Thus, a woman's fertility begins to decline significantly after the age of 35. Spontaneous and successful pregnancies after the age of 40 become less common, and after the age of 45 are rare (but not zero!). The woman seeking natural conception at this age will find that the facts are fairly discouraging. While 80% of women between 40 and 45 can potentially conceive with different methods, this percentage falls off dramatically after the age of 45. The average woman notices that by this time her cycle is already less regular.
An externality is a consequence of an economic activity experienced by unrelated third parties; it can be either positive or negative. Pollution emitted by a factory that spoils the surrounding environment and affects the health of nearby residents is an example of a negative externality.
High transaction costs are very often at the root of the problems discussed under the heading of externalities, especially in those situations where the external costs or benefits accrue to very large numbers of third parties and therefore a contractual agreement to internalize the externality is extremely costly to negotiate.