Typically, a woman only ovulates once per month, but there are some cases in which multiple ovulation takes place. This occurs when two eggs are released within a 24-hour period. When this happens, the woman cannot get pregnant twice, but she can create fraternal twins. Women who calculate ovulation by analyzing their symptoms may notice their telltale ovulation signs occurring for longer than usual, though many women never notice that they release more than one egg until they become pregnant with fraternal twins.

Some may assume that since multiple ovulation exists, a woman can get pregnant two separate times in one cycle. This, however, is false. If the first egg results in pregnancy, the only way the second egg can also end in pregnancy is if it was released 24 hours or less after the initial egg, sometimes resulting in fraternal twins. This is because about a day after the first egg is released, the first ovarian follicle will signal for the release of progesterone, which will prohibit other follicles from maturing. The process makes it virtually impossible to get pregnant more than once in a single cycle.

A woman who suspects multiple ovulation during a cycle may not necessarily get pregnant at all, let alone end up with twins. This is because it is possible for neither egg to be fertilized, causing the cycle to end in menstruation. It is also possible that either the first or second egg will be the only one fertilized, usually resulting in pregnancy of a singleton. Finally, both eggs could be fertilized, ending in the development of fraternal twins. Some women who initially get pregnant with twins experience vanishing twin syndrome, in which one is miscarried.

Certain women have a higher chance of multiple ovulation than others. Hyper-ovulation, or the tendency to release more than one egg in a cycle, runs in families. The main way to determine if this gene exists in a family is to count the number of sets of fraternal twins, as a family with at least one set carries the gene. For this reason, female fraternal twins are much more likely than most women to have twins, as well. Additionally, women over the age of 35 tend to experience multiple ovulation since their body releases more than one egg at a time in order to attempt to procreate before menopause.