How is the sex of the baby determined?

The sex of a baby is determined genetically at conception through the combination of sex chromosomes from both parents. Every human cell contains 23 pairs of chromosomes, with the 23rd pair being the sex chromosomes that determine biological sex. Females have two X chromosomes (XX configuration), while males have one X and one Y chromosome (XY configuration).

When reproductive cells (gametes) form, they undergo meiosis and receive only one chromosome from each pair. Therefore, all eggs produced by the mother contain an X chromosome, but sperm from the father can carry either an X or a Y chromosome—approximately 50% of each. When fertilization occurs, if an X-carrying sperm fertilizes the egg, the result is XX (female). If a Y-carrying sperm fertilizes the egg, the result is XY (male). The critical gene on the Y chromosome is called SRY (sex-determining region Y), which activates around week 7 of embryonic development and triggers the formation of testes and subsequent male development. In its absence, the default developmental pathway produces female anatomy. This process is entirely random, which is why the natural ratio of male to female births is approximately 1:1, though slightly more males are born (about 105 males for every 100 females).