Baby gender determination. Coloured conceptual image of a normal male karyotype (set of chromosomes) superimposed on a 3-D ultrasound scan of a male foetus. The presence of a X and a Y chromosome (lower right) show that this is a male, females have two X chromosomes. Humans have forty- six chromosomes, arranged into twenty-three pairs. Chromosomes are made up of deoxyribonucleic acid (DNA). DNA contains sections, called genes, which encode the body's genetic information. Genes determine each cell's structure, function and behaviour. The foetus is in the third trimester of pregnancy.