By the time a woman reaches puberty, this potential egg supply has dropped to about 300,000.
A woman is most fertile in her mid-twenties and around the age of 27 her fertility starts to decline. If you are sexually active at the time of ovulation and are not using birth control, you have a 20-30% chance of becoming pregnant during each menstrual cycle.
The production of good quality eggs drops rapidly after the age of 35. By age 40 the chances of becoming pregnant naturally are reduced to about 5%. This is why you must monitor your ovulation pattern to increase your chances of getting pregnant sooner.