Biological sex development begins at conception when chromosomes from the egg and sperm combine to create a unique genetic blueprint. Research shows this process unfolds through several interconnected stages that determine physical sex characteristics.
At fertilisation, the egg contributes an X chromosome whilst the sperm contributes either an X or a Y chromosome. This chromosomal combination typically guides the development of internal reproductive organs during early pregnancy. Evidence indicates that XX chromosomes generally lead to the development of a womb and ovaries, while XY chromosomes typically result in the formation of testes. These reproductive organs become the foundation for hormone production that will influence development throughout life.
During puberty, the reproductive organs begin producing sex hormones that drive the development of secondary sex characteristics. Guidelines recognise that oestrogen promotes changes such as breast development and wider hips, whilst testosterone encourages deeper voice changes, increased muscle mass, and body hair growth. This hormonal influence creates the physical features that form what medical professionals call assigned sex at birth.
Understanding this biological process helps explain the complex interplay between chromosomes, organs, and hormones in human development. People often ask about these mechanisms when exploring questions about sex and gender, and recognising this natural variation can provide valuable context for many important conversations about identity and health.