The lifespan of an unfertilized egg is between 12 to 24 hours after it has left the ovary, as reported by the American Pregnancy Association. A single mature egg is released by the ovary and pushed down the fallopian tube where it is made available to be fertilized by sperm each month. If the egg is unfertilized, it will disintegrate, be absorbed by the uterus lining, which contains all of the nutrients in preparation for the fertilized egg, and will be shed, along with blood, during menstruation.
Each women is born with millions of eggs that are waiting to be released by the ovaries each month. The average monthly cycle that most women can expect will range anywhere from 28 to 32 days, although cycle times can vary. Ovulation of the egg typically happens between days 11 to 21 of the menstrual cycle, and ovulation can be easily affected by hormonal changes within the body that have been caused by stress, illness or even unexpected disruptions to one's normal routines.
If the egg is fertilized by sperm, it may implant itself to the uterine lining about 6 to 12 days after ovulation and an embryo will begin to develop within the uterus.