Explain how sex is determined in the unborn baby.

Sex determination in an unborn baby occurs at the moment of fertilization and depends on which sex chromosome is carried by the sperm that fertilizes the egg. Human cells normally contain 23 pairs of chromosomes, with the 23rd pair being the sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

The mother's egg always contributes an X chromosome, since that's all she has to give. The father's sperm, however, can carry either an X or a Y chromosome. If a sperm carrying an X chromosome fertilizes the egg, the resulting combination is XX, and the baby develops as female. If a sperm carrying a Y chromosome fertilizes the egg, the combination is XY, and the baby develops as male. The Y chromosome contains a gene called SRY (sex-determining region Y) that triggers the development of testes and male characteristics around the seventh week of pregnancy. Without this gene, the default developmental pathway produces female characteristics. This means the father's genetic contribution technically "determines" the baby's sex, though this is a random process. Modern ultrasound technology can usually identify the baby's sex by around 18-20 weeks of pregnancy by visualizing physical differences.