Biological sex development begins at the moment of conception when an egg and sperm combine to create your chromosomes. The egg always contributes an X chromosome, whilst the sperm contributes either an X or Y chromosome. This chromosomal combination becomes the blueprint for biological sex development.
Research shows that if you receive XX chromosomes, your body will typically develop a womb and ovaries during foetal development. If you receive XY chromosomes, your body will develop a penis and testicles. These reproductive organs play a crucial role in determining which hormones your body will produce when puberty begins. The hormones testosterone and oestrogen then drive the development of secondary sex characteristics including body hair distribution, voice changes, and breast development during adolescence.
Evidence indicates that this process usually follows predictable patterns, though biological development can sometimes vary from these typical pathways. Understanding sex development helps explain how our bodies change throughout different life stages and why hormonal changes have such significant effects during puberty. This knowledge forms the foundation for understanding both typical development and the various ways human biology can naturally differ from expected patterns.