They only have 23 chromosomes each. When a woman becomes pregnant, her egg joins with the father's sperm to make a new cell. This cell will grow and divide into a baby. This first cell has 46 chromosomes, 23 from the mother and 23 from the father. All the instructions for how the child will grow are in the genes on these chromosomes. Each cell of the body will contain exact copies of the 23 pairs of chromosomes that were in the first cell that was formed by the egg and sperm.