Selection model helps explain why most human pregnancies are singletons

Each month during a woman's menstrual cycle, an ovary prepares 10–20 antral follicles, fluid-filled sacs that hold immature eggs, for maturation. In most cycles, only one follicle is selected to undergo maturation, eventually releasing an egg ready for fertilization. In fact, only 2–3% of natural pregnancies result in fraternal twins, or two eggs being released and fertilized. For natural pregnancies, it is most common for only one egg to be released and fertilized.