Why do humans menstruate while most mammals do not? Here, we present our answer to this long‐debated question, arguing that (i) menstruation occurs as a mechanistic consequence of hormone‐induced differentiation of the endometrium (referred to as spontaneous decidualization, or SD); (ii) SD evolved because of maternal–fetal conflict; and (iii) SD evolved by genetic assimilation of the decidualization reaction, which is induced by the fetus in non‐menstruating species. The idea that menstruation occurs as a consequence of SD has been proposed (...) in the past, but here we present a novel hypothesis on how SD evolved. We argue that decidualization became genetically stabilized in menstruating lineages, allowing females to prepare for pregnancy without any signal from the fetus. We present three models for the evolution of SD by genetic assimilation, based on recent advances in our understanding of the mechanisms of endometrial differentiation and implantation. Testing these models will ultimately shed light on the evolutionary significance of menstruation, as well as on the etiology of human reproductive disorders like endometriosis and recurrent pregnancy loss. (shrink)

In lieu of an abstract, here is a brief excerpt of the content:Reviewed by:Bleed: Destroying Mythsand Misogyny in Endometriosis Care by Tracey LindemanSarah Seabrook (bio)Bleed: Destroying Myths and Misogyny in Endometriosis Care by Tracey Lindeman. Toronto: ECW Press, 2023Endometriosis—a complex and painful health condition in which tissue similar to the endometrium tissue that normally lines the inside of a person's uterus grows outside—has gained considerable attention over the last few years. Because endometriosis patients' concerns about their reproductive health and (...) menstrual experiences are often dismissed by healthcare professionals (Wahl 2021; Alberta Women's Health Foundation 2022), there has been a groundswell of conversations about abnormal menstruation and endometriosis, not by government or medical professionals but rather by patients who validate each other's experiences and relate to one another's struggles in seeking access to care (EndoAct Canada 2024). Experiences about the trials and tribulations endometriosis patients endure seeking medical care and diagnosis are shared through op-eds and Facebook support groups and have appeared on special feature segments on the local news. With increased attention around endometriosis, there remains a lot of work to do around dispelling what endometriosis truly is and why people are not being adequately treated.In BLEED: Destroying Myths and Misogyny in Endometriosis Care, author and endometriosis patient Tracey Lindeman (2023), takes on the work of that dispelling. With a focus on the North American context (largely Canada and the United States), Lindeman shines a light on the deeply embedded systemic discrimination that shapes experiences of endometriosis and other gendered health conditions that prevent women, trans, and non-binary people from receiving a diagnosis and/or adequate treatment. This compelling part memoir, part investigation of the systemic barriers to care for endometriosis patients, reveals the long-standing and ongoing history of misogyny in contemporary gynecology and healthcare (Davis-Floyd 1997; Wetherell, Taylor, and Yates 2001; Young, Fisher, and Krikman 2019), including how pharmaceutical funding and interests are prioritized at the expense of endometriosis patients' health and well-being, and how sexism and racism, among other markers of discrimination, are [End Page 181] institutionalized features in healthcare systems that strip women, trans, and non-binary people of their agency over their bodies and decision-making capacity.BLEED is a particularly timely and important contribution to feminist health scholarship because it offers an intimate account of how the historical medicalization of female-specific health issues has normalized power asymmetries between patients and medical professionals. She demonstrates how the contemporary dynamics in healthcare settings leave women and gender-nonconforming people to navigate the intersections of misogyny and myths surrounding endometriosis. Lindeman does so by discussing her own experiences, including being asked by medical professionals to provide a written letter of support from her male partner indicating his approval of her requested hysterectomy.As both a researcher and someone with endometriosis, Lindeman is able to take the narrative of BLEED in several directions. She offers readers the intimacy of her perspective and experiences with endometriosis in Canada's health system while also offering readers a broader view of how age, race, gender identity, sexual orientation, and geography inform the experiences of others who have endometriosis. Her ability to situate her experiences as a cis-white woman as the focal point of her analysis provides readers with a strong reference point to recognize and understand the heightened systemic discrimination experienced by her nonwhite, non-cis interviewees. In particular, the contrast between Lindeman's experiences of being prescribed sixty-tablet refills of the painkiller naproxen for endometriosis pain and the experiences of her interviewee, Denise, a black cis-woman who was only given five naproxen pills a month provides insight into racialized dimensions of endometriosis care (39).Recognizing that a history unlearned results in history being repeated, Lindeman opens BLEED by situating the painfully paternalistic and misogynistic experiences of endometriosis patients in the healthcare system in the context of the evolution of gynecology as a subfield. Lindeman highlights how the medicalization of endometriosis as a condition went hand in hand with the professionalization of gynecology, which has since created and perpetuated a power asymmetry between patients and their care providers. Whereas gynecological care was historically provided by women—witches, midwives, and healers—the long male-dominated... (shrink)

Given an equal sex ratio at conception, the excess of human males at birth can only be explained by greater loss of females during pregnancy. It is proposed that the bias against females during human development is the result of a greater degree of genetic and metabolic “differentness” between female embryos and maternal tissues than for similarly aged males, and that successful implantation and placentation represents a threshold dichotomy, where the acceptance threshold shifts depending on maternal condition, especially stress. Right (...) and left ovaries are not equal, and neither are the eggs and follicular fluid that they produce, and it is further hypothesized that during times of stress, the implantation threshold is shifted sufficiently to favor survival of females, most likely those originating from the right ovary, and that this, rather than simply a greater loss of males, explains at least some of the variability in the human sex ratio at birth. (shrink)