There is a 10% chance of getting pregnant after any scene that includes a vaginal creampie (assuming Lust Doll has a vagina). A fertility pill raises it to 100% for any scene. Please note that one pill guarantees only one pregnancy and must be taken again to re-apply the effect after delivery.
If the impregnation occurs successfully, the player receives a notification about the Pregnant: Early Stage effect. The early stage has no inherent effects on its own, but after a while it progresses to Pregnant: Late Stage. The last stage directly affects stats, adds a visible belly to Lust Doll's portrait, and modifies some encounters.
The last stage of pregnancy never goes away on its own; The only way to give birth is to visit the nursery in New Ark City. Some characters can be impregnated by the player. Unlike the player, those characters must take a fertility pill to get pregnant, which can be gifted to them; it is impossible to imbue them until some story progression is achieved with them.
Your pregnancy varies in the time it takes to develop, with some taking longer than others. In the last stage, its appearance changes and additional scenes become available.
Currently, the following characters can be impregnated: Alissa, Cassie, Fawn, Linda, Rinny.