By the end of a mother’s first trimester of pregnancy, a baby girl has all the eggs she’ll ever carry in her lifetime. They lie dormant in the ovaries until she reaches puberty, when they’re released once a month. Each month of a cycle, several eggs begin to mature in the ovaries, but normally only one developed egg is released. This happens during ovulation. The egg has a very short lifespan and has to be fertilised within 12 to 24 hours, otherwise it disintegrates, and is expelled during your period. If two eggs are released during ovulation and both are fertilised, you get non-identical twins. If a single fertilised egg splits during the early stages of cell division, you get identical twins, which have their own placenta. If they split in the later stages of cell division, they share a placenta. The egg is the largest cell in the body and is similar in size to that of a grain of sand.