Olfaction in RPL: Circumstantial Evidence for a Human Bruce Effect?

Background: About 50% of RPL remains unexplained. In the Bruce effect, exposure to the odor of non-stud males triggers miscarriage. The Bruce effect is evident in rodents, but has yet to be investigated in humans. Given evidence for human reliance on social chemosignals in mate selection and menstrual synchronization, we set out to probe for circumstantial evidence of a human Bruce effect in unexplained RPL.

Objective and Methods: Test whether women prone to idiopathic RPL differ in their olfactory capacities from a control group with no known previous miscarriages. We measured olfactory perception of ordinary odors, putative human social chemosignals, and body odors, concurrently with physiological monitoring, in 21 RPL and 21 matched control women.

Results: RPL women had better olfactory abilities, yet decreased physiological responses to odor. The RPL group were 55% better at detecting putative human chemosignals (F_1,40=6.4, p=0.015). Moreover, the ability to successfully detect chemosignals was correlated with number of abortions (r₂₁=0.46, p=0.035). In turn, whereas exposure to men`s body odor drove increased Galvanic Skin Response (GSR), cortisol, and testosterone levels in controls, no such increases were evident in RPL (F_1,38=6.38, p=0.016, F_1,36=3.82, p=0.06). Whereas the above uncovers altered olfaction in RPL women, we also investigated the body odor of men in RPL couples. Strikingly, we found that an independent group of 35 women rated body odor of RPL men as different from that of control men (p<0.001).

Conclusion: RPL women had altered responses to body odors. RPL men had altered body odor. We speculate a model where a particular female susceptibility combines with particular male body odor to put in motion a cascade that ends in miscarriage. Our current data provide only circumstantial support for this hypothesis, yet we outline a path towards a causal mechanistic exploration of a Bruce effect in humans.