There are two pregnancy endings, a good end with a CG and a not-so-good end without; which one you get depends on her trust level (and maybe other factors I am not aware of). The day you get the ending does not matter, just finish inside her and wait a few days.
Trust level and the amount of money on hand (kids are expensive). Good or bad, pregnancy ending is permanent ending and kicks to title scene.
This is not like those RPG-VX games where the heroine can be impregnated, give birth, and then the kid disappears, get pregnant again, wash, rinse, repeat. And it's not like monochrome where she apparently gets pregnant in the bad end and goodbye forever, bad end, or the good end "oops, thought I was really pregnant and the test was even positive, but it was just a scare, tee-hee, and guess what, for some reason, I really can't get pregnant so come inside all you want, you hopeless onii-chan!" and the game keeps going until you get bored or unlock all the scenes, whichever comes first.