The normal size of a woman’s egg is around 20mm. An egg smaller than 18mm is considered an immature egg, and an egg larger than 25mm is probably just an empty sac, an egg without quality. A woman only has one mature egg in 4 weeks and this egg must be larger than 20mm to be considered a mature egg and mature before it can be expelled. However, there are differences in each woman’s body type and some women may have eggs as long as 15mm before they can be expelled. The development of a woman’s eggs begins right after menstruation and the eggs reach their mature size when the woman enters ovulation. An egg can survive for about 18 hours after ovulation, waiting to unite with sperm for 48 hours. If it does not unite with sperm within 48 hours to form a fertilized egg, it will die within 48-72 hours.