Abstract: Female age is the most important factor affecting fertility. Women are born with all the eggs they will ever have and the number of eggs available decreases each day from birth onwards. In young women the decline is fairly gradual (only a few eggs are ‘lost’ each day), but as women approach their mid to late 30s, the decrease gets much steeper (many more eggs are ‘lost’ each day). In addition to this decrease in the number of eggs available, the quality of the eggs also declines as women get older. This reduction in both the quantity and quality of available eggs means that older women are less likely to get pregnant and, if they do get pregnant, they are more likely to have a miscarriage. Male fertility may also decrease with age although to a much lesser degree.