Rice University Apr 22 2026 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.