The timing doesn't quite work out unless several months are going to pass. Since the time jump is 20 years and most girls are 18-years-old at the start of the game (and the ones that are 19+ have accounted for fathers), we'd need a minimum of 3 months to pass before any of the past LIs become pregnant (since, for example, 18 years and 11 months old plus 9 months pregnancy is still less than 20-year time jump).
Certainly a possibility I've considered though.
Just to help you out a little, the natural length of gestation for a human pregnancy is 9 full months, meaning that the due date is technically the start of the 10th month. Thought that might be helpful in figuring out the math for when the MC may or may not have impregnated some or all of the moms.
It's also entirely possible that the MC will make multiple additional trips, that the specific time he travels to may shift, and that he may spend an extended time there at .
Obviously that part is pure supposition. No way of knowing before it appears in the story.