special adaptations to signal estrus through such cues—or whether the cues are “leaking” to sexually discriminating men as unselected side effects of cycle physiology. Distinguishing between estrous “signals” and “leaked cues” may be difficult in practice because estrous females (seeking extra-pair copulations with good-gene males) and extra-pair males (offering good genes) may have shared interests in female fertility signals being “conspiratorial whispers” that are accurate but inconspicuous (Pagel, 1994). In serially monogamous species such as ours, women's estrous signals may have evolved an extra degree of plausible deniability and tactical flexibility to maximize women's ability to attract high-quality extra-pair partners just before ovulation, while minimizing the primary partner's mate guarding and sexual jealousy. For these reasons, we suspect that human estrous cues are likely to be very flexible and stealthy—subtle behavioral signals that fly below the radar of conscious intention or perception, adaptively hugging the cost–benefit contours of opportunistic infidelity.

When Chess Bitch came out, Rowson's NIC review of it made a point something like 'feminists shouldn't shy away from a discussion of the effects of menstruation on chess because people do talk/think about it, and better to have a reasonable discussion*.' It seems like this should be pretty easy to look at, if you have a dozen female players who have regular cycles and are willing to tell you about them and let you compare to their rating history. (I tried looking at my own before writing this, but despite playing ten reasonably long tournaments this year, it seems like I didn't have my period at any of them. what a shame. )

Let me just add, I think you might see no effect at all on performance, i.e. I'm not suggesting it's a terribly important study, just a relatively easy one to do, at least non-rigorously. (Certainly one should expect to see far far less of an effect than you would see in a stripclub.)