Multiple ovulation is when two or more eggs are released in the same cycle (this is how fraternal twins are born!). No matter how many eggs are released, they will all be released within a 24-hour period. So, once you confirm ovulation with a Temperature Shift that is sustained for at least three days, you can be certain that you won’t ovulate again until your next cycle.
If you’re using the Fertility Awareness Method to avoid pregnancy, the possibility of multiple ovulation is why you need to wait until your Temperature Shift has been sustained for three days before you can consider yourself infertile. This is because if one egg is released at ovulation, and another one is released 24 hours later, and THAT egg lives for 24 hours, you’re fertile for that entire three-day period.
Multiple ovulations happen in as many as 5-10% of all ovulatory cycles – so if you don’t want to become pregnant, you need to assume that multiple ovulation may have happened and observe the Temp plus 3 rule every time you ovulate.
You might be wondering why, if multiple ovulation occurs so often, there aren’t WAY more fraternal twins out there. This is because many fraternal fetuses miscarry in what is called the “vanishing twin phenomenon”, and only one fetus is carried through to term. Only about 1 in 90 natural births actually results in fraternal twins.