Just keep repeating cumming inside over and over (say 5/6 times with ctrl key to speed up) and they will get pregnant. The way it is coded, everytime you cum inside probability of each orgasm making the girl pregnant goes up. It is not just that your chances are higher by more random raws, but the probability of each draw goes up. Just to be sure, before hitting the botton to continue the story save, so that if she is not pregnant after that, reload and cum inside her a few more times. Eventually she will be pregnant.
The only one this does not work is wife and in her sex scene you can only cum inside once.