This US 2018 review lead author was a gynecologic oncologist in private practice:
“Sexual orientation is biologically conferred in the first trimester of pregnancy. Gender identity is biologically conferred during the middle trimester of pregnancy.
Since the genitals differentiate in the first trimester, and the brain becomes imprinted in the latter half of gestation, it is possible for the fetal brain to be imprinted differently than the genitals. As children mature, this innate imprinting expresses as genital anatomy, gender identity, sexual orientation and other physiologic capabilities and natural preferences along a continuum, between masculine and feminine.
The evidence shows that both orientation and identity are biologic features that co-vary with a very large number of other biologic sexually dimorphic traits.”
1. A fetus’ development is influenced by survival reactions to their environment. Although fetal and placental responses to environmental stressors are relevant to sexual orientation and gender identity, the reviewers didn’t explore the subject.
2. Epigenetic adaptations to the prenatal environment involving microRNA were mentioned in a small subsection. But the reviewers didn’t cite relevant studies involving DNA methylation, chromatin and histone modifications for epigenetic causes of and effects on sexual orientation and gender identity.
3. The reviewers included a half-dozen anecdotal quotations from personal correspondence that promoted their narrative. These came across as appeals to authority rather than evidence for scientific understanding of the subject.
It was insufficient for the review to note “a continuum between masculine and feminine” without also exploring evidence for an individual’s placement on the continuum. The question of possible epigenetic causes for sexual orientation and gender identity remains.
https://www.sciencedirect.com/science/article/pii/S009082581731510X “Biological origins of sexual orientation and gender identity: Impact on health” (not freely available)