To be very specific, there is only two ways to guarantee pregnancy. One is taking a pill beforehand, and the other is losing (or surrendering) to corrupted scientists. But the second method will not have any special flavor text or scenes related to it.
Otherwise, every creampie taken is a 10% chance to make a baby.
As for impregnating the various female NPCs, you must first take them on a date, and then convince them to take a fertility pill.