Elucidation of the Vaginal Microbiome During Gestation and Its Involvement with the Fraternal Birth Order Effect and Male Homosexuality

The vaginal microbiome plays a crucial role in protecting its host from bacterial, viral, and fungal pathogens and serves as a first line of immune system defense for the lower reproductive tract. The composition of the microbiome is relatively understudied despite its integral role in women’s reproductive health. Maintenance of the correct abundances of bacteria can help prevent many different infections. Additionally, the microbiota also serve to initiate and support the immune system of a newborn in the case of vaginal birth. The introduction of these organisms can mediate many interactions in the developing brain and nervous system of the infant (Günther, V. et al. 2022). Similarly, research on biological causes of sexual orientation is a new field that severely lacks extensive research. There are a few theories related to the development of homosexuality that have been backed by research. Synthesizing what we know from previous research on these two fields, we aim to bridge the gap between these two areas of study by postulating a relationship between the composition of the female vaginal microbiome during pregnancy and the fraternal birth order effect leading to male homosexuality. We propose that during pregnancy, changes in the maternal vaginal microbiome result in a change in the vaginal microbiome that makes them more susceptible to the development of anti-NLGN4Y antibodies responsible for the immune attack on the Y-linked NLGN4Y protein responsible for male brain development (Bogaert, A. et al. 2017), specifically in the anterior hypothalamus. We propose that mothers who have already given birth to males are especially susceptible to this antibody development, providing a possible explanation for the Fraternal Birth Order effect.