Table 1 Definitions of terms

Gender incongruence

The International Classification of Diseases 11th Revision (ICD-11) [3] defines gender incongruence as “Gender incongruence is characterized by a marked and persistent incongruence between an individual’s experienced gender and the assigned sex. Gender variant behavior and preferences alone are not a basis for assigning the diagnoses in this group.”

Transgender people

T’Sjoen et al. [1] define transgender people as “An umbrella term to describe individuals whose gender identity differs from the sex assigned at birth based on their sexual characteristics.”

Transgender female

T’Sjoen et al. [1] define a transgender female as “A person who self-identifies as female, but whose sex was assigned male at birth.”

Transgender male

T’Sjoen et al. [1] define a transgender male as “A person whose sex was assigned female at birth (based on sexual characteristics) but self-identifies as male.”

Cisgender

T’Sjoen et al. [1] define cisgender as “A person whose identity matches the sex assigned at birth.”

Gender-affirming hormone (GAH) therapy^a

For GAH therapy, we will adopt Wikipedia’s definition for transgender hormone therapy or cross-sex hormone therapy.

Wikipedia [4] defines transgender hormone therapy, or cross-sex hormone therapy, as “a form of hormone therapy in which sex hormones and other hormonal medications are administered to transgender or gender nonconforming individuals for the purpose of more closely aligning their secondary sexual characteristics with their gender identity.”

Scoping review

Colquhoun et al. [5] define a scoping review as “A scoping review or scoping study is a form of knowledge synthesis that addresses an exploratory research question aimed at mapping key concepts, types of evidence, and gaps in research related to a defined area or field by systematically searching, selecting, and synthesizing existing knowledge.”

Craniofacial

The Merriam-Webster medical dictionary defines “craniofacial” as “relating to, or involving both the cranium and the face” [6]. In this manuscript, we will apply the following definition for “craniofacial”: “relating to, or involving the cranium or the face or both.”