Polyovulation is the production of more than one ovum, or egg, during a single menstrual cycle. This paper examines the probability of the human ovarian system ovulating k eggs during a single cycle, for k ≥ 0. In order to obtain precise estimates for the probability of polyovulation, we use U.S. birth data from the 1950s (before the introduction of artificial reproductive technologies). However, to utilize birth data, we model the various processes that eggs undergo in order to result in a live birth, including fertilization, possible division, implantation, and potential miscarriage. Our model produces novel estimates for the probability that a fertilized egg divides, as well as for the zygosity type frequencies of twins, triplets, and quadruplets.