Fertility

The Fertility rating is one - or rather two - of the PCs stats. It determines the likelihood of the PC becoming pregnant when having vaginal sex with an NPC able to impregnate her. There are actually two different fertility ratings, base and bonus, but there is no practical difference between them. They just add up to determine the total fertility rating and increasing one has the same effect as increasing the other.

Initial Fertility Rating

A male PC starts the game with a base fertility rating of 5 but can't of course actually get pregnant unless he is transformed into a female or a hermaphrodite.

A female PC usually starts the game with a fertility rating of 10. If she has the Fertile Starter Perk, she starts with a fertility rating of 35.

Increasing Base Fertility

Base fertility rating can be increased by consuming:

Succubi Milk - Always increases fertility to 6 if it is lower. In practice this means that succubi milk only affects the fertility rating of a PC who started off as male and hasn't previously used any fertility increasing potion.
Goblin Ale - Can increase fertility by 1-6 if it is less than 40.
Pink Egg - Can increase fertility by 5 if it is less than 40.
Neon Pink Egg - Can increase fertility by 1-6 if it is less than 50.
Large Pink Egg - Always increases fertility by 10 if it is less than 70.

Except for the succubi milk which always does exactly the same, the effects of these potions are not capped at the maximum rating, it just stops once the rating goes beyond it. It is for example possible to reach a fertility rating of 45 with goblin ale if you have 39 to start with and get the maximum effect from a single bottle. If you already have a fertility rating of 69 and eat a large pink egg, fertility rating will reach 79 which is the highest that can be achieved through potions.

Visiting a Corrupted Glade can under certain circumstances increase base fertility rating by 1 until it reaches 90. This is however quite rare and even if you have the right stats to qualify, you have to expect to visit the Deepwoods or the Forest (where the glades can be found) several hundred times (typically around 700 times according to tests done) to increase base fertility from 79 to 90. Increasing bonus fertility rating is much easier and has exactly the same effect.

Pregnancy will under certain conditions increase base fertility. Exactly which conditions are required need to be investigated. What is known, is that giving birth to a litter of Amily's children will increase fertility by 1 at least up to 40.

Increasing Bonus Fertility

Bonus fertility rating has exactly the same effect as base fertility rating. The two numbers are just added together to determine total fertility. Bonus fertility can be achieved through various perks and through fertite piercings.

Perks that boosts fertility are:

The easiest way to increase fertility beyond what is achievable with large pink eggs is to get your ears pierced with fertite jewelry, have them removed, get new ones and so on until you've reached the rating you want. This method may cost a lot of gems and toughness points but these are relatively easy to regain.

Maximum Fertility Rating

There doesn't seem to be any limit to the total fertility rating (the highest confirmed rating is 180). But a rating of 100 means that pregnancy is guaranteed (provided the partner and other conditions are right of course) so increasing it beyond that has no effect.

Temporary Fertility Increasers

A PC who is in heat will have their fertility temporarily increased.

Reducing Fertility

Being raped by a worm-infested Hellhound can decrease fertility, but only if you don't have a penis
A Brown Pill will make you infertile. You can regain your fertility by eating a Pink Pill.

Notes

There may be some other fertility increasing drugs or events in addition to the ones listed here.