The teacher is only telling half of the truth of this innocent one, deep down she does know Lily can do both Light and Dark, is it a stroke of luck that Dark then Light appears in order?
There's a bit more that shows it's in fact not from pregnancy just yet.
For one she still has a strong apetite, she also does not feel ill at all (no stomachache whatsoever for these chapters). Her body growing to the front might be more related to how brutal their bed exercises are, she also...
After 23 chapters there are still people who are confused why this family line keeps this boyfailure around, not realizing how dangerless this loser is despite being a despicable pervert. Great read as always.