She was showing obvious signs or pregnancy with things like eating habits/craving etc and the guard lady picked up on it when she was held hostage and no doubt would have told him (●'◡'●)ﾉ
It may also have been hinted at when he mentioned his surprise after having sec in the early stages which could mean there were complications or they were warned off it by the doctor
Damn, wish the author had actually showed that instead of just skipping everything #－.－)
i think Karin inform him beforehand.
When did he find out about the pregnancy? She told him? When was that? Did they really skip that or I missed a chapter?