Day 0: Pronuclear stage
Fertilization of an egg by a sperm doesn’t happen instantaneously. The two gradually unite over the course of 4–6 hours. The first sign of fertilization is the presence of two round bodies in the egg’s center — the female pronucleus and the male pronucleus. Each contains 23 chromosomes, representing an equal genetic contribution from each parent. In a process known as syngamy, the two cells come together, creating a cell with 46 chromosomes.