Getting pregnant in ATLust has three major requirements. If any of these are not met, pregnancy will not be possible.
Caveat: Womb pregnancies are limited to one-at-a-time. Anal and stomach pregnancies do not have this limit. This limitation includes oviposition.
Pregnancy cannot occur if there are no eggs/ova present. Ova can only be produced by wombs, but can be obtain from the egg bank as well (See Surrogacy below).
Ova/eggs are released only in certain circumstances.
Each womb has a limited number of ova it can release. Functions that induce ovulation have a 1 day cool down. Triggering them within that 1 day window will not replenish the available ova.
Using birth control greatly lowers the chance of releasing ova in all situations, but it does not reduce the conception chance if an ova is released.
Ova will use the womb and character's attributes at the time of release. This includes gestation duration, cross species compatibility, mother/father balance and the source character's species. Changing any of these after an ovum was released will not affect it.
An ova's life span is 7% of the womb's cycle time. If cycle time is 0, the life span defaults to 1 day. In a normal 28-day cycle, this would mean the ova will live for 2 days.
There are two major components to compatibility: Species matching and cross species compatibility.
This is how your species is compared with another's. Each part of the species is compared, Class, family and species. Variation does not affect compatibility. Compatibility outside of a full match is determined by the cross-species compatibility.
Species are compared literally. This means that mammal/canine/wolf does not match mammal/canidae/wolf. Even though they mean the same thing, the text does not match up. This would result in only a class match, and thus have very low compatibility.
In mixed species, the percent of the species will affect the compatibility score. Being 50% wolf compared to someone who is 100% wolf would only create a 50% compatibility (Assuming 0% cross-species compatibility).
This affects your conception chances with those outside of your own species. If set to zero, there is virtually no chance to become pregnant from someone outside your species. (But the chance is not zero, just to keep things interesting 😉)
There are two types of surrogacy in ATLust, direct and indirect. In either case, children born through surrogacy do not go into the surrogates child list. They also do not count as births, but do count as pregnancies (IE: You can satisfy your Get Impregnated desire). Surrogacy can be done in the stomach (Oviposit into the mouth), anus and of course another womb.
A character that is already pregnant uses an ovipositor to transfer the pregnancy to you.
An indirect surrogacy is essentially having the egg and sperm fertilized inside your body, rather than receiving an already in-progress pregnancy. This can be done through any combination of using the egg bank, sperm bank, oviposition (Using unfertilized eggs) and natural insemination. This can be done to any of the aformentioned organs. For example, your character could inject ova from the egg bank into their anus, then have a male character inseminate their anus. Given high enough compatibility, fertilization will occur and you will become pregnant.