This begins about eight days into her cycle. For reasons that I don't believe are understood, one developing egg becomes dominant, and the others fade away. Occasionally, this doesn't happen, and more than one egg is released, which may result in fraternal twins if pregnancy occurs. Around day 14 of the cycle, a surge in the pituitary hormones causes the release of the egg, which begins to travel down the Fallopian tube to the uterus. The fertilized egg then passes into the uterus, where it implants, or sticks on the wall, and begins to grow.
Ovulation only occurs once in each cycle.
Oral contraceptives interfere with this process in several ways. They inhibit the surges of pituitary hormones, and therefore prevent ovulation. They also alter the mucus at the mouth of the uterus, making it more difficult for sperm to get through, decreasing the likelihood of a fertilized egg becoming implanted in the uterus. Missing a pill may permit ovulation to occur, especially with the modern low-dose pills. If only one is missed, the Pill may still prevent fertilization or implantation.