At the beginning of puberty a woman has 300,000-500,000 eggs that she will ovulate over her lifetime. Leaving her with about 25,000 eggs by the time she is 37 years old. Since a woman does not continue to produce eggs the chances of her conceiving naturally will begin to decline as she advances in age.
Beginning at age 22, when the likelihood of achieving a pregnancy is the highest, you only have a 25% chance of pregnancy each month. This is why even for two healthy individuals, it may take more than one cycle to conceive. The chances begin to drop at an accelerated rate beginning at age 30. If you are under the age of 35, and have not achieved pregnancy after 6 months of trying we recommend you seek the advice of a fertility specialist. At age 40, the rate of decline decelerates until the chances of pregnancy drop below 5%. Most women above the age of 40 will need to seek the advice of a fertility specialist to achieve a healthy pregnancy.
The chart below details the chances of pregnancy per month naturally based on age.